IP overlapping

[olzhik]

I am  trying to configure this, however, once I get to g0/0.20 and try to set up the IP address it tells me that 172.17.18.0 overlaps with g0/0.10 . What should I do?

[Reza Sharifi · Accepted solution]

The message is correct because you can not assign 2 IPs from the same network segment to 2 different interfaces.

172.17.18.1/24 and 172.17.18.19/24 are both in the same /24 segment. Each interface needs to be in a different segment. Example:

172.17.18.1/25

172.17.18.129/25

HTH

[olzhik]

thanks!

[M02@rt37]

Hello @olzhik,

Router route networks/subnets. You can not have same subnet/IP on differents router's interfaces.

Depend of what you want, you could use VRF in order to use same IP/subnet on the Router, but on a different VRFs.

[Yous]

Guys, Have you encountered a situation where two different interfaces are assigned the same subnet, but one interface is configured within its own VRF? In this case, the VRF is configured before assigning the IP address, and errors are encountered, indicating that the subnet overlaps with another interface. This setup is on an ASR1000 router running firmware version Cisco IOS XE Software, Version 16.06.09. Thank you all. 

[Joseph W. Doherty]

I have not seen that issue, which if as you describe, would, I think, be a software defect.  (To be clear, you're seeing an overlapping interface IP address assignment, although interfaces are in different VRFs, and there's no incorrect route leaking between the different VRFs?)

[Yous]

Hi Joseph, that is indeed correct. I guess i will need to use a new subnet or upgrade the Routers to mitigate the bug. 

[Joseph W. Doherty]

"What should I do?"

Either change network address block allocations such that they do not overlap (all four IPs are all in the same network, 172.17.18.0/24, i.e. a /29 would work for each) or (as mentioned by M02@rt37) put each interface into a different VRF.