The MPLS WG Archive

[Abhijit <gabhijit@ee.iitb.ernet.in>]

Hi all 

In draft-ietf-mpls-arch-06.txt , in section 4.1.3, the example given says
that 4.1.3. Using the Hop by Hop path as the LSP

Following lines are included from the draft
-----> 
   If the hop-by-hop path that packet P needs to follow is <R1, ...,
   Rn>, then <R1, ..., Rn> can be an LSP as long as:

      1. there is a single address prefix X, such that, for all i,
         1<=i<n, X is the longest match in Ri's routing table for P's
         destination address;

      2. for all i, 1<i<n, Ri has assigned a label to X and distributed
         that label to R[i-1].

   Note that a packet's LSP can extend only until it encounters a router
   whose forwarding tables have a longer best match address prefix for
   the packet's destination address. At that point, the LSP must end and
   the best match algorithm must be performed again.

   Suppose, for example, that packet P, with destination address
   10.2.153.178 needs to go from R1 to R2 to R3.  Suppose also that R2
   advertises address prefix 10.2/16 to R1, but R3 advertises
   10.2.153/23, 10.2.154/23, and 10.2/16 to R2.  That is, R2 is
   advertising an "aggregated route" to R1.  In this situation, packet P
   can be label Switched until it reaches R2, but since R2 has performed
   route aggregation, it must execute the best match algorithm to find
   P's FEC.          
<------------

My question is R3 in this case should not advertise prefix 10.2/16 to R2 
as when the Label Request for FEC 10.2/16 comes to R2, it
can assign a binding to it and can forward that request to R3,  wherein R3
can in turn give the binding to R2 again, but when the labelled packet
arrives at R2 it forwards it to R3 and there is no way for R3 to know now
where it should forward that packet. 

In Other words how can R2 know that it has better prefix match than
10.2/16 and thus issue a new request to R3. and stop Label Switching the
packets there? 


Thanks in advance

Abhijit