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[mpls] One LDP Implementation specific question of receive labelmapping for prefix FECs

  • From: Eric W Gray <ewgray2k@netscape.net>
  • Date: Tue, 11 Apr 2006 07:02:00 -0400
  • Cc: mpls@ietf.org
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Title:
The implementation MUST do an exact match, as downstream LSRs will forward based
only on the label.  Because they do not (typically) perform a route look-up on the label
encapsulated inner IP packet, there is no reason to expect a downstream LSR to be able
to separate a flow based on longest match when the routes further downstream are distinct
for "longer matches".

Dutta, Pranjal wrote:
Hi Eric,
          As per RFC 3036 Label mapping receive procedures, when Ru
received a label mapping from Rd for a FEC x, Ru need to find an "exact"
match of the FEC x(IPv4/V6 prefix) in its route table. In Inter-IGP-area
or inter-AS case the routes from Rd would be summarized to Ru and in
such a case Ru MAY not find the exact match for such FEC x. In terms of
RFC 3036 then Ru MAY not find exact match for the FEC received from Rd
in its route table and no further action at Ru. My question was why Ru
can't we do a longest prefix match for the FEC x as LPM means it can be
reached by a downstream Rd? 

Thanks,
Pranjal

-----Original Message-----
From: Eric Rosen [mailto:erosen@cisco.com] 
Sent: Monday, 10 April, 2006 11:39 PM
To: Dutta, Pranjal
Cc: mpls@ietf.org
Subject: Re: [mpls] One LDP Implementation specific question of receive
label mapping for prefix FECs 


  
When Ru receives a label mapping for a FEC of type IPv4/v6 prefix from
    
Rd,
  
the next hop  for the FEC can be determined after  route lookup. Now
    
here,
  
for looking  up the  FEC at Ru  do we  need to do  exact match  or
    
Longest
  
prefix match? It is quite possible that the prefix as FEC mapped by Rd
    
may
  
be within a supernetted prefix at Ru. 
    

It's a  little hard  to understand what  you're asking.  By  definition,
one
cannot change the FEC in mid-LSP.  If  the FEC is an address prefix, and
you
reach a  point where that address prefix  is not in the  routing table,
then
the LSP terminates. 



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