On an NBMA network, the many-to-many
direct interaction between routers is not guaranteed unless virtual
circuits are configured from each router to all other routers.
The primary reason that most organizations
implement a hub-and-spoke topology is because of cost
considerations. In a hub-and-spoke topology, multiple remote sites
have virtual circuits connected to one or more routers at a central
site. Figure
illustrates an example of next-hop behavior in a nonbroadcast
multiaccess environment.
The only difference between the
environments illustrated is that the medium in Figure
is a Frame Relay cloud that is NBMA. RTC is the hub router; RTA and
RTB are the spokes. Note that the virtual circuits are laid out
between RTC and RTA, and between RTC and RTB, but not between RTA
and RTB. This is a partially meshed topology.
RTA gets a BGP routing update about
11.11.11.0/24 from RTC and would try to use RTB (10.10.10.3) as the
next hop (the same behavior as on multiaccess media).
Routing will fail because no virtual circuit exists between RTA and
RTB.
Cisco IOS supports a special-case
parameter that remedies this situation. The next-hop-self keyword
forces the router (in this case, RTC) to advertise 11.11.11.0/24
with itself as the next hop (10.10.10.2). RTA then directs its
traffic to RTC to reach destination 11.11.11.0/24. The syntax for
this option is as follows:
Router(config-router)#neighbor
ip-address next-hop-self
For RTC, you would issue the command:
RTC(config-router)#neighbor
10.10.10.1 next-hop-self
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