Route redistribution is a very important topic that every CCIE candidate should master before attempting their lab. In some scenarios when dealing with redistribution, one or multiple routing loops can occur. In the CCIE R&S lab, if a routing loop is possible then you can be almost certain that there will be one, so you better know how to deal with them. This post is meant to help people understand, identify and remove these loops from the network.
First, let’s start by breaking down the different kind of loops that you will see and the best tools to identify them.
This kind of loop is the easiest one to identify and can be seen when you have data constantly looping in your network until TTL expires. This can be for known or unknown traffic. “Known traffic” will be the traffic that you have a prefix in the RIB for and “Unknown traffic” will be for the traffic that only matches a default route. The best tool to identify these kind of loops is the traceroute or ping command. Sending ICMP packets to these prefixes will always fail and traceroute will clearly show a pattern of where the traffic is looping.
Control-plane loops are when you have routing information looping in the network. These are the most devious kind of loops because sending ICMP packets will not always help you to identify them. The reason for this is that the routing prefixes will sometimes be in a constant race to enter and be removed from the RIB between two protocols. The tool to identify these is actually running a “debug ip routing” on the redistribution routers. In a stable and loop-free environment, this debug command will be almost completely silent. However, if you are having control-plane loop problems, you will several debug messages indicating that routes being added and withdrawn from the RIB.
When can a routing loop occur?