[c-nsp] Sup720-3bxl MPLS Ingress PE/P ttl mess

[Christoph Loibl]

Hello,

I was reading those threads about using the c6500 or c7600 with  
sup720-3bxl as P/PE routers. In my experience those devices work just  
fine with MPLS. But they also do some very nasty things with the ttl  
of the processed frames:

When having "no tag-switching ip propagate-ttl forwarded" configured  
on the ingress PE and encapsulating IP-traffic (no VPNs - only one  
label is pushed in that case). A Sup720 P switch that needs to pop  
this single label (PHP) also decrements the IP-ttl of the resulting  
IP-packet before sending it to the egress router. This behaviour  
leads to ugly traceroutes where the egress router shows up twice in  
the path. (The PHP P switch sends an IP-frame with ttl=0 :-(, it  
simply should not touch the IP-frame at all when doing label  
operations and the ip-ttl is lower than the label's-ttl).

In contrast, having a Sup720 equipped c6500 ingress PE with "no tag- 
switching ip propagate-ttl forwarded" enabled (same situation as  
above, just a single label is pushed). The egress PE does not show up  
in your traceroute at all.

This at least holds true for 12.2(18)SXF9 and as far as I understand  
is covered in CSCef16357. Cisco says this is a hardware issue and  
can't be fixed. But the bug report does not look like they really  
investigated that very much (it does not describe the hole issue very  
precise).

Is there anyone out there who also saw this bug and probably found a  
practical workaround? (Like disabling PHP -> using explicit-null  
labels, but this probably does not fix the problem and needs packet  
re-circulation - I don't know how much this affects the switching  
performance of the overall system in an CFC environment)?

Are there any newer hardware-revisions where this bug is fixed?

Regards

Christoph

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