One of the main issues with distance-vector routing protocols is that they are susceptible to routing loops – a direct result of their slow convergence times. A routing loop can occur in the distance vector world because of the way routers exchange information. For example, let’s say that we have a network as shown in the figure below. Three routers exist in this example, connecting a total of four networks. We’ll begin with a network that is fully converged – that is, all routers are aware of all networks, as shown in the diagram. On a fully converged network, everything works well.
Routing loops become a potential issue when our network experiences a problem. For example, imagine that network 192.168.2.0 experiences a failure – maybe the switch it was connected to malfunctioned, or somebody simply disconnected the cable. At any rate, Router C recognizes that network 192.168.2.0 is unavailable, and passes this information to Router B in its next routing table update. Once the update arrives, Router B removes the entry for network 192.168.2.0 from its routing table. So far, things are going well. However, there is one little problem – Router A is also sending out routing table updates, and is telling Router B that network 192.168.0.0 is available through it, with a hop count of two, as shown in the figure below. See a problem developing?
Now we have a network where Router B thinks it can get to network 192.168.2.0 via Router A. Without anything else occurring, think about what happens here. When Router B gets a packet destined for network 192.168.2.0, it will send it to Router A. Router A will look at the packet, and will send it back to Router B – that is the direction of network 192.168.2.0 as far as Router A is concerned, after all. The packet will actually end up being passed back and forth forever and ever. This is certainly not a very comforting thought.
The problem just described is a directly related to slow convergence. Router C did its job in getting information about the unavailable network to Router B, but unfortunately Router A also sent an update to Router B prior to the “correct” information arriving at Router A. On a larger network, the problem would be even worse. Remember that distance vector routing protocols are not terribly discerning – they simply trust their neighbors to provide them with correct information. Thankfully, distance vector routing protocols take a number of steps to try to avoid the routing loops just described.