Routing Loops

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.

Figure: Fully converged network, prior to routing loop.

Routing loops become a potential issue when our network experiences a problem. For example, imagine that network 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 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 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 is available through it, with a hop count of two, as shown in the figure below. See a problem developing?

Figure: Update from Router A creates a routing loop between Routers A and B.

Now we have a network where Router B thinks it can get to network via Router A. Without anything else occurring, think about what happens here. When Router B gets a packet destined for network, 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 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.

Author: Dan DiNicolo

Dan DiNicolo is a freelance author, consultant, trainer, and the managing editor of He is the author of the CCNA Study Guide found on this site, as well as many books including the PC Magazine titles Windows XP Security Solutions and Windows Vista Security Solutions. Click here to contact Dan.