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Traceroute through MPLS BackBone doesn't work properly, because mpls ttl propagation mode is not configured correctly.

Publication Date:  2012-07-27 Views:  5 Downloads:  0
Issue Description
There is part of MPLS BB network scheme:

Client’s traffic are forwarded through MPLS BB in Global Routing Table (not VPN-Instance)
As You can see on the pic 1. Traceroute output doesn’t include all intermediate hops from Internet. This is the problem!


Alarm Information
traceroute centre.ru
1 gw-servers.net.orn.ru (80.76.176.17) 0.175 ms 0.136 ms 0.105 ms
2 gw-bgp.net.orn.ru (80.76.176.33) 0.139 ms 0.135 ms 0.133 ms
3 85.26.196.85 (85.26.196.85) 0.578 ms 0.310 ms 0.298 ms <- -Ingress-PE
4 ms2.centre.ru (194.186.45.229) 11.232 ms 11.266 ms 11.265 ms<- - Destination


There are no any hops from Internet! After Ingress-PE (85.26.196.85) comes destination (centre.ru)!


Handling Process
For solving this problem You should do following:
  1. disable mpls ttl propagation on the Ingress-PE router;
  2. disable mpls ttl propagation on the Egress-PE router;
  3. disable mpls ttl propagation on the Last P router;
  4. reset mpls ldp all.
For example:


Root Cause
Because Ingress-PE device (Cisco 7609) has disabled mpls ttl propagation function, ”no mpls ip propagate-ttl forwarding” (for Huawei device, it should be "undo mpls ttl propagation"). MPLS TTL field in packet has value 255 at the Ingress PE. Then hop by hop MPLS TTL value is decreased one by one at P devices on the LSP. At the last P router (NE5000E) POP operation is performed and labeled information is removed before sending ip packet to the Egress-PE. During POP operation last P router copies value of MPLS TTL field into IP TTL (252) that is why all intermediate hops from Internet can’t be displayed in Traceroute output. This is root cause of this problem!
 


Suggestions
You should remember that last P device (before Egress-PE) removes labeled information of ip packet, if PHP is used.
After changing mpls configuration of LSRs, You have to reset mpls ldp on these routers, otherwise changing has not effect.


END