You already know that in distance-vector routing protocol, split horizon and route-poisoning are the two techniques which are used to prevent the formation of loops in the route under observation. This cannot happen in case of link-state protocols, because in this case, only the state of the node is exchanged, unlike distance-vector protocol where the entire topology is exchanged. Instead, in link-state protocol, any network change is calculated using the Dijkstra’s algorithm, and therefore there is not any loops that are allowed to form. The video highlights the importance of the hub and spoke configuration of the OSPF networks, and also the methods to configure such networks. You should pay attention, because when a hub and spoke configuration is built over an NBMA technology, which is similar to the Frame Relay you normally work with, every bit of attention is required to prevent any unwanted errors.
Octet
The network used in the video is a little bigger when compared to the networks you have been working on. In the network, routers 1, 2, and 3 are connected to a Frame Relay cloud over the NBMA, thereby forming the area 0, and acting as the backbone. Every non-backbone area must have a minimum of one router connected to the interface of area 0. In this tutorial, we reconfigure the networks which having the following network numbers as the last octet:
- Frame Relay Network: 172.12.123.0/24
- Ethernet Segment: 172.23.23.0/24
Also note that each router over the Frame Relay network may employ a different kind of interface, and each router also has a loopback having its own number for every octet.
Both Connections
What you have to ensure is that the hub is acting as the DR, and that not any backup designated routers are present. In this tutorial, R1 acts as the hub. This task can be accomplished by setting the OSPF interface priority to zero on the spoke routers. You will also need to configure each neighbor statement on the hub because on an NBMA network, the neighbors are not dynamically discovered by the hub. At the end of video, the viewer should be able to configure NBMA OSPF networks over both the Frame Relay as well as Ethernet connections.