Flooding in full-mesh topology is a big concern for network-design experts, especially in large-scale OSPF deployments. When the link or node fails in an OSPF network, failure information is flooded everywhere in the same area. If Flat OSPF network design is used, then the problem gets bigger. Each router receives at least one copy of the new information from each neighbor.
If there are only 2 routers in the topology, the total number of links between them is 1; if 3 routers, the number of links is 3; if 4 routers, there are 6 links in the typology; if 5 routers, there are 10 links in the full mesh network.
This is the formula: for N routers, there are (N) (N-1)/2 links; If there is only 2 routers in the topology, the total number of link between them is 1; if 3 routers, there are 3 links; if 4 routers, there are 6 links; and if 5 routers, there are 10 links. Because in the above topology there are 6 routers, there are 15 links. Even if one loopback is added to any one of these routers, that loopback information is flooded in all the routers over all the links.
Mesh group is one mechanism that reduces the amount of flooding in a full-mesh topology. With mesh group, we can designate couple routers in the topology to flood.
Those routers will be responsible for flooding event. In this topology, Router A and B can be assigned as the flooders. Important consideration is to select powerful devices as flooders in the mesh group since their duty is to flood the LSAs in all the networks .
Full-mesh topologies are complex, and OSPF and IS-IS need to be configured to work optimally in a full-mesh topology.
In fact, IGRP works better in full-mesh topology than does OSPF and IS-IS; however, full-mesh topologies are generally found in the datacenter network. Since more factors need to be considered in choosing routing protocols (see the table below), neither EIGRP nor IS-IS is a suitable choice for the datacenter networks.