IP (routed) interworking in MPLS

Unanswered Question
Mar 13th, 2008

When IP payload is taken out from one AC in IP (routed) interworking MPLS, is TTL touched or its is preserved !?

I have this problem too.
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ric_asomgdata Mon, 03/17/2008 - 04:52

Thanx Mikael

This document speakes @ Qos and EXP bits. I wanted to know whether TTL is touched upon or not.

mheusing Tue, 03/18/2008 - 02:17

Hi,

"It depends" :-)

What does it depend on? Your setting on the edge LSR first inserting MPLS labels determines the subsequent behavior.

The behavior can be configured with "[no] mpls ip propagate-ttl [local|forwarded]"

http://www.cisco.com/en/US/docs/ios/12_3/switch/command/reference/swi_m2.html#wp1058956

Default behavior of an edge LSR: copy the TTL value of the processed IP packet into the TTL field of all imposed labels.

Behavior, if the labels are removed: take the smaller value of label and IP TTL field.

This way, by default, a traceroute will show the same sequence of hops as in a pure IP case. In other words: Turning on MPLS in an IP network will not change the order/number of hops seen from a traceroute command.

Hope this helps! Please use the rating system.

Regards, Martin

ric_asomgdata Mon, 03/24/2008 - 20:21

Thanx Martin,

My question is on L2 Interworking i.e. Ethernet to PPP.Is TTL reduced when IP Packet is taken out from one AC(Ethernet) and switched to other AC (PPP)?

Hope my question is clear to u

mheusing Tue, 03/25/2008 - 10:14

Hi,

L2 does not touch IP, which is only payload.

Regards, Martin

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