The MPLS WG Archive

["Igor Achkinazi" <achkinazi@hotmail.com>]

To avoid confusion I want to rephrase my question and clarify the scenario I 
have a question about.

Let’s think about path established using LDP, and a network administrator 
sending MPLS trace-route Echo request from ingress. This MPLS Ping packet 
ends up at some LSR A in the middle of MPLS cloud:

                 B ----------> (after topology change)
                /
-------------> A
                \
                 C ----------> (before topology change)

When MPLS trace-route Echo request (not regular MPLS ping) comes to LSR A 
before topology changes, LSR A reports C as a downstream router in 
Downstream Mapping TLV. Immediately after that IGP reroute causes LDP to 
reconfigure it’s downstream information on LSR A. When ingress sends a new 
MPLS Echo request with TTL incremented LSR A forwards it to B, but MPLS Echo 
request has old Downstream Mapping TLV reported by A before, which has label 
and IP address from C. So B will report FEC/Label mismatch.

I cannot find how the draft covers such scenario

Thank you,
Igor Achkinazi




>From: "Igor Achkinazi" <achkinazi@hotmail.com>
>To: mpls@UU.NET
>Subject: MPLS Ping question
>Date: Mon, 05 Apr 2004 18:42:02 +0000
>
>Hi,
>
>There were several discussions about MPLS Ping already, but I could not 
>find the answer to following question about trace-route mode: assume that 
>just before reroute some midpoint LSR X responds with its current 
>downstream information (label, interface, hashing addresses), so MPLS Ping 
>initiator will build a packet meant to go to the old downstream in respect 
>to LSR X. However at the time LSR X gets new MPLS Ping packet (with TTL 
>incremented by one), its hardware will be configured with a new forwarding 
>information and the packet will be forwarded to a new downstream router 
>that will complain about FEC/Label mismatch.
>
>Since trace-route fails because of topology change, it should be repeated 
>again. But network operator sending MPLS Ping from ingress router might not 
>know that some LSR along the way just did a reroute, so he would assume 
>that the LSP is broken. How the operator can differentiate between topology 
>changes and broken LSP, since LDP topology changes are not visible to him?
>
>Thank you,
>- Igor Achkinazi
>

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