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5.2 | |
Configuring Frame Relay | |
| 5.2.3 | |
Reachability issues with routing updates in NBMA |
By default, a Frame Relay network provides non-broadcast multi-access (NBMA)
connectivity between remote sites. An NBMA environment is viewed like other
multiaccess media environments, such as Ethernet, where all the routers are
on the same subnet. However, to reduce cost, NBMA clouds are usually built
in a hub-and-spoke topology. With a hub-and-spoke topology, the physical
topology does not provide the multi-access capabilities that Ethernet does.
 The physical
topology consists of multiple PVCs.
A Frame Relay NBMA topology may cause two problems:
Split-horizon updates reduce routing loops by not allowing a routing
update received on one interface to be forwarded out the same interface. If
Router B, a spoke router, sends a broadcast routing update to Router A, the
hub router, and Router A has multiple PVCs over a single physical interface,
then Router A cannot forward that routing update through the same physical
interface to other remote spoke routers. If split-horizon is disabled, then the
routing update can be forwarded out the same physical interface from which
it came. Split-horizon is not a problem when there is a single PVC on a
physical interface. This would be a point-to-point Frame Relay connection.
Routers that support multiple connections over a single physical interface have many PVCs that terminate in a single router. This router must replicate broadcast packets such as routing update broadcasts, on each PVC, to the remote routers. The replicated broadcast packets can consume bandwidth and cause significant latency to user traffic. It might seem logical to turn off split-horizon to resolve the reachability issues caused by split-horizon. However, not all network layer protocols allow split-horizon to be disabled and disabling split-horizon increases the chances of routing loops in any network. One way to solve the split-horizon problem is to use a fully meshed topology. However, this will increase the cost because more PVCs are required. The preferred solution is to use subinterfaces.
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