Unless special consideration is given to the operation of OSPF in a
Non-Broadcast Multi-Access (NBMA) environment such as Frame Relay, routing
problems will occur. This is because OSPF expects to be able to multicast all
routers within a subnet, or broadcast domain. In a Frame Relay hub and spoke
topology this will not be possible. Consequently, some routers and routes will
be inaccessible.
The problem can be solved by forcing RTB to become the
designated router.
This is achieved
by configuring 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. As long as RTB can exchange hellos with RTA and RTC it can
maintain a complete view of the network and pass this routing information to
both RTA and RTC.
Alternatively, subinterfaces could be used on the hub router making each
link a point to point link. Recall that no DR election occurs in a point to
point configuration. The downside to this technique is the creation of a new
subnet or network.
Still another technique to manage OSPF over hub and
spoke NBMA networks is to explicitly configure OSPF as a Point-to-multipoint
network. Point-to-multipoint networks have the following properties.
Adjacencies are established between all neighboring routers. There is no DR
or BDR for a point-to-multipoint network. When originating a router LSA, the
point-to-multipoint interface is reported as a collection of point-to-point
links to all the interface's adjacent neighbors, together with a single
stub link advertising the interface's IP address with a cost of 0.
When flooding out a nonbroadcast interface, the LSU or LSAck packet must be
replicated to be sent to each of the interface's neighbors. To configure
point-to-multipoint, manually override the detected OSPF network type with the
following syntax:
Router(config-if)#ip ospf network
point-to-multipoint
The interface should also be
configured with a frame-relay map ip command, as in the
following syntax:
Router(config-if)#frame-relay map ip address dlci
[broadcast]
The broadcast
keyword permits the router to send broadcasts via the specified DLCI to the
mapped neighbor or neighbors. If the point-to-multipoint configuration is
applied to the example network, two separate frame-relay map statements would
have to be configured on the hub router, RTB.
