multi access network topology

OSPF defines five types of network. The types are point-to-point, Broadcast Multi-access, Non-broadcast Multi-access (NBMA), point-to-multi-point and virtual networks. The Multiaccess networks can create two challenges for OSPF regarding the flooding of LSAs:

• Multiple adjacencies– Open shortest path first (OSPF) try to find the best path between the source and the destination router.  There are multiple OSPF enabled routers may interconnect over a common link. The OSPF router tries to create adjacency with each OSPF enabled router. Creating adjacencies with each router is unnecessary and unwanted. This would lead to an unnecessary number of LSAs exchanged between routers on the same network.
• Extensive LSA flooding– OSPF enabled the router to flood their link-state packets (LSP) when OSPF is initialized, or when there is a change in the network topology. This flooding can become excessive.

We can calculate the number of required adjacencies using the following formula.

n (n – 1) / 2

n is the number of OSPF enabled router in the network. The figure below illustrates the simple topology of three routers, all routers are attached to the same multiaccess Ethernet network. These routers will form six adjacencies:

n(n-1) /2

4(4-1)/2 =6

If we add a router to the multiaccess network, the adjacencies will increase dramatically. To decrease the adjacencies we required some special technique which will be discussed the incoming article.