Virtual Links - Foundry Networks NetIron M2404C User Manual

Foundry NetIron M2404C and M2404F Metro Access Switches
Point-to-Point networks, since only two routers exist on a segment. They exchange link state
information and routes as peers across the common subnet. This is the default network type for
point-to-point frame relay sub-interfaces.
Non-Broadcast Multi-Access (NBMA)
The NBMA network type was developed for OSPF to run on media such as X.25, frame relay, and
ATM, where the network cannot dynamically forward broadcast packets to all other routers in a
virtual network. Other than manual configuration of OSPF neighbors, router behavior configured
with this network type is identical to that of broadcast (i.e., Hello protocol elects DR/BDR and
adjacency with all non-DR/BDR routers is formed). Again, it is important to ensure that a hub
router is elected to be the DR on a hub-and-spoke partially meshed frame relay network to ensure
that adjacencies can be formed with every spoke.
Point-to-Multipoint (Pt-to-Mp)
The Point-to-Multipoint network type was developed for OSPF to run on NBMA networks such
as Frame relay and ATM. Routers are addressed out of a common IP subnet on WAN IP interfaces
but full meshing is not required, as DR/BDR election is not done on point-to-multipoint segments.
This network type is well suited for frame relay hub-and-spoke networks where conservation of IP
addresses or minimizing resource impact of logical IP interfaces on hub routers is an issue. Any-to-
any spoke connectivity in a partially meshed PVC environment is possible since the hub router will
advertise itself as the next hop forwarding address to all spokes for all routes.

Virtual Links

The backbone must be contiguous. However, it need not be physically contiguous. Backbone
connectivity can be established/maintained through the configuration of virtual links.
If an area cannot be physically connected to the backbone area, the user can use a virtual link to
connect to the backbone through a non-backbone area, known as a transit area. The transit area
must have full routing information - it cannot be a stub area.
Virtual links can be configured between any two backbone routers that have an IP interface to a
common non-backbone area. Virtual links belong to the backbone. The protocol treats two routers
joined by a virtual link as if they were connected by an unnumbered point-to-point backbone
network.
For example, in the configuration shown in
backbone fails, the connection via ABR2 provides redundancy so that the discontiguous area can
continue to communicate with the backbone using the virtual link.
Open Shortest Path First (OSPF)
Figure
Figure 2: Virtual Link Providing Redundancy
© 2008 Foundry Networks, Inc
Configuring Advanced Routing Information (Rev. 03)
2, if the connection between ABR1 and the
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