Overlapping interfaces permitted?

CriscoSystems · ‎10-12-2007

I'm doing some lab exercises that involve my flaky old 2521.

I think it might be even flakier than I thought, because I noticed that I configured both S0 and S1 interfaces with the exact same IP address, 192.168.2.1/24.

(Elsewhere in my lab is network 192.168.1.0/24 which is what I was going for on one of them.)

In any case, when I configured the second interface with that exact same address, shouldn't I have gotten an "Overlaps with Serialx" error message; rather than being allowed to proceed?

I've got 12.2(1d); the exact image name is c2500-jk8os-l.122-1d.bin

Anyone got insights?

Thanks.

Richard Burts · ‎10-12-2007

Seth

What you have found is not an issue with the 2521 (they are wonderful machines) but is a consistent (but not well known or well understood) behavior of IOS. IOS uses a somewhat different set of rules for LAN interfaces (like Ethernet) and for point to point serial interfaces. Your assumption that you should get an error message about overlapping addresses is quite true for LAN interfaces. But for serial point to point interfaces it is allowed. I have never seen an authoritative explanation of why this is, but I have observed (for a long time) that this is a consistent behavior. So it is not a bug, it is a feature :)

HTH

Rick

HTH

Rick