The MPLS WG Archive

[Jean Philippe Vasseur <jvasseur@cisco.com>]

At 11:19 28/02/2002 -0500, Sachin Kalra wrote:
>Snigdho:
>
>Please find my comments in-line.
>
>At 11:04 AM 2/27/02 -0800, Snigdho Bardalai wrote:
>>Hi,
>>
>>Could you please help me understand how the NNHOP
>>Bypass tunnel works as described in Secion 6.5.
>>
>>Consider the network below (from Section 1.2):
>>
>>                 [R8]
>>                     \
>>               [R1]---[R2]----[R3]----[R4]---[R5]
>>                          \\          //   \
>>                           [R6]===[R7]     [R9]
>>
>>Say, labels available at R2 are L1 for the working
>>tunnel from R3 and L2 for the bypass tunnel from R6.
>>
>>Again, labels available at R3 are L3 for the working
>>tunnel from R4.
>>
>>Now when a packet is switched from the working tunnel
>>onto the bypass tunnel at R2 the label stack (as
>>described) will be L2->L1.
>
><SK> No, this is incorrect. when switching a packet from working tunnel 
>onto bypass tunnel at R2, the label stack will be L2->L3. So, R7 will pop 
>up L2 and R4 will get L3.

More precisely R7 will pop the top label which will not be L2 (as the top 
label would have been changed when received by R7) and R4 will receive L3 
as expected.

JP.

>This is done using "Label recording" in the RRO "Record route object" when 
>Primary LSP ("working tunnel": your terminology) is getting setup. Thus, 
>R2 will know that R4 expects L3, so any Bypass terminating at R4 should 
>have L3 as the top label.
>
>
>>When the packet leaves R7 (the penultimate hop) the
>>label stack is L1.
>>
>>At R4 it is expecting to L3, whereas it sees L1, won't
>>that be an error ?
>>
>>Would appreciate a reply,
>>Thanks,
>>Snigdho
>>
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>
>
>Regards,
>Sachin Kalra