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I have created an ipv6 tunnel by using the following commands:

ip link add gre_test type ip6gre remote 2001:4860:4860::8888 local ffff:ffff:ffff:ffff:ffff:ffff:ffff:ffff ttl 255

removed actual ipv6

After this I turned up this interface:

ip link set dev gre_test up

And then finally when I am trying to add the interface under the bridge default_lan I am facing this issue

brctl addif default_lan gre61 can't add gre61 to bridge default_lan: Invalid argument

So basically I am making an ipv6 tunnel and terminating it on a bridged interface But I am getting the error Invalid argument when I am trying to add the bridge. It works if gre interface is purely ipv4 and if we use 'gretap'.

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It works if gre interface is purely ipv4 and if we use 'gretap'.

That works because you use 'gretap', not because the interface is purely ipv4.

Standard GRE interfaces, whether IPv4 or IPv6, are not bridgeable because they're "layer 3" tunnels – they do not carry the L2 headers that would be necessary for an L2 bridge to function.

Recall that a bridge works by forwarding packets according to their L2 destination (MAC address) and a packet that arrives through ordinary GRE doesn't have that, so there is nothing that a bridge could do with it.

(If you expect the bridge to look at the L3 (IP) destination addresses instead – well, that wouldn't be a bridge anymore, that would be a router.)

So if you want to bridge networks over IPv6, you need a L2 tunnel like 'gretap' or 'ip6gretap'. (The two types are depending on the underlying network, not on the tunneled traffic – all GRE tunnels can carry both kinds of IP, as can all L2 tunnels).

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