Tech Tips
Network Analysis Tip # 66 – Determining the Origin of Each Route
Why is this important?
It is a good practice to periodically check the routing tables to make sure that the appropriate routes exist and that the purpose of each route is known. Quite often you may find a surprise route that looks like it shouldn’t be in your routing tables. Such a route might indicate a rogue routing entry, or perhaps an entry left over from a service or connection that has been discontinued. Once such a route is identified, you need to determine which routers are originating that route.
Manual determination
Tracking down a route involves following the route distribution, hop by hop across the network. This can be achieved using Cisco CLI command ‘show ip route’. Look for the router from which the route was learned, telnet to it, and repeat the process. The manual task of tracking down a route is often a tedious and error-prone, plus it only provides information about one potential source of the route. If multiple sources exist, then each one must be found in turn.
Automatic determination
NetMRI collects routing tables from all routers and determines the originating routers for each route. The process of identifying the origin of each route uses the following priority:
- Any directly connected interface. This will always be the best origin.
- Static routes are identified next
- Routes with the lowest metric for each routing protocol, sequencing through multiple protocols in the same order as is used by Cisco’s Administrative Distance Algorithm.
- Any routes imported from outside your network, as identified by the CIDR address blocks, will be identified with the protocol that imported them.
Sample results, viewed below, shows the resulting routing summary as well as a list of routers originating the selected route. In the example, nine routers each have an interface on the 10.20.1.0/24 subnet and may be advertising that route to the rest of the network, if the routing configuration allows.
