Router topology configuration

When configuring a router topology, the default is broadcast network. There are also other topologies which are non broadcast network (NBMA), point to multi-point network, and point to point network. Point to multi-point is actually a group of point-to-point networks. For point to multi-point (broadcast) network, the neighbor routers do not need to be configured. For point to multi-point (non-broadcast) network, the neighbor routers do need to be configured. On a point to point or point to multi-point configuration, there is no DR and BDR, simply because there is no need to do so. There is no other router to broadcast to in a point to point route.

OSPF can be configured in an NBMA network, where the router's configuration is in a 'hub-and-spoke' network using a non-broadcast network. To configure OSPF on NBMA network, only the hub needs to be configured for neighbors.

Steps are:

1. Configure OSPF on hub router with the commands:
(config)#router ospf (ospf number)

(config-router)#network area

(config-router)#neighbor // apply for other neighbors too, if available

2. Configure spoke OSPF router with the command:
(config)#router ospf (ospf number)

(config-router)#network area

3. Set spoke router priorities to 0 (zero) as they may not become a DR or BDR router.

A router that is not in an ospf area 0, needs to have access to a router in ospf area 0 in order to communicate with each other. This can be configured by using a virtual link, transit/stub area done in both sides of the link. This will also be considered as a point to point network.
The command for setting this link will be:
(config)#router ospf
(config-router)#area virtual link

To verify and show virtual link use the command: #show ip ospf virtual-link

-- 4 January 2011 --