DHCPv6 IA_NA and default route selection on a CPE router

[insideshell]

Hello,

As you probably know, DHCPv6 doesn't provide default route to the client (by design). ICMPv6 RA does, and the client instantiates a default route only if it considers itself as a host, as per rfc4861#section-6.3.6.

Now, let's say the client is a CPE router (meaning, ipv6 unicast-routing is configured). It gets a Global Unicast Address (GUA) from a BNG through IA_NA option of DHCPv6. Even if the BNG sends RA messages, the CPE won't instantiate a default route, as it considers itself a router and not a host.

In this case, how to dynamically obtain a default route? I cannot disable ipv6 unicast-routing since the CPE has to provide IPv6 parameters (on-link prefix and other info) to LAN-side devices through ICMPv6 RA.

In the case of a PPP access, adding a static connected default route to the Dialer interface solves the problem. But what about an IPoE access?

I was testing this on a Cisco IOSv 15.9(3)M3 CPE router, so maybe I'm biased by the model behavior.

Thanks for reading me!

[Harold Ritter · Accepted solution]

Hi @insideshell ,

You can configure "ipv6 nd autoconfig default-route" on the wan interface to force the router to install a default route according to the received router advertisement.

https://www.cisco.com/c/en/us/td/docs/ios-xml/ios/ipv6/command/ipv6-cr-book/ipv6-i3.html#wp4085876610

Regards,

[insideshell]

Shame on me I wasn't aware of this command. I found strange the RFC 4861 only defined default route selection for hosts and not routers. This somewhat contradicts the RFC 7084:

W-3: Absent other routing information, the IPv6 CE router MUST use
Router Discovery as specified in [RFC4861] to discover a
default router(s) and install a default route(s) in its routing
table with the discovered router's address as the next hop.

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Thanks for the answer!

[Harold Ritter]

You are very welcome @insideshell and thanks for the feedback