GRE tunnels between two routers with same source and destination

[sqambera]

Hello,

I have two routers directly connected together through a single physical Ethernet link and I want to create multiple GRE tunnels between them using the same source and destination addresses. The tunnels are formed but the problem I am facing is that when I try to run OSPF on those tunnels in different areas adjency is not formed and stays in INIT state.

I'd appreciate if someone could please advise to make this work.

Thanks,

Qamber

[Peter Paluch]

Hi Qamber,

Can you please post the configuration of the tunnel interfaces from both routers? Also the show ip ospf neighbor would be helpful.

The OSPF INIT state means that we hear Hello packets from the other router, but these packets do not contain our own RID in the "seen routers" section, meaning that the other router does not hear us. We will need to find the reason for this behavior.

Best regards,
Peter

[sqambera]

Thanks Peter for helping. Attached are the configurations of two routers.

Looking forward to hear from you.

Regards,

Qamber

[Peter Paluch · Accepted solution]

Hi Qamber,

An obvious problem with your configuration is that your tunnels are missing a so-called key - a numerical value that is carried in GRE headers which helps the receiving router to properly identify the incoming tunneled packet with the appropriate Tunnel interface. Tunnel keys are not necessary if you have just a single tunnel interface toward a neighbor; however, if you have multiple tunnel interfaces anchored to the same source and destination, the key value is crucial to properly demultiplex incoming tunneled packets into appropriate tunnel interfaces.

Please modify your configuration so that on respective routers, add tunnel key 1 to the Tunnel11 and Tunnel21 configuration, and tunnel key 2 to the Tunnel 12 and Tunnel22 configuration.

Please let us know if this helped.

Best regards,
Peter

[sqambera]

Many thanks Peter. Yes tunnel key was the issue and configuring it instantly solved the problem.

I appreciate your time and effort to help me out.

Regards,

Qamber

[Calin C.]

Hi,

I assume you tried already to use ping over the tunnel p2p subnet and it's successful, right? Just to be sure that the tunnels are working properly, not that they are just "up".

What is the "show ip ospf int Tun11" (or Tun22 on the other side) saying?

Cheers,

Calin

[sqambera]

Hi Calin,

Thanks for joining. Peter was right about it. The problem was due to tunnel key.

I appreciate your help.

Regards,

Qamber