Query on GRE tunneling

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In the CCNP Encore video on GRE tunneling, Anthony used different tunnel number at the source and destination. He used:
R1 (config)# int tunnel 10
R4 (config)# in tunnel 99
I always assumed that one has to use the same tunnel number at both R1 and R4. But, in the video, I saw that tunnels came up. So does this mean, the 2 different tunnels are created like 2-way traffic highways, each with a specific traffic direction?

Hi Avinash!

We need to be very careful with assumptions! :-) The tunnel interface ID is only significant to the local device and will NOT impact the behavior of the tunnel in any way.

Now- with this said, it would NOT be a good idea to use 10 on one side and 99 on another...why? It will get too confusing when you are troubleshooting as you will be forgetting which you used on which device.

In practice, we will often pick an ID that communicates meaning. For example, if my tunnel subnet space is 10.10.10.0/24 - we might opt for tunnel 10 on each device. For the 10.20.20.0/24 subnet, we would of course use tunnel interfaces of ID 20.

Thanks for the great question.

Hi Anthony,

Thank you for sharing that explanation.