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What is the reason that you cannot set the tunnel's other side as destination address? Is this because it is virtual only?

192.168.1.0/30 is the subnet I'm using and this is my configuration:

tunnel1
ip add 192.168.1.1 255.255.255.252
tunnel source (doesn't matter)
tunnel dest 192.168.1.2

tunnel dest 192.168.1.2 -> Why is this invalid?

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  • on a side note, whether the tunnel destination is virtual or not does not matter. The tunnel destination could be a physical interface on a router or it could be a SVI/vlan interface. The tunnel destination is simply and IP that says "when traffic gets there, de-encapsulate the IP from the GRE headers."
    – John Kennedy
    Commented Jan 16, 2014 at 21:37
  • Did any answer help you? if so, you should accept the answer so that the question doesn't keep popping up forever, looking for an answer. Alternatively, you could provide and accept your own answer.
    – Ron Maupin ♦
    Commented Aug 8, 2017 at 15:49

3 Answers 3

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Yes,

The tunnels IP addresses need to be on their own subnet. The tunnel source and destination identify the points on the network where routers should encapsulate or de-encapsulate the traffic that is sent thru the tunnels. Having a route to your tunnel destination is a requirement for a tunnel interface to show "UP/UP" so really the 192.168.1.0/30 subnet doesn't even exist on the network until your source and destinations are configured.

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The tunnel destination can't be the subnet of the tunnel itself, which is what you have configured. And tunnel source does matter.

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  • why can't it be? I know the source does matter I just want to understand why cant I use the the tunnel's other address in my case .2 as destination?
    – user3916
    Commented Jan 16, 2014 at 21:51
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    How would the router know what to do with it? Suppose it has a packet addressed to 192.168.1.2. Is that a packet that is supposed to go in the tunnel, or is it a GRE packet itself? Another way to think of it is a GRE tunnel acts like a subnet. So you have the endpoint of the tunnel, on a subnet, but you also have the tunnel subnet itself with overlapping addresses. Does that make any more sense?
    – Ron Trunk
    Commented Jan 16, 2014 at 23:04
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For simple GRE tunnels:

The destination address used to build the tunnel should be the actual address of the device that you are trying to reach. Also, while it is quick to use an interface name as the source for the tunnel, using the actual address will help when it comes to troubleshooting.

In order for the tunnel to work the destination address of the tunnel should be pingable.

You will also need to configure the tunnel from the other device back to this one. But if you used addresses for configuring the one end of the tunnel, the other end of the tunnel is simply switching the source and destination.

Ref: https://supportforums.cisco.com/docs/DOC-2569

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