The MPLS WG Archive

[Nagendra Singh Tomar <nagendras@netbrahma.com>]

Hi!
   I cannot understand why an LSR (downstream) will need to withdraw  a
label mapping for a particular FEC from its upstream peer and send a new
mapping for the same FEC this time with a different label
(withdraw-and-map message). It can as well use the old (incoming) label
for whatever purpose it's planning to use the new label.
The only reason can be setting up of multiple equal-cost paths, which the
RFC says is not supported now.
I would like to know if there are any other implications.
 
-- tomar


On Tue, 29 May 2001, Chris Kuszmaul wrote:

> 
>  Before a label can be mapped to an FEC that has already been mapped, the LSR
> must release the old label (prompted perhaps by a withdrawal message). Thus, LDP
> calls for a break-before-make on LSP updates.  It would seem a straightforward
> matter for the protocol to include a withdraw-and-map message that performs this
> sequence atomically, and so preserves a make-before-break condition.
> 
>   Is there some reason for not providing for this kind of functionality?
> 
>  Thanks for any comments.
> 
> CLK
> 
> PS: The careful reader will realize that I have mentioned this issue before in
> the thread [Re: Receive Label Receive !!], but have not seen any reply. This is
> an attempt to focus the question and broaden the pool of possible respondents.
>