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

[Greg Mirsky <[e-mail]>]

Hi Wenhu,
I think that you are overlooking the fact that B advertises at LSInfinity
its ALL links, including to PE2 in your diagram. Thus, as I understand OSPF,
there's no way A will select B for its path to PE2. Yes, you've found
another way "to slice the cake" but, as I've mentioned, the main issue is
when to advertise the real IGP cost being handled by both documents
identically.

Regards,
Greg

On Mon, Mar 1, 2010 at 4:15 PM, Wenhu Lu <[e-mail]> wrote:

>  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
>