How to enable " ipv6 unicast-routing" to permit traceroute from 6PE to ipv6-cloud

Mark Tinka mtinka at
Wed Sep 7 04:43:26 CEST 2011

On Tuesday, August 09, 2011 06:36:27 PM Herlianto wrote:

> Dear  IPv6 Expert,

Very late response...

> We are on development IPv6 over MPLS using 6PE schenario.
> Our P using CRS-1 (IOS-XR).
> I Would like to know how to ensure/to test using
> traceoute from one of 6PE to IPv6 Internet cloud?
> Traceroute not permit because of TTL ,  but there are
> suggest to permit traceroute at P can be enable " ipv6
> unicast-routing"
> but that command is only for IOS, not for IOS-XR.
> I need your advice how to ensure using traceoute from one
> of PE or CE to the IPV6 internet cloud with our P using
> IOS-XR (CRS-1)

Well, first things first, IOS XR is unlike IOS/IOS XE. In 
IOS XR, the v6 stack is enabled by default (or what I really 
mean is - you don't need to manually turn it on). The only 
thing you need to do is assign an interface a v6 address and 
you're good to go.

Regarding why traceroute fails in your scenario - well, with 
the way traceroute works in MPLS, when an MPLS label's TTL 
is decremented to 0 during transit, the router on which the 
TTL expires needs to generate the ICMP Unreachable message, 
which should be sent down the original LSP along which the 
MPLS frame was received.

In your scenario, your core router should generate an ICMPv6 
Unreachable message toward the source, but since your router 
isn't running v6 (which is the whole point of 6PE), this 
will not work, and you'll just see [* * *] in that portion 
of the traceroute output.

There has been mention of enabling v6 in IOS (don't know 
about IOS XR) and using v4-mapped v6 address to source 
ICMPv6 Unreachable messages in a 6PE topology; but if you 
ask me, it will be far easier to simply turn on v6 inside 
your core and be done with it :-).

Hope this helps.


Mark <= who doesn't believe in 6PE :-).
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