MPLS TTL propagation of FRR

oldcreek12 · ‎02-21-2008

Hi, all,

I am wondering what is default behavior of MPLS TTL propagation in PLR when traffic is diverted to by pass tunnel and an extra label is imposed? is inner MPLS label TTL copied to TE label TTL? and what is the behavior on PHP (or MP if explicit null is advertised), is TE label TTL copied back to original LSP label TTL?

ebreniz · ‎02-27-2008

You must find the link that has the lower mtu and adjust it (raise the mtu).You may have to enable ttl propagation through the routers and then traceroute to find the what the hops are-then go to each hop and see which link has the lower mtu.

http://www.cisco.com/en/US/tech/tk436/tk428/technologies_tech_note09186a008020a42a.shtml

In some cases, the PHP can expose the IP packet to the penultimate hop. This happens when an MPLS packet arriving at the penultimate hop has only one label. In this case, the penultimate LSR and the edge LSR do not have access to the EXP value that the packet carried before the MPLS header was removed. To preserve the EXP value in this case, the edge LSR needs to advertise an explicit NULL label (a label value of zero).

http://www.cisco.com/en/US/tech/tk828/technologies_q_and_a_item09186a00800a43f5.shtml#wp15329