IPv6 Default Route with outgoing interface

[mabarian86]

Hi all,

I have my hands on the following scenario:

P1R1 and P1R3 are connected through their respective FastEthernet 0/0 interfaces.The IPv6 address of the fa0/0 of P1R1 is 2001:0:1:1::1 (this will be the next-hop address of the default route) P1R1 has a loopback 0 as 1:1:1:1::1/64.

I wanted to add a default route from P1R3 to P1R1 using the outgoing interfce (FastEthernet 0/0). For that I use this command:

P1R3(config)#ipv6 route ::/0 fa0/0

If I try to ping from P1R3 to P1R1 loopback interface, it doesn't work. Although, if I add the same defafult route but instead of using the outgoing interface, using the destination IPv6 address, it works:

P1R3(config)#ipv6 route ::/0 2001:0:1:1::1

My question is, why the default route doesn't work when I specify an outgoing interface? It works with the next-hop address and yes, I made sure that I am correctly following the topology.

Thanks for the help!

[Peter Paluch · Accepted solution]

Hi,

When you define your static default route out Fa0/0 interface without specifying the next hop address, you are forcing your P1R3 router to believe that the default route - the whole IPv6 internet - is directly connected to Fa0/0. Because of that, P1R3 tries to resolve every destination IPv6 address into a MAC address using ICMPv6 Neighbor Discovery (ND). However, Cisco routers do not support Proxy ND that would allow them to answer ND Neighbor Solicitations on behalf of other reachable destinations. Even though P1R3 asks for the MAC address of every destination in outgoing IPv6 packets, P1R1 is not going to answer any of those solicitations unless the destination is P1R1 itself. As a result, P1R3 is unable to forward the packets because it is unable to encapsulate them into frames properly - it does not know what destination MAC address to use.

With the next hop specified explicitly, the process is straightforward - the default route is no longer considered to be directly connected. P1R3 will resolve 2001:0:1:1::1 into MAC address and encapsulate all internet-bound packets into frames toward this MAC address.

On multi-access interfaces like Ethernet, you should never, ever, define a static route out the interface without specifying the next hop. Defining routes out interfaces is only suitable for point-to-point interfaces such as tunnels or serial interfaces using HDLC or PPP.

Feel welcome to ask further!

Best regards,
Peter

[mabarian86]

Thank you very much Peter! Now everything is clear.

BR,

Mario