Subject: A question about Constraint-based Routing

From: "Brian Hou" <[email-masked]>
Date: Tue, 19 Dec 2000 23:00:28

Hi:

I am new to MPLS and to this group, so please forgive me if my question has 
been answered before.

What I have learned so far about constrained based routing is that we need 
to compute an LSP at the ingress node and ensure that the path satisfies 
some constraints.  I also learn that we need a mechanism to distribute 
network topology and link attributes (such as available bandwidth) 
throughout a network so that we have enough information to perform 
constraint-based routing calculation.

My question is, when do we actually perform constraint-based routing 
calculation?  Is it at the time when a request to establish an LSP is made?  
I think so because this is when constraints can be specified, and since each 
LSP could have different constraints.  Then, as network topology or link 
state/attributes change, the path needs to be recomputed, and the MPLS 
signaling protocol should be updated.  As an LSP is torn down, we should 
also remove the constraints so that we don't perform unnecessary 
calculation.

I would appreciate it if someone can tell me if my thinking is in the 
ballpark or correct me if I am wrong.

Thanks.

Brian
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