An engineer configured a GRE tunnel to carry native IPv6 between two sites but IPv6 traffic is not forwarded through the tunnel. The tunnel was created with the default command 'tunnel mode gre ip'. What single change fixes the problem?
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Correct answer: Change the tunnel mode to ipv6ip (tunnel mode ipv6ip) and assign IPv6 addresses on the tunnel interfaces.
Why this is the answer
The correct answer is to change the tunnel mode to ipv6ip and assign IPv6 addresses on the tunnel interfaces. The default tunnel mode gre ip encapsulates IPv4 packets over a GRE tunnel. To tunnel native IPv6 traffic, the tunnel mode must be explicitly set to ipv6ip, which encapsulates IPv6 packets. Additionally, the tunnel interfaces themselves need IPv6 addresses to route IPv6 traffic. Applying an IPv6 ACL to the physical interface is incorrect because the issue is with the tunnel's encapsulation type, not a firewall blocking traffic. Enabling ipv6 unicast-routing is necessary for IPv6 routing but doesn't resolve the incorrect tunnel encapsulation mode. Configuring an IPv6 crypto map is for securing the tunnel with IPsec, which is a separate function from enabling basic IPv6 tunneling.
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