It is possible to repeat source ip in GRE tunnel?

[sir_ulrick]

Hi everybody, 

I have a question about GRE tunnels. I have the follow topology connected using dinamic routing (whichever)

and I would like create a GRE tunnel between each router (must be point to point GRE tunnel, to avoid different problems with the topology, I can't use DMVPN) so I need define a GRE tunnel between router 0 -> router 1, router 0 -> router 2, router 0 -> router 3, router 1 -> router 2, .... etc.  My question is, can I use same ip to define tunnel betwen different interfaces? I mean, GRE tunnel between router 0 -> 1 will have ip 1.1.1.1 (router 0) and 1.1.1.2 (router1). Can I define same ip between router 0 and router 3 or it's necessary to add new ip?

Tunnel between router 0 and router 1:

interface tunnel 5

tunnel mode gre ip

ip address 1.1.1.1 255.255.255.0

ip mtu 1400 

tunnel source gi0/0/0 // 10.10.10.1

tunnel destination 10.10.10.2

Tunnel between router 0 and router 3:

interface tunnel 6

tunnel mode gre ip

ip address 1.1.1.1

ip mtu 1400

tunnel source gi0/1/0

tunnel destination 10.10.20.2

I thought to use loopback interface as source address, but I read that doing this causes routing problems

[Giuseppe Larosa · Accepted solution]

Hello @sir_ulrick ,

you can have multiple tunnels using the same source interface.

You can multiplex / demultiplex using the command tunnel key <number>

However, the IP address assigned to the GRE Tunnel must be different in each tunnel (the internal address). Each p2p GRE tunnel will need a different IP subnet for internal addresses.

Hope to help

Giuseppe

[sir_ulrick]

Thanks a lot for your reply Giuseppe Larosa! I think that tunnel key it's what I need!!

[Joseph W. Doherty]

BTW, on some platforms (like 6500s I recall), reusing the same source-IP inhibits performance.