TTL mpls

edge7mpls · ‎02-18-2013

Hi community.

My question is about mpls ttl.

Let's see this traceroute:

traceroute [(192.168.1.3:33456) -> (109.112.224.94:33457)], protocol icmp, algo hopbyhop, duration 25 s
 1  192.168.1.1 (192.168.1.1)  4.187 ms !T0   1.266 ms !T0   1.303 ms !T0 
 2  192.168.100.1 (192.168.100.1)  163.767 ms   285.182 ms   616.070 ms 
 3  172.18.33.149 (172.18.33.149)  163.678 ms   286.867 ms   616.051 ms 
 4  172.18.32.37 (172.18.32.37)  164.892 ms   286.937 ms   718.885 ms 
 5  172.17.7.105 (172.17.7.105)  179.347 ms   304.307 ms   616.026 ms 
 MPLS Label 16240 TTL=255
 6  172.17.8.173 (172.17.8.173)  180.509 ms !T2   305.315 ms !T2   616.039 ms !T2 
 MPLS Label 16240 TTL=255
 7  151.99.75.163 (151.99.75.163)  177.436 ms   299.976 ms   616.053 ms

As you can see, Hop 5 and Hop 6 are inside a mpls domain.

In this domain I think, likely, there is TTL propagation and ICMP extension is enabled too.

But, TTL=255 ? Why TTL has so high value?

Other question is:

Is possibile that 2 routers inside a mpls domain use same label , Hop 5 and Hop 6 use same label, that is 16240.

Thanks for your usefull answers!

Bye

Michael Toth · ‎02-18-2013

labels are local to a router, so the same label can be on every router

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Alessio Andreoli · ‎03-10-2013

Ciao Enrico,
As Michael said labels are locally significant to a router and there is no requirement for having their values unique. However, if you want to better distinguish the hop you are talking about you can define the range of labels that can be assigned:

R2#
Conf t
MPLS label range 2000 2999
End
!

In this case you would tell to R2 it can assign only labels between the values of 2000 and 2999. In a trace you would see that when a packet traverses R2 an outer label value of 2xyz is assigned making clear to you where that packet is in that moment. Clearly this would speed up verifications in most of the cases including MPLS-TE.

Hth

Alessio

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Fredrik Lönnman · ‎03-10-2013

The TTL=255 seems to indicate that ttl propagation is disabled.