Re: [mpls] working group last call on draft-ietf-mpls-ldp-igp-sync-bcast-00.txt

[Wenhu Lu <[e-mail]>]

Hi Greg,
"I don't think that A has or can to do anything." - that's exactly my point. i.e. A cannot manipulate its cost to favor C or B.
Now B can't either !
If you increase B's cost (to the LAN) to LSInfinity, it still doesn't change A's decision.
"A->B->PE2" is still shorter than "A->C->D->PE2".

The traffic thus will be directed to B, hence the traffic loss.

Regards,
-wenhu

________________________________
From: Greg Mirsky [mailto:[e-mail]]
Sent: Monday, March 01, 2010 3:32 PM
To: Wenhu Lu
Cc: [e-mail]
Subject: Re: [mpls] working group last call on draft-ietf-mpls-ldp-igp-sync-bcast-00.txt

Hi Wenhu,
I've snipped text to leave only your question for further discussion.



[luw] Let me ask a simple question. In the following diagram, let's assume the link cost is 10 everywhere.

                          |
                          |    +---+           +---+
                          |----| B |-----------|PE2|
                          |    +---+           +---+
        +---+    +---+    |                      |
        |PE1|----| A |----|                      |
        +---+    +---+    |                      |
                          |    +---+    +---+    |
                          |----| C |----| D |----+
                          |    +---+    +---+

 Originally the primary LSP from PE1 to PE2 is "PE1-A-B-PE2". The backup LSP is "PE1-A-C-D-PE2".
Now after B's link to the broadcast network (that connects A, B, and C) restores to UP from being DOWN,
"A" wants the traffic continues to flow through the backup path "PE1-A-C-D-PE2" for a while, so that it gains
time to recover the primary LSP(s).
With the RFC5443 method, could you let me know what "A" should do to keep traffic from flowing through "B"?

Regards,
-wenhu

I don't think that A has or can to do anything. Per RFC 5443, as well as per your work, it is responsibility of up-and-coming node to try not to disturb existing topology. The node B can, per RFC 5443, advertise its link to the broadcast network (DR in OSPF) with LSInfinity (I'd prefer divide it in half but that's me). The real problem, and RFC 5443 clearly acknowledges this, is when the node B can advertise the real IGP cost (irrespective of type of its IGP interface). Yes, the delay timer is a workaround and sort of a "black magic". The LDP End-of-Lib, as both documents point out, is better and more appropriate trigger.

Regards,
Greg