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Because a full-mesh topology is costly, many organizations implement a
partial-mesh topology instead. A partial-mesh topology is any
configuration in which at least one router maintains multiple connections
to other routers, without being fully meshed. The most cost-effective
partial-mesh topology is a hub-and-spoke topology, in which a single
router (the hub) connects to multiple spoke routers.
The hub-and-spoke topology is a cost-effective WAN solution that
introduces a single point of failure (the hub router). Organizations
typically deploy Frame Relay because it is inexpensive, not because it is
fault-tolerant. Since dedicated leased lines (not Frame Relay links)
typically carry mission-critical data, an economical Frame Relay topology,
such as hub-and-spoke, makes sense.
Unfortunately, the
neighbor
command that worked with a full-mesh topology does not work as well with
the hub-and-spoke topology. The hub router in Figure
sees all the spoke routers and can send routing information to them using
the neighbor
command, but the spoke
routers can send hellos only to the hub.
The DR/BDR election will be held, but only the hub router sees all of the
candidates. Because the hub router must act as the DR for this OSPF
network to function properly, you could configure an OSPF interface
priority of 0 on all the spoke routers. Recall that a priority of 0 makes
it impossible for a router to be elected as DR or a BDR for a network.
A second approach to dealing with this topology is to avoid the DR/BDR
issue altogether by breaking the network into point-to-point connections.
Point-to-point networks
will not elect a DR or a BDR.
Although they make OSPF configuration straightforward, point-to-point
networks have major drawbacks when used with a hub-and-spoke topology.
Subnets must be allocated for each link, which in turn can lead to WAN
addressing that is complex and difficult to manage. The WAN addressing
issue can be avoided by using IP unnumbered, but many organizations have
WAN-management policies that prevent using this feature. Are there any
viable alternatives to a point-to-point configuration? Fortunately, the
Cisco IOS offers a relatively new alternative. A hub-and-spoke physical
topology can be manually configured as a point-to-multipoint network type,
as described in the following section.
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