Discontiguous Network

dppagc

What will a router do if it encounters a discontiguous network?

Will it forward the packet out both interfaces or will it drop the packet?

ccie14023

It would help if you posted a diagram of what you mean. First off, you can't configure overlapping networks on multiple interfaces of a router, so it's not an issue with directly attached networks. If the router receives two different identical routes pointing to two different places, then it decides which one to use based on:

- Metric, if they are learned via the same protocol.
- Admin distance (Cisco) or route preference (Juniper) if they are learned via different protocols

If the router has two truly equal cost routes, then what it does depends on how it's configured. It will probably do equal-cost multi-path among the routes, i.e., load balance between them. Remember, the router doesn't know they are discontiguous--it just thinks it is receiving two different paths to the same place. If it does ECMP you will see half of your traffic dropped. Some protocols won't do ECMP and might pick one of the routes as the best path.

In general, you don't want to do this. When it's a matter of, for example, merging two companies that have some of the same networks, we either renumber or NAT.

dppagc

Thanks. You answered my question. It will load balance.