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...
OSPF allows sets of networks to be grouped together. Such a
grouping is called an area. The topology of an area is hidden
...
... destination and mask. Two
different subnets of the same IP network number may have
different sizes (i.e., different masks). This is commonly
referred to as variable length subnetting. A packet is routed
...
... Network ...
... subnet/supernet. It is possible
for one physical network to be assigned multiple IP
network/subnet numbers. We consider these to be separate
...
... for one physical network to be assigned multiple IP
network/subnet numbers. We consider these to be separate
networks ...
... IP
network/subnet numbers. We consider these to be separate
networks. Point-to-point physical networks ...
... networks. Point-to-point physical networks are an exception
- they are considered a single network no matter how many
...
... physical networks are an exception
- they are considered a single network no matter how many
(if any at all) IP network/subnet ...
... - they are considered a single network no matter how many
(if any at all) IP network/subnet numbers are assigned to
them.
...
... Network mask ...
... range of IP addresses
residing on a single IP network/subnet/supernet. This
specification displays network ...
... IP network/subnet/supernet. This
specification displays network masks as hexadecimal numbers.
...
... For example, the network mask for a class C IP network is
displayed as 0xffffff00. Such a mask is often displayed
elsewhere in the literature as 255.255.255.0.
...
... Point-to-point networks ...
...
A network that joins a single pair of routers. A 56Kb
serial line is an example of a point-to-point ...
... Networks supporting many (more than two) attached routers,
together with the capability to address ...
... they exist. Each pair of routers on a broadcast network is
assumed to be able to communicate directly. An ethernet is
...
... Non-broadcast networks ...
... neighboring router, in turn. An X.25 Public Data Network
(PDN) is an example of a non-broadcast network ...
... OSPF runs in one of two modes over non-broadcast networks.
The first mode, called non-broadcast multi-access or NBMA ...
... simulates the operation of OSPF on a broadcast network. The
second mode, called Point-to-MultiPoint, treats the non-
...
... Point-to-MultiPoint, treats the non-
broadcast network as a collection of point-to-point links.
Non-broadcast ...
... networks or
Point-to-MultiPoint networks, depending on OSPF's mode of
operation over the network ...
... connection between a router and one of its attached
networks. An interface has state information associated
...
... routing protocol itself. An interface to
a network has associated with it a single IP address and
mask (unless the network ...
... network has associated with it a single IP address and
mask (unless the network is an unnumbered point-to-point
network ...
... network is an unnumbered point-to-point
network). An interface is sometimes also referred to as a
link ...
... Two routers that have interfaces to a common network.
Neighbor relationships are maintained by, and usually
...
... link state advertisements of all routers and
networks forms the protocol's link state database.
Throughout this memo, link state advertisement ...
... neighbor relationships. On broadcast networks the Hello
Protocol can also dynamically discover neighboring routers.
...
... Each broadcast and NBMA network that has at least two
attached routers has a Designated Router ...
... Router generates an LSA for the network and has other
special responsibilities in the running of the protocol.
The Designated Router ...
... broadcast or NBMA
network. This in turn reduces the amount of routing
protocol traffic and the size of the link-state ...
... link-state routing protocol was developed for use in
the ARPANET packet switching network. This protocol is
described in [Ref3]. It has formed the starting ...
... traffic
reduction when operating over broadcast networks. This is
accomplished by election of a Designated Router for each
...
... graph. The vertices of the graph consist of routers and
networks. A graph edge connects two routers when they are
...
... an interface on the network. Networks can be either transit or
stub networks. Transit networks ...
... network. Networks can be either transit or
stub networks. Transit networks are those capable of carrying
data traffic ...
... Networks can be either transit or
stub networks. Transit networks are those capable of carrying
data traffic that is neither locally originated nor locally
...
... data traffic that is neither locally originated nor locally
destined. A transit network is represented by a graph vertex
having both incoming and outgoing edges. A stub network ...
... network is represented by a graph vertex
having both incoming and outgoing edges. A stub network's vertex
has only incoming edges.
...
... routers having an interface to
the network. Three cases are depicted in Figure 1a. Rectangles
indicate routers. Circles and oblongs indicate networks ...
... network. Three cases are depicted in Figure 1a. Rectangles
indicate routers. Circles and oblongs indicate networks.
Router names are prefixed with the letters RT ...
... Router names are prefixed with the letters RT and network names
with the letter N. Router interface names are prefixed by the
...
... routers indicate point-to-point
networks. The left side of the figure shows networks with their
connected routers ...
... point-to-point
networks. The left side of the figure shows networks with their
connected routers, with the resulting graphs shown on the right.
...
... N3 *
Stub networks
**FROM**
...
... in each direction. Interfaces to point-to-point networks need
not be assigned IP addresses. When interface ...
... address. Optionally, an IP subnet can be assigned to the point-
to-point network. In this case, both routers advertise a stub
link ...
... network with only one attached
router (i.e., a stub network). In this case, the network appears
on the end of a stub connection ...
... router (i.e., a stub network). In this case, the network appears
on the end of a stub connection in the link-state ...
... database graph shows all routers bidirectionally
connected to the network vertex. This is pictured at the bottom
of Figure 1a.
...
... network (stub or transit) in the graph has an IP address
and associated network mask. The mask indicates the number of
nodes on the network ...
... network mask. The mask indicates the number of
nodes on the network. Hosts attached directly to routers
...
... (referred to as host routes) appear on the graph as stub
networks. The network mask for a host route ...
... host routes) appear on the graph as stub
networks. The network mask for a host route is always
...
... Representation of non-broadcast networks ...
... Hello
protocol and flooding work over the non-broadcast network,
and the way that the network is represented in the link ...
... OSPF emulates operation over a broadcast
network: a Designated Router is elected for the NBMA
...
... Designated Router is elected for the NBMA
network, and the Designated Router originates an LSA for the
...
... Designated Router originates an LSA for the
network. The graph representation for broadcast networks and
...
... networks and
NBMA networks is identical. This representation is pictured
in the middle of Figure 1a.
...
... routers attached to the NBMA network to be able to
communicate directly. This restriction may be met on some
non-broadcast ...
... communicate directly. This restriction may be met on some
non-broadcast networks, such as an ATM subnet utilizing
...
... SVCs. But it is often not met on other non-broadcast
networks, such as PVC-only Frame Relay networks ...
... networks. On non-
broadcast networks where not all routers can communicate
directly you can break the non-broadcast ...
... routers can communicate
directly you can break the non-broadcast network into
logical subnets, with the routers ...
... misconfiguration. It is probably better to run such a non-
broadcast network in Point-to-Multipoint mode.
...
... router connections over the non-broadcast network as if they
were point-to-point links. No Designated Router ...
... point-to-point links. No Designated Router is elected
for the network, nor is there an LSA generated for the
network ...
... network, nor is there an LSA generated for the
network. In fact, a vertex for the Point-to-MultiPoint
network ...
... network. In fact, a vertex for the Point-to-MultiPoint
network does not appear in the graph of the link-state
database ...
... database representation
of a Point-to-MultiPoint network. On the left side of the
figure, a Point-to-MultiPoint network ...
... network. On the left side of the
figure, a Point-to-MultiPoint network is pictured. It is
assumed that all routers can communicate directly, except
...
... IP interface addresses on the Point-to-MultiPoint network.
In the graphical representation of the link-state database ...
... routers that can communicate directly over the Point-to-
MultiPoint network are joined by bidirectional edges, and
...
... interface. Arcs having no
labelled cost have a cost of 0. Note that arcs leading from
networks to routers always have cost 0; they are significant
nonetheless. Note also that the externally derived routing ...
... neighborhood of each router or transit
network is represented in a single, separate LSA. Figure 4
shows these LSAs ...
... routers. The cost of all links from Network N6 to its
attached routers is 0. Note that the LSA ...
... attached routers is 0. Note that the LSA for Network N6 is
actually generated by one of the network's attached routers ...
... LSA for Network N6 is
actually generated by one of the network's attached routers:
the router ...
... Figure 3: The resulting directed graph
Networks and routers are represented by vertices.
An edge ...
... The tree gives the entire path to any destination network or
host. However, only the next hop ...
... separate route for each end of a numbered point-to-point network
(in this case, the serial line between Routers RT6 and RT10).
...
...
Routes to networks belonging to other AS'es (such as N12) appear
as dashed lines on the shortest path tree ...
... Edges that are not marked with a cost have a cost of
of zero (these are network-to-router links). Routes
...
... router links). Routes
to networks N12-N15 are external information that is
considered in Section 2.3
...
... route to destination Network N12. Router RT7 is preferred since
it is advertising N12 at a distance of 10 (8+2) to Router ...
... were advertising Type 2 external routes. Then all traffic
destined for Network N12 would be forwarded to Router RT7, since
2 < 8. When several equal-cost Type 2 routes exist, the
...
... suppose in Figure 2 there is an additional router attached to
Network N6, called Router RTX. Suppose further that RTX does
not participate in OSPF ...
...
OSPF allows collections of contiguous networks and hosts to be
grouped together. Such a group ...
... routers having
interfaces to any one of the included networks, is called an area.
Each area runs a separate copy of the basic link-state routing
...
... link as if they were connected by an
unnumbered point-to-point backbone network. On the graph of the
backbone, two such routers ...
... advertising external routes are chosen. Each area border router
in an area summarizes for the area its cost to all networks
external to the area. After the SPF tree ...
...
A router with all directly connected networks belonging to
the same area. These routers run a single copy of the basic
...
...
Figure 6 shows a sample area configuration. The first area
consists of networks N1-N4, along with their attached routers
...
... N1-N4, along with their attached routers
RT1-RT4. The second area consists of networks N6-N8, along with
their attached routers RT7, RT8, RT10 and RT11. The third area
...
... their attached routers RT7, RT8, RT10 and RT11. The third area
consists of networks N9-N11 and Host H1, along with their
attached routers ...
... attached routers RT9, RT11 and RT12. The third area has been
configured so that networks N9-N11 and Host H1 will all be
grouped into a single route ...
... backbone LSAs are shown in
Table 4. These summaries show which networks are contained in
Area 1 (i.e., Networks N1 ...
... Table 4. These summaries show which networks are contained in
Area 1 (i.e., Networks N1-N4), and the distance to these
networks ...
... Networks N1-N4), and the distance to these
networks from the routers RT3 and RT4 respectively.
...
... backbone; these are the dashed stubs that
appear in Figure 8. Remember that the third area has been
configured to condense Networks N9-N11 and Host H1 into a single
route ...
... Host H1 into a single
route. This yields a single dashed line for networks N9-N11 and
Host H1 in Figure 8. Routers ...
... summaries from all other area border routers. It then forms a
picture of the distance to all networks outside of its area by
examining the collected LSAs, and adding in the backbone ...
... backbone. This gives the distances to all other area border
routers. Also noted are the distances to networks (Ia and Ib)
and AS boundary routers ...
...
Next, by looking at the area summaries from these area border
routers, RT3 and RT4 can determine the distance to all networks
outside their area. These distances are then advertised
internally to the area by RT3 and RT4. The advertisements that
...
... Router RT1 would use RT4 for
traffic to Network N6, RT3 for traffic to Network N10, and would
...
... IP class A, B, or C
network number can be broken up into many subnets of various
sizes. For example, the network ...
... network number can be broken up into many subnets of various
sizes. For example, the network 128.185.0.0 could be broken up
into 62 variable-sized subnets: 15 subnets ...
... subnets of size 256, and 32 subnets of size 8. Table 7 shows
some of the resulting network addresses together with their
masks.
...
... There are many possible ways of dividing up a class A, B, and C
network into variable sized subnets. The precise procedure for
doing so is beyond the scope of this specification. This
...
... specification however establishes the following guideline: When
an IP packet is forwarded, it is always forwarded to the network
that is the best match for the packet's destination. Here best
...
... of IP supernetting. For example, a single physical network
segment could be assigned the [address ...
... address,mask] pair
[192.9.4.0,0xfffffc00]. The segment would then be single IP
network, containing addresses from the four consecutive class C
...
... addresses from the four consecutive class C
network numbers 192.9.4.0 through 192.9.7.0. Such addressing is
now becoming commonplace with the advent of CIDR ...
... address range is defined as an [address,mask]
pair. Many separate networks may then be contained in a single
address range, just as a subnetted network ...
... networks may then be contained in a single
address range, just as a subnetted network is composed of many
separate subnets. Area border routers ...
... address range. The cost of the route is
the maximum cost to any of the networks falling in the specified
range.
...
...
For example, an IP subnetted network might be configured as a
single OSPF area. In that case, a single address range ...
... address range could be
configured: a class A, B, or C network number along with its
natural IP mask. Inside the area, any number of variable sized
...
... subnets could be defined. However, external to the area a
single route for the entire subnetted network would be
distributed, hiding even the fact that the network is subnetted
...
... route for the entire subnetted network would be
distributed, hiding even the fact that the network is subnetted
at all. The cost of this route is the maximum of the set of
...
... edges.[1] Each edge
of the graph connects to a network, or is itself a point-to-
point network. In either case, the edge ...
... of the graph connects to a network, or is itself a point-to-
point network. In either case, the edge is colored with the
network ...
... broadcast and point-to-point
networks, the router dynamically detects its neighboring routers by
...
... multicast address AllSPFRouters.
On non-broadcast networks, some configuration information may be
necessary in order to discover neighbors ...
... timer execution. When multiple routers are attached to a
single network, all doing broadcasts, this can lead to the
synchronization ...
... single area only.
________________________________________________________
2 Network-LSAs Originated for broadcast
...
... list of routers connected
to the network. Flooded
throughout a single area only.
________________________________________________________
...
... divide a single IP class A, B, or C network number into many
subnets of various sizes. This is commonly called
...
... IP class A, B, and C
networks into larger quantities called supernets.
Supernetting has been proposed as one way to improve the
scaling of IP routing ...
...
Lower-level protocol support
The lower level protocols referred to here are the network
access protocols, such as the Ethernet data link layer.
...
... Indications must be passed from these protocols to OSPF as
the network interface goes up and down. For example, on an
ethernet it would be valuable to know when the ethernet ...
... Non-broadcast lower-level protocol support
On non-broadcast networks, the OSPF Hello Protocol can be
aided by providing an indication when an attempt is made to
...
... backbone, and behave as if they were
unnumbered point-to-point networks between the two routers. A
virtual link ...
... link-state
database. A network belongs to a single area, and a router interface
connects to a single area. Each router ...
... neighbor's Hello Packet. On broadcast and NBMA networks,
the Hello Protocol elects a Designated Router ...
...
On NBMA networks some configuration information may be necessary
for the operation of the Hello Protocol ...
... Designated Router has a list of all other
routers attached to the network. A router, having Designated
Router potential, sends Hello Packets to all other potential
...
... Designated Routers when its interface to the NBMA network first
becomes operational. This is an attempt to find the Designated
Router for the network ...
... network first
becomes operational. This is an attempt to find the Designated
Router for the network. If the router itself is elected
Designated Router ...
... Designated Router, it begins sending Hello Packets to all other
routers attached to the network.
...
... communication ensured, and (if on a broadcast or NBMA network) a
Designated Router elected, a decision is made regarding whether
...
... the Designated Router itself) currently attached to the
network. The Link State ID for this LSA (see Section
...
... IP interface address of the Designated
Router. The IP network number can then be obtained by using
the network's subnet ...
... Designated
Router. The IP network number can then be obtained by using
the network's subnet/network mask.
...
... Designated Router becomes adjacent to all other routers
on the network. Since the link state databases are
...
... router's interface to a network first becomes functional, it
checks to see whether there is currently a Designated Router for
...
... checks to see whether there is currently a Designated Router for
the network. If there is, it accepts that Designated Router,
regardless of its Router ...
... the highest Router Priority on the network. A more detailed
(and more accurate) description of Designated Router election is
...
... order to optimize the flooding procedure on broadcast networks,
the Designated Router multicasts ...
... nodes are labelled with their
Router ID. Transit network nodes are actually labelled with the
IP address ...
... Designated Router. It follows that when the
Designated Router changes, it appears as if the network node on
the graph is replaced by an entirely new node ...
... the graph is replaced by an entirely new node. This will cause
the network and all its attached routers to originate new LSAs.
...
... Priorities should be configured so that
the most dependable router on a network eventually becomes
Designated Router.
...
... Designated Router is also adjacent
to all routers on the network, and becomes Designated Router
when the previous Designated Router ...
... Designated Router and all other routers attached to the
network. Part of the adjacency forming process is the
synchronizing of link-state databases ...
... link-state databases, which can potentially
take quite a long time. During this time, the network would not
be available for transit data traffic. The Backup Designated
...
... network-LSA for
the network. (If it did, the transition to a new Designated
Router would be even faster. However, this is a tradeoff
between database ...
... Hello
Protocol. Each Hello Packet has a field that specifies the
Backup Designated Router for the network.
...
... routers have
in common. If two routers have multiple networks in common,
they may have multiple adjacencies between them.
...
...
One can picture the collection of adjacencies on a network as
forming an undirected graph. The vertices consist of routers,
...
...
Two graphs are possible, depending on whether a Designated
Router is elected for the network. On physical point-to-point
...
... broadcast and NBMA
networks only the Designated Router and the Backup Designated
Router become adjacent to all other routers ...
... Router RT3 the Backup
Designated Router, for the Network N2. The Backup Designated
Router performs a lesser function during the flooding ...
... Appendix D. A different authentication procedure can be
used for each IP network/subnet. Autype indicates the type
...
... destination is always set to the address AllSPFRouters. On all
other network types (including virtual links), the majority of
OSPF packets ...
... unicasts are on broadcast networks; on these networks Hello
packets are sent to the multicast ...
... unicasts are on broadcast networks; on these networks Hello
packets are sent to the multicast destination ...
... directly to the neighbor. On multi-access networks, this means
that retransmissions should be sent to the neighbor ...
... Interfaces to unnumbered point-to-point
networks have no associated IP address. On these interfaces,
...
... virtual links act precisely the same as
unnumbered point-to-point networks. However, each virtual link
does have an IP interface ...
... an OSPF protocol packet on its interface to Network N8, it may
want to associate the packet with the interface to Area 2, or
...
... Therefore, the packet's IP source address is required to
be on the same network as the receiving interface. This
...
... Point-to-MultiPoint network or NBMA network the sender
is identified by the IP source address ...
... interface connects to a point-to-point
network or a virtual link, the sender is identified by the
...
... interface is the connection between a router and a network.
We assume a single OSPF interface ...
... We assume a single OSPF interface to each attached network/subnet,
although supporting multiple interfaces ...
... subnet,
although supporting multiple interfaces on a single network is
considered in Appendix F. Each interface structure has at most one
...
... OSPF interface can be considered to belong to the area that
contains the attached network. All routing protocol packets
originated by the router ...
... interface. Note
that a number of these items are actually configuration for the
attached network; such items must be the same for all routers
connected to the network ...
... IP interface address that identifies the attached
network. Masking the IP interface address with the IP interface ...
... address with the IP interface
mask yields the IP network number of the attached network. On
point-to-point ...
... IP interface
mask yields the IP network number of the attached network. On
point-to-point networks ...
... virtual links, the IP interface mask
is not defined. On these networks, the link itself is not
assigned an IP network ...
... networks, the link itself is not
assigned an IP network number, and so the addresses of each side
of the link ...
... Area ID
The Area ID of the area to which the attached network belongs.
All routing protocol packets originating from the interface ...
... unsigned integer. When two routers attached to a
network both attempt to become Designated Router, the one with
the highest Router ...
... Router Priority is set to 0 is ineligible to become Designated
Router on the attached network. Advertised in Hello packets
sent out this interface.
...
... timer fires every HelloInterval seconds. Note
that on non-broadcast networks a separate Hello packet is sent
to each qualified neighbor.
...
... state, and as a consequence select a Designated Router
on the network. The length of the timer is RouterDeadInterval
seconds.
...
... routers
The other routers attached to this network. This list is formed
by the Hello Protocol. Adjacencies will be formed to some of
...
... Designated Router
The Designated Router selected for the attached network. The
Designated Router is selected on all broadcast ...
... Designated Router is selected on all broadcast and NBMA networks
by the Hello Protocol. Two pieces of identification are kept
...
... IP interface
address on the network. The Designated Router advertises link
state for the network ...
... network. The Designated Router advertises link
state for the network; this network-LSA is labelled with the
...
... Designated Router advertises link
state for the network; this network-LSA is labelled with the
Designated Router ...
... Hello Protocol. All routers on the
attached network become adjacent to both the Designated Router
and the Backup Designated Router ...
... identity of the (Backup) Designated Router for the network.
To do this, the router monitors the Hello Packets it
...
... interface is to a broadcast or NBMA network on which
another router has been selected to be the Designated
Router ...
... state, the router itself is the Backup Designated
Router on the attached network. It will be promoted to
Designated Router when the present Designated Router ...
... router establishes adjacencies to all other routers
attached to the network. The Backup Designated Router
performs slightly different functions during the Flooding ...
... router itself is the Designated Router
on the attached network. Adjacencies are established to all
other routers attached to the network ...
... network. Adjacencies are established to all
other routers attached to the network. The router must also
...
... routers (including the
Designated Router itself) attached to the network. See
Section 7.3 for more details on the functions performed by
the Designated Router ...
...
InterfaceUp
Lower-level protocols have indicated that the network
interface is operational. This enables the interface to
transition out of Down state ...
... router has detected the existence or non-existence of a
Backup Designated Router for the network. This is done in
one of two ways. First, an Hello Packet may be received
from a neighbor ...
... interface is now
looped back to itself. This indication can be received
either from network management or from the lower level
protocols.
...
... longer looped back. As with the LoopInd event, this
indication can be received either from network management or
from the lower level protocols.
...
... periodic sending of Hello packets out the interface.
If the attached network is a physical point-to-point
...
... Designated Router. In this case, in an attempt to
discover the attached network's Designated Router
the interface ...
... shot Wait Timer is started. Additionally, if the
network is an NBMA network examine the configured
...
... state: Depends upon action routine.
Action: Calculate the attached network's Backup Designated
Router and Designated Router, as shown in Section
...
... state: Depends upon action routine.
Action: Calculate the attached network's Backup Designated
Router and Designated Router, as shown in Section
...
... state: Depends upon action routine.
Action: Recalculate the attached network's Backup Designated
Router and Designated Router, as shown in Section
...
... Action: Since this interface is no longer connected to the
attached network the actions associated with the
above InterfaceDown event are executed.
...
... This section describes the algorithm used for calculating a
network's Designated Router and Backup Designated Router. This
...
... initial time a router runs the election algorithm for a network,
the network's Designated Router ...
... Router X. The list of
neighbors attached to the network and having established
bidirectional communication with Router ...
... Router X's neighbors (on
this network) whose state is greater than or equal to 2-Way (see
Section 10.1). Router ...
... Calculate the new Backup Designated Router for the network
as follows. Only those routers on the list that have not
...
... Calculate the new Designated Router for the network as
follows. If one or more of the routers have declared
...
... If the attached network is an NBMA network, and the router
itself has just become either Designated Router ...
... relationships.[6] On broadcast and NBMA networks, Hello Packets
are also used to elect the Designated Router and Backup
...
... RouterDeadInterval must be the same for all routers attached to
a common network. The Hello packet also contains the IP address
mask of the attached network ...
... network. The Hello packet also contains the IP address
mask of the attached network (Network Mask). On unnumbered
point-to-point ...
... IP address
mask of the attached network (Network Mask). On unnumbered
point-to-point networks ...
... Network Mask). On unnumbered
point-to-point networks and on virtual links this field should
be set to 0.0.0.0.
...
... routers, the Hello packet contains the list of all routers on
the network from which Hello Packets have been seen recently.
The Hello packet also contains the router's current choice for
...
... networks and physical point-to-point networks,
Hello packets are sent every HelloInterval seconds to the IP
multicast address ...
... end of the virtual link) every HelloInterval seconds. On Point-
to-MultiPoint networks, separate Hello packets are sent to each
attached neighbor every HelloInterval seconds. Sending of Hello
...
... neighbor every HelloInterval seconds. Sending of Hello
packets on NBMA networks is covered in the next section.
...
... for the Hello Protocol to function on non-broadcast networks
(see Sections C.5 and C.6). On NBMA networks ...
... networks
(see Sections C.5 and C.6). On NBMA networks, every
attached router which is eligible to become Designated
Router ...
... router which is eligible to become Designated
Router becomes aware of all of its neighbors on the network
(either through configuration or by some unspecified
mechanism). Each neighbor ...
... OSPF router
and another router have multiple attached networks in common,
multiple conversations ensue, each described by a unique neighbor
...
... neighbor's Hello packets, this item is used when selecting the
Designated Router for the attached network.
Neighbor ...
... router's interface to the
attached network. Used as the Destination IP address when
protocol packets are sent as unicasts ...
... router-LSAs as the Link ID for the attached network
if the neighboring router is selected to be Designated Router ...
... from the neighbor. On NBMA networks, Hello packets may
still be sent to "Down" neighbors, although at a reduced
...
... neighbors attached to NBMA
networks. It indicates that no recent information has been
received from the neighbor, but that a more concerted effort
...
... neighbor is now unreachable. For example, on an X.25
network this could be indicated by an X.25 clear indication
...
... networks,
Point-to-MultiPoint networks and virtual links always become
adjacent. On broadcast ...
... adjacent. On broadcast and NBMA networks, all routers become
adjacent to both the Designated Router ...
... neighbor, and secondly, when
the identity of the attached network's (Backup) Designated
Router changes. If the decision is made to not attempt an
adjacency, the state ...
... The underlying network type is point-to-point
...
... The underlying network type is Point-to-MultiPoint
...
... The underlying network type is virtual link
...
... OSPF packet
header. Next, the values of the Network Mask, HelloInterval,
and RouterDeadInterval fields in the received Hello packet must
be checked against the values configured for the receiving ...
... interface. Any mismatch causes processing to stop and the
packet to be dropped. In other words, the above fields are
really describing the attached network's configuration. However,
there is one exception to the above rule: on point-to-point
...
... there is one exception to the above rule: on point-to-point
networks and on virtual links, the Network Mask in the received
...
... networks and on virtual links, the Network Mask in the received
Hello Packet should be ignored.
...
... broadcast, Point-to-
MultiPoint or NBMA network the source is identified by the IP
source address found in the Hello's IP header. If the receiving ...
...
On NBMA networks, receipt of an Hello Packet may also cause an
Hello Packet to be sent back to the neighbor in response. See
...
... and A.2). The E-bit should be set if and only if the attached
network belongs to a non-stub area. Unrecognized bits in the
Options field ...
... Routers RT1
and RT2 are both connected to a broadcast network. It is
assumed that RT2 is the Designated Router for the network ...
... network. It is
assumed that RT2 is the Designated Router for the network, and
that RT2 has a higher Router ID than Router ...
... Router RT1's interface to the
network becomes operational. It begins sending Hello Packets,
although it doesn't know the identity of the Designated Router ...
... Destination type is either "network" or "router". Only network
entries are actually used when forwarding IP data traffic ...
... identifier or name. This depends on the
Destination Type. For networks, the identifier is their
associated IP address ...
... area border
routers share multiple areas in common. For destinations of
type "network", only the set of paths associated with the best
area (the one providing the preferred route) is kept.
...
... destination. For example, if the destination is a transit
network, this is the transit network's network-LSA ...
... Point-to-MultiPoint and NBMA
networks, the next hop also includes the IP address of the next
...
... considered "active" if the range contains one or more networks
reachable by intra-area paths.) The destination ...
... routing table is shown in Table 12. Destination types are
abbreviated: Network as "N", Router as "R".
...
... external routes to be calculated to the destinations advertised
by RT5 and RT7 (i.e., Networks N12, N13, N14 and N15). It is
assumed all AS-external-LSAs ...
... Routing information is condensed
at area boundaries. In this example, we assume that Area 3 has
been defined so that networks N9-N11 and the host route to H1
...
... link, RT11 would be
unable to advertise a route for networks N9-N11 and Host H1 into
the backbone ...
... Host H1 into
the backbone, and there would not be an entry for these networks
in Router RT4's routing table ...
...
In this example there are two equal-cost paths to Network N12.
However, they both use the same next hop (Router ...
... LSAs describe how an
area's routers and networks are interconnected. Summary-LSAs
provide a way of condensing an area's routing information ...
... consult Section 12.4.1.
________________________________________________
2 These are the network-LSAs.
They describe the set of routers ...
... They describe the set of routers
attached to the network. For
more information, consult
Section 12.4.2.
...
... area border
routers, the Type 3 summary-LSAs
describe routes to networks while the
Type 4 summary-LSAs describe routes to
...
... AS-
external-LSA for the network 10.0.0.0 with mask of
255.0.0.0, the Link State ID can be set to anything in the
...
... router to originate separate
LSAs for two networks having the same address but different
masks. See Appendix E for details.
...
... LSA is describing a network (LS type = 2, 3 or 5),
the network's IP address is easily derived by masking the
Link State ...
... IP address is easily derived by masking the
Link State ID with the network/subnet mask contained in the
body of the LSA ...
... LSAs, this field is identical to the
Link State ID field. Network-LSAs are originated by the
network ...
... router is also the
Designated Router for any of the area's networks, it will
originate network-LSAs ...
... Designated Router for any of the area's networks, it will
originate network-LSAs for those networks.
...
... LSAs as pictured in Figure 7). If
RT4 has been selected as Designated Router for Network N3, it
will also originate a network-LSA ...
... AS-external-LSAs (one for each of the networks N12-N14). These
will be flooded throughout the entire AS, assuming that none of
...
... AS-external-LSAs for
networks N12-N14 will not be flooded into area 3 (see Section
3.6). Instead, Router RT11 would originate a default summary-
...
... router itself
is now the Designated Router, a new network-LSA should be
produced. If the router ...
... produced. If the router itself is no longer the Designated
Router, any network-LSA that it might have originated for
the network ...
... network-LSA that it might have originated for
the network should be flushed from the routing domain (see
...
... links) to the area. Each
link is typed according to the kind of attached network.
Each link is also labelled with its Link ID ...
... routing table calculation, see Section 16.1.1). For
links to stub networks, this field specifies the stub
network's IP address ...
... links to stub networks, this field specifies the stub
network's IP address mask. For unnumbered point-to-point
links, the Link ...
... link is configurable. With the
exception of links to stub networks, the output cost must
always be non-zero.
...
... Loopback, add a Type 3
link (stub network) as long as this is not an interface
to an unnumbered point-to-point ...
... interface
to an unnumbered point-to-point network. The Link ID
should be set to the IP interface ...
... address. For
unnumbered point-to-point networks, the Link Data
field should specify the interface ...
... state), a Type 3 link (stub
network) should be added. There are two forms that
this stub link can take:
...
... interface is Waiting, add a Type
3 link (stub network) with Link ID set to the IP
network number of the attached network ...
... link (stub network) with Link ID set to the IP
network number of the attached network, Link Data
...
... network) with Link ID set to the IP
network number of the attached network, Link Data
set to the attached network ...
... network, Link Data
set to the attached network's address mask, and cost
equal to the interface ...
... Else, there has been a Designated Router elected for
the attached network. If the router is fully
adjacent to the Designated Router ...
... router, add a single Type 2 link
(transit network) with Link ID set to the IP
interface address ...
... Link ID set to the IP
interface address of the attached network's
Designated Router (which may be the router ...
... pictured in Figure 6. The area containing Router RT3
(Area 1) has been redrawn, with actual network
addresses, in Figure 15. Assume that the last byte of
...
... Router RT4 has been
selected as the Designated router for network 192.1.1.0.
RT3's router-LSA for Area 1 is then shown below. It
...
... connections to Area 1, the
first a link to the transit network 192.1.1.0 and the
second a link to the stub network ...
... network 192.1.1.0 and the
second a link to the stub network 192.1.4.0. Note that
the transit network is identified by the IP interface ...
... link to the stub network 192.1.4.0. Note that
the transit network is identified by the IP interface of
its Designated Router ...
... Link Data = 192.1.1.3 ;RT3's IP interface to net
Type = 2 ;connects to transit network
# TOS metrics = 0
...
... NBMA network. (A transit network is a network having two or
more attached routers). The network ...
... LSA only if it is fully
adjacent to at least one other router on the network. The
network-LSA ...
... router on the network. The
network-LSA is flooded throughout the area that contains the
transit network ...
... network-LSA is flooded throughout the area that contains the
transit network, and no further. The network-LSA lists
...
... LSA is flooded throughout the area that contains the
transit network, and no further. The network-LSA lists
those routers ...
... address of the Designated Router. This value, masked by the
network's address mask (which is also contained in the
network ...
... network's address mask (which is also contained in the
network-LSA) yields the network's IP address ...
... router that has formerly been the Designated Router for a
network, but is no longer, should flush the network-LSA that
...
... Designated Router for a
network, but is no longer, should flush the network-LSA that
it had previously originated. This LSA ...
... router may have no idea what it's
previous Router ID might have been, these network-LSAs are
indicated by having their Link State ...
...
Again consider the area configuration in Figure 6.
Network-LSAs are originated for Network N3 in Area 1,
...
... Network-LSAs are originated for Network N3 in Area 1,
Networks N6 and N8 in Area 2, and Network ...
... LSAs are originated for Network N3 in Area 1,
Networks N6 and N8 in Area 2, and Network N9 in Area 3.
Assuming that Router ...
... Network N3 in Area 1,
Networks N6 and N8 in Area 2, and Network N9 in Area 3.
Assuming that Router RT4 has been selected as the
...
... Router RT4 has been selected as the
Designated Router for Network N3, the following
network-LSA ...
... Designated Router for Network N3, the following
network-LSA is generated by RT4 on behalf of Network N3
...
... network-LSA is generated by RT4 on behalf of Network N3
(see Figure 15 for the address assignments):
...
... Router = 192.1.1.4 ;RT4's Router ID
Network Mask = 0xffffff00
Attached Router = 192.1.1.4 ;Router ID ...
... The destination described by a summary-LSA is either an IP
network, an AS boundary router or a range ...
... destination, with Link State ID equal to the network's
address (if necessary, the Link State ...
... address (if necessary, the Link State ID can also have
one or more of the network's host bits set; see Appendix
...
... intra-area route to a
network. This means that the network is contained in
one of the router ...
... route to a
network. This means that the network is contained in
one of the router's directly attached areas. In
...
... bits set;
see Appendix E for details) and cost equal to the
largest cost of any of the component networks. When the
range's status indicates DoNotAdvertise, the Type 3
...
... range's status indicates DoNotAdvertise, the Type 3
summary-LSA is suppressed and the component networks
remain hidden from other areas.
...
...
By default, if a network is not contained in any
explicitly configured address range, a Type 3 summary-
...
... LSA is generated with Link State ID equal to the
network's address (if necessary, the Link State ID can
...
... address (if necessary, the Link State ID can
also have one or more of the network's "host" bits set;
...
... traffic (i.e.,
its TransitCapability is set to TRUE), routing
information concerning backbone networks should not be
condensed before being summarized into the area. Nor
should the advertisement of backbone networks ...
... backbone networks should not be
condensed before being summarized into the area. Nor
should the advertisement of backbone networks into
transit areas be suppressed. In other words, the
backbone ...
... host routes Ia and Ib into a single summary-LSA.
Finally, the routes to networks N9,N10,N11 and Host H1
are advertised by a single summary-LSA ...
... Router RT4
follow. The actual IP addresses for the networks and
routers in question have been assigned in Figure 15.
...
... LSA's Link State ID is set to the destination network's IP
address (if necessary, the Link State ID can also have one
...
... IP
address (if necessary, the Link State ID can also have one
or more of the network's "host" bits set; see Appendix E for
...
... Router RT7 originates two AS-external-LSAs, for networks
N12 and N15. Assume that RT7 has learned its route to
...
... 16. There are three OSPF routers (RTA, RTB and RTC)
connected to a common network. Only one of these
routers, RTA, is exchanging BGP ...
... Link State Acknowledgment packets sent by the
various routers attached to a common network. The fixed
interval between a router's delayed transmissions must be short
...
... acknowledgments are sent immediately when the duplicate is
received. On multi-access networks, these acknowledgments are
sent directly to the neighbor's IP address ...
... associated with the interface. On broadcast networks, this is
accomplished by sending the delayed Link State Acknowledgment
...
... destination AllDRouters is used. On non-broadcast
networks, delayed Link State Acknowledgment packets must be
unicast ...
... The reasoning behind sending the above packets as multicasts is
best explained by an example. Consider the network
configuration depicted in Figure 15. Suppose RT4 has been
elected as Designated Router ...
... elected as Designated Router, and RT3 as Backup Designated
Router for the network N3. When Router RT4 floods a new LSA to
...
... Router RT4 floods a new LSA to
Network N3, it is received by routers RT1, RT2, and RT3. These
routers ...
... sent directly to the neighbor. On multi-access networks, this
means that retransmissions are sent directly to the neighbor ...
... The virtual link is treated as if it were an unnumbered point-to-
point network belonging to the backbone and joining the two area
border routers. An attempt is made to establish an adjacency over
...
... virtual links (see
TransitCapability in Sections 6 and 16.1). Such an area requires
special treatment when summarizing backbone networks into it
(see Section 12.4.3), and during the routing calculation (see
...
... links
between routers and transit networks. Then the stub networks
are incorporated into the tree ...
... between routers and transit networks. Then the stub networks
are incorporated into the tree. During the area's shortest-path
...
... first stage, only links between routers and transit networks are
considered. Using the Dijkstra algorithm, a tree ...
... represented as a directed graph. The graph's vertices are
routers, transit networks and stub networks. The first stage of
the procedure concerns only the transit vertices (routers ...
... routers, transit networks and stub networks. The first stage of
the procedure concerns only the transit vertices (routers and
...
... the procedure concerns only the transit vertices (routers and
transit networks) and their connecting links. Throughout the
shortest path calculation, the following data is also associated
...
... 32-bit number which together with the vertex type (router
or network) uniquely identifies the vertex. For router
vertices the Vertex ID is the router ...
... router-LSA. For transit networks, this
is a network-LSA (which is actually originated by the
network ...
... network-LSA (which is actually originated by the
network's Designated Router). In any case, the LSA's Link
State ...
... router-LSAs and
network-LSAs). One path is said to be "shorter" than
another if it has a smaller link state ...
... link in V's LSA. Links to stub networks will be
considered in the second stage of the shortest path
calculation.
...
... (b) Otherwise, W is a transit vertex (router or transit
network). Look up the vertex W's LSA (router-LSA or
...
... that when there is a choice of vertices closest to the root,
network vertices must be chosen before router vertices in
order to necessarily find all equal-cost paths. This is
...
... (similar to the calculation in Section 16.1.1).
If the newly added vertex is a transit network, the routing
table entry for the network is located. The entry's
...
... If the newly added vertex is a transit network, the routing
table entry for the network is located. The entry's
Destination ID is the IP network ...
... network is located. The entry's
Destination ID is the IP network number, which can be
obtained by masking the Vertex ID (Link State ID) with its
...
... associated subnet mask (found in the body of the associated
network-LSA). If the routing table entry already exists
...
... destination installed in the routing table), multiple
vertices have mapped to the same IP network. For example,
this can occur when a new Designated Router is being
...
...
If there is no routing table entry for the network (the
usual case), a routing table entry for the IP network ...
... network (the
usual case), a routing table entry for the IP network should
be added. The routing table entry's Link State ...
... algorithm by returning to Step 2.
The stub networks are added to the tree in the procedure's
second stage. In this stage, all router ...
... LSA is then examined, and the following steps are executed:
(1) Calculate the distance D of stub network from the root. D
is equal to the distance from the root ...
... root to the router vertex
(calculated in stage 1), plus the stub network link's
advertised cost. Compare this distance to the current best
...
... link's
advertised cost. Compare this distance to the current best
cost to the stub network. This is done by looking up the
stub network's current routing table ...
... cost to the stub network. This is done by looking up the
stub network's current routing table entry. If the
calculated distance D is larger, go on to examine the next
...
... routing table entry. If the
calculated distance D is larger, go on to examine the next
stub network link in the LSA.
...
... LSA.
(2) If this step is reached, the stub network's routing table
entry must be updated. Calculate the set of next hops ...
... entry must be updated. Calculate the set of next hops that
would result from using the stub network link. This
calculation is shown in Section 16.1.1; input to this
...
... calculation is shown in Section 16.1.1; input to this
calculation is the destination (the stub network) and the
parent vertex (the router vertex). If the distance D is the
...
... itself). This means that the destination is either a
directly connected network or directly connected router.
The outgoing interface ...
... Data field
belonging to the Point-to-MultiPoint network provides an IP
address of the next hop router ...
... router. If the destination is a
directly connected network, or a router which connects to
the calculating router ...
...
In the second case, the parent vertex is a network that
directly connects the calculating router to the destination ...
... destination described by a summary-LSA is either a network (Type
3 summary-LSAs) or an AS boundary ...
... Active" means that there
are one or more reachable (by intra-area paths) networks
contained in the area range.
...
... LSA
describes a route through a transit area Area A to a Network N
(N's address is obtained by masking the LSA ...
... has been configured between routers RT1 and RT4. On the right
side of the figure, Network N1 belongs to the backbone. The
...
... N1. However, since Router RT5
is so much closer to Network N1, all routers internal to Area 1
...
... routers internal to Area 1
(e.g., Routers RT2 and RT3) will forward their Network N1
traffic ...
... LSAs by the above calculation, Router
RT1 will also forward Network N1 traffic towards RT5. Note that
...
... Autonomous System (Destination ID = DefaultDestination,
network/subnet mask = 0x00000000). For each AS-external-LSA ...
... masking the LSA's Link State ID with the network/subnet mask
contained in the body of the LSA ...
... masking the LSA's Link State ID with the network/subnet mask
contained in the body of the LSA ...
... routing table entry has changed.
If the destination described by this entry is a Network or
AS boundary router ...
...
[1]The graph's vertices represent either routers, transit networks,
or stub networks. Since routers ...
... routers, transit networks,
or stub networks. Since routers may belong to multiple areas, it is
not possible to color the graph's vertices.
...
... be addressed to, interfaces to unnumbered point-to-point networks.
This is regardless of such an interface's state ...
... [5]It is instructive to see what happens when the Designated Router
for the network crashes. Call the Designated Router for the network
...
... for the network crashes. Call the Designated Router for the network
RT1, and the Backup Designated Router RT2. If Router ...
... Router RT1 crashes
(or maybe its interface to the network dies), the other routers on
the network ...
... network dies), the other routers on
the network will detect RT1's absence within RouterDeadInterval
seconds. All routers may not detect this at precisely the same
...
...
[6]On point-to-point networks, the lower level protocols indicate
whether the neighbor is up and running. Likewise, existence of the
...
...
[9]The address space of IP networks and the address space of OSPF
Router IDs may overlap. That is, a network ...
... IP networks and the address space of OSPF
Router IDs may overlap. That is, a network may have an IP address
which is identical (when considered as a 32-bit number ...
... partial information. This is during the routing table calculation,
when a network-LSA must be found based solely on its Link State ID.
...
... The lookup in this case is still well defined, since no two
network-LSAs can have the same Link State ...
... interface will actually be received over the interface (which is
useful for diagnostic purposes) and c) it allows network
bootstrapping of a neighbor, without requiring that the bootstrap ...
... Perlman, R., "Fault-Tolerant Broadcast of Routing Information", Computer Networks, December 1983. ...
... McCloghrie, K., and M. Rose, "Management Information Base for network management of TCP/IP-based internets: MIB-II", STD ...
... deSouza, O., and M. Rodrigues, "Guidelines for Running OSPF Over Frame Relay Networks", RFC 1586, March 1994. ...
... Rosen, E., "Vulnerabilities of Network Control Protocols: An Example", Computer Communication Review, July 1981. ...
... OSPF runs directly over the Internet Protocol's network layer. OSPF
packets are therefore encapsulated ...
... fragmentation when transmitting packets larger than
the network MTU. If necessary, the length of OSPF packets can be up
...
... OSPF is IP protocol number 89. This number has been registered
with the Network Information Center. IP protocol number
assignments are documented in [Ref11 ...
...
All routers connected to a common network must agree on certain
parameters (Network mask, HelloInterval and RouterDeadInterval).
...
... routers connected to a common network must agree on certain
parameters (Network mask, HelloInterval and RouterDeadInterval).
These parameters are included in Hello packets, so that differences
can inhibit the forming of neighbor ...
... Authentication |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Network Mask |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| HelloInterval | Options | Rtr Pri |
...
... if the interface is to a class B network whose third byte is
used for subnetting, the network mask is 0xffffff00.
...
... class B network whose third byte is
used for subnetting, the network mask is 0xffffff00.
Options
...
... The identity of the Designated Router for this network, in the
view of the sending router. The Designated Router ...
... here by its IP interface address on the network. Set to 0.0.0.0
if there is no Designated Router.
...
... The identity of the Backup Designated Router for this network,
in the view of the sending router. The Backup Designated Router ...
... is identified here by its IP interface address on the network.
Set to 0.0.0.0 if there is no Backup Designated Router.
...
... router from whom valid Hello packets have
been seen recently on the network. Recently means in the last
RouterDeadInterval seconds.
...
... LSA. The contents of this field
depend on the LSA's LS type. For example, in network-LSAs the
Link State ...
... IP interface address of the
network's Designated Router (from which the network's IP address ...
... network's Designated Router (from which the network's IP address
can be derived). The Link State ...
... router that originated the LSA. For
example, in network-LSAs this field is equal to the Router ID of
...
... link being described. It may
be a link to a transit network, to another router or to a stub
network ...
... network, to another router or to a stub
network. The values of all the other fields describing a router
link ...
... links to stub networks this
field specifies the network's IP address mask. For other link types
...
... Note that host routes are classified as links to stub networks
with network mask of 0xffffffff.
...
... LSA (i.e., another router or a transit
network) the Link ID is equal to the neighboring LSA's Link
State ...
... stub networks, Link Data specifies the network's IP address
mask. For unnumbered point-to-point connections ...
... network in the area which supports two or
more routers. The network-LSA is originated by the network's
...
... LSA describes all routers attached to the
network, including the Designated Router itself. The LSA's Link
State ...
...
The distance from the network to all attached routers is zero. This
is why metric fields need not be specified in the network ...
... network to all attached routers is zero. This
is why metric fields need not be specified in the network-LSA. For
details concerning the construction of network ...
... checksum | length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Network Mask |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Attached Router ...
... The Router IDs of each of the routers attached to the network.
Actually, only those routers that are fully adjacent to the
...
... Type 3 summary-LSAs are used when the destination is an IP network.
In this case the LSA's Link State ...
... In this case the LSA's Link State ID field is an IP network number
(if necessary, the Link State ID can also have one or more of the
...
... (if necessary, the Link State ID can also have one or more of the
network's "host" bits set; see Appendix E for details). When the
...
... checksum | length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Network Mask |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 0 | metric |
...
... LSA's Link State ID
is always set to DefaultDestination (0.0.0.0) and the Network Mask
is set to 0.0.0.0.
...
... LSAs, this indicates the destination
network's IP address mask. For example, when advertising the
location of a class ...
... IP address mask. For example, when advertising the
location of a class A network the value 0xff000000 would be
used. This field is not meaningful and must be zero for Type 4
summary-LSAs ...
... For these LSAs the Link State ID field specifies an IP network
number (if necessary, the Link State ID can also have one or more of
...
... number (if necessary, the Link State ID can also have one or more of
the network's "host" bits set; see Appendix E for details). AS ...
... default route, the Link State ID is always set to
DefaultDestination (0.0.0.0) and the Network Mask is set to 0.0.0.0.
...
... checksum | length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Network Mask |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|E| 0 | metric |
...
... destination. For
example, when advertising a class A network the mask 0xff000000
would be used.
...
... LSAs. Its value is the
IP address 0.0.0.0. Its associated Network Mask is also always
0.0.0.0.
...
... routers in an area must agree on that
area's parameters, and all routers attached to a network must agree
on that network's IP network ...
... Area
ID of 0.0.0.0 is reserved for the backbone. If the area
represents a subnetted network, the IP network number of the
subnetted network ...
... backbone. If the area
represents a subnetted network, the IP network number of the
subnetted network may be used for the Area ID ...
... IP addresses contained
in the address range. Networks and hosts are
assigned to an area depending on whether their
...
... Routers are viewed as belonging to
multiple areas, depending on their attached
networks' area membership.
Status Set to either Advertise or DoNotAdvertise. Routing
information ...
... Unadvertised ranges allow the existence of certain
networks to be intentionally hidden from other
areas. Status is set to Advertise by default.
...
...
As an example, suppose an IP subnetted network is to be its
own OSPF area. The area would be configured as a single
...
... IP address is the address of the
subnetted network, and whose mask is the natural class A, B,
or C address ...
... route would be advertised
external to the area, describing the entire subnetted
network.
ExternalRoutingCapability
...
... address and subnet mask) actually imply properties of
the attached networks, and therefore must be consistent across
all the routers attached to that network ...
... networks, and therefore must be consistent across
all the routers attached to that network. The parameters that
must be configured for a router interface are:
...
... internet. An IP
address is not required on point-to-point networks. Such a
point-to-point network ...
... IP interface mask
Also referred to as the subnet/network mask, this indicates
the portion of the IP interface address ...
... IP interface address that identifies the
attached network. Masking the IP interface address with the
...
... address with the
IP interface mask yields the IP network number of the
attached network. On point-to-point ...
... IP interface mask yields the IP network number of the
attached network. On point-to-point networks and virtual
links ...
... attached network. On point-to-point networks and virtual
links, the IP interface mask is not defined. On these
...
... virtual
links, the IP interface mask is not defined. On these
networks, the link itself is not assigned an IP network
...
... networks, the link itself is not assigned an IP network
number, and so the addresses of each side of the link ...
... round-trip
delay between any two routers on the attached network. The
setting of this value should be conservative or needless
retransmissions ...
... setting of this value should be conservative or needless
retransmissions will result. Sample value for a local area
network: 5 seconds.
InfTransDelay
...
... interface. It must be greater than 0. Sample value for a
local area network: 1 second.
Router ...
... unsigned integer. When two routers attached to a
network both attempt to become Designated Router, the one
with the highest Router ...
... ineligible to become Designated Router on the attached
network. Router Priority is only configured for interfaces ...
... router's Hello Packets. It must be the
same for all routers attached to a common network. The
smaller the HelloInterval, the faster topological changes
will be detected; however, more OSPF routing protocol ...
... traffic will ensue. Sample value for a X.25 PDN network: 30
seconds. Sample value for a local area network: 10 seconds.
...
... X.25 PDN network: 30
seconds. Sample value for a local area network: 10 seconds.
RouterDeadInterval
...
... Identifies the authentication procedure to be used on the
attached network. This value must be the same for all
routers attached to the network ...
... network. This value must be the same for all
routers attached to the network. See Appendix D for a
discussion of the defined authentication ...
... network much like it treats a broadcast
network. Since there may be many routers attached to the
network ...
... network. Since there may be many routers attached to the
network, a Designated Router is selected for the network. This
...
... network, a Designated Router is selected for the network. This
Designated Router then originates a network ...
... Router
Priority for the network is non-zero), and then only if no
automatic procedure for discovering neighbors ...
... The list of all other routers attached to the NBMA network.
Each router is listed by its IP interface ...
... When an interface to a NBMA network comes up, the router
sends Hello Packets only to those neighbors ...
... rate PollInterval, which should be much larger than
HelloInterval. Sample value for a PDN X.25 network: 2
minutes.
...
... C.6 Point-to-MultiPoint network parameters ...
...
On Point-to-MultiPoint networks, it may be necessary to
configure the set of neighbors that are directly reachable over
...
... neighbors that are directly reachable over
the Point-to-MultiPoint network. Each neighbor is identified by
its IP address ...
... its IP address on the Point-to-MultiPoint network. Designated
Routers are not elected on Point-to-MultiPoint networks ...
... network. Designated
Routers are not elected on Point-to-MultiPoint networks, so the
Designated Router eligibility of configured neighbors ...
... Alternatively, neighbors on Point-to-MultiPoint networks may be
dynamically discovered by lower-level protocols such as Inverse
ARP ...
... Host routes are advertised in router-LSAs as stub networks with
mask 0xffffffff. They indicate either router interfaces to
...
... authentication type is configurable on a per-interface (or
equivalently, on a per-network/subnet) basis. Additional
authentication data ...
... authentication type means that routing exchanges
over the network/subnet are not authenticated. The 64-bit ...
... authentication type, a 64-bit field is configured on
a per-network basis. All packets sent on a particular network
must have this configured value in their OSPF ...
... 64-bit field is configured on
a per-network basis. All packets sent on a particular network
must have this configured value in their OSPF header ...
... domain; each router must first
be configured with its attached networks' passwords before it
can participate in routing ...
... Ref16]). Anyone with physical
access to the network can learn the password and compromise the
security ...
... shared secret key is
configured in all routers attached to a common network/subnet.
For each OSPF protocol ...
... OSPF protocol packet and the secret key. Since the secret
key is never sent over the network in the clear, protection is
provided against passive attacks.
...
... authentication key expiration" notification to the network
manager and treat the key as having an infinite lifetime until
the lifetime ...
... the lifetime is extended, the key is deleted by network
management, or a new key is configured.
...
... LSAs and summary-LSAs is usually
set to the described network's IP address. However, if necessary one
or more of the network ...
... network's IP address. However, if necessary one
or more of the network's host bits may be set in the Link State ...
... This allows the router to originate separate LSAs for networks
having the same address, yet different masks. Such networks ...
... networks
having the same address, yet different masks. Such networks can
occur in the presence of supernetting and subnet 0s (see [Ref10 ...
... requirement on the algorithms used is that the
network's IP address should be used as the Link State ID whenever
...
... terminates. Otherwise,
(2) Obtain the network mask from the body of the already existing
AS-external-LSA ...
... together with all the bits that are not set in NM1, which is
network [NA,NM1]'s broadcast address ...
... Link State ID of NA) to reference the new
network [NA,NM1] by incrementing the sequence number,
...
...
changing the mask in the body to NM1 and inserting the cost
of the new network. Then originate a new LSA for the old
network ...
... network. Then originate a new LSA for the old
network [NA,NM2], with Link State ID equal to NA ...
... The above algorithm assumes that all masks are contiguous; this
ensures that when two networks have the same address, one mask is
more specific than the other. The algorithm ...
... more specific than the other. The algorithm also assumes that no
network exists having an address equal to another network's
...
... AS-external-LSA, try to
use the network number as the Link State ID. If that produces a
conflict, examine the two networks ...
... network number as the Link State ID. If that produces a
conflict, examine the two networks in conflict. One will be a subset
of the other. For the less specific network, use the network ...
... conflict, examine the two networks in conflict. One will be a subset
of the other. For the less specific network, use the network number
as the Link State ...
... networks in conflict. One will be a subset
of the other. For the less specific network, use the network number
as the Link State ID and for the more specific use the network ...
... network number
as the Link State ID and for the more specific use the network's
broadcast address ...
... host" bits to 1). If
the most specific network was originated first, this will cause you
to originate two LSAs at once.
...
... [10.0.0.0,255.0.0.0].
(c) The network [10.0.0.0,255.255.255.0] keeps its Link State ID
of 10.0.0.255.
...
... remove looping LSAs from the network (see Section 13.3), and b)
routers refuse to accept LSA ...
... authentication; the type of authentication in use
can be configured on a per network segment basis. One of OSPF's
...