SVI and a Loopback with the same IP range in the same router.

[Jose Vidal Diez]

Hi everybody,

I have a doubt about a SVI and a Loopback with the same IP range in the same router.

Some days ago I tried to configure a SVI and a Loopback in the same router and the CLI answered me that it is no posible to have a SVI and a Loopback in the same router in the samre IP range, and I would want to know what is the technical answer for this behaviour.

Thank you very much for the support.

Best Regards.

José

[Julio E. Moisa · Accepted solution]

Hi

You cannot have 2 IP addresses of the same network segment because it will create an overlaping, on the most of the cases the IP addresses configured on Physical/Logical Interfaces are related to default gateways of LANs, in order to avoid inconveniences with the traffic.

The exception is when you use 'secondary' word at the end of the addressing but over the same interface only, for example:

interface loopback 0

ip address 1.1.1.1 255.255.255.255 

ip address 1.1.1.2 255.255.255.255 secondary

or 

Interface FastEthernet 0/0

ip address 10.12.0.1 255.255.255.0

ip address 10.12.0.45 255.255.255.0 secondary

Hope it is useful

:-)

[Jose Vidal Diez]

Thank you very much Julio.

[Julio E. Moisa]

You are welcome, have a great day!

:-)

[Joseph W. Doherty]

BTW, if your router supports VRFs, then you can have overlapping networks, as long as they are in different VRFs. (You can also have overlapping IPs too.)

As to why Cisco router's disallows this, I suspect Cisco considers it a configuration error which could lead to other issues if allowed. For example, consider you want to assign two routed ports to the same subnet, so you can "double' your bandwidth to/from that network. It would probably work fine for traffic to/from other end hosts, but what if a routing protocol is enabled on those ports, would those ports try to create an adjacency between themselves? (Etc. . .)

[Jose Vidal Diez]

Hi Joseph,

Thank you very much for your answer.

Very instructive.

Best regards.