The MPLS WG Archive

[Sachin Kalra <skalra@opnet.com>]

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. 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