The MPLS WG Archive

[Eric Rosen <erosen@cisco.com>]

> 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

If I  understand your  example, Ru has  the summary  route, Rd has  the more
specific routes, and  Rd is sending Ru label bindings  for the more specific
routes  but not  for the  summary route.   You don't  say anything  about Ru
itself  distributing  labels  bindings  for  the more  specific  routes,  so
presumably the only thing  at issue is Ru should label an  IP packet that it
receives.  What will happen is that  the IP packet gets sent unlabeled to Rd
which then labels it according to the specific route which is the best match
to the packet's IP destination address.

The label imposition procedure here is: 

- get the IP destination address
- find  the prefix  in the routing  table which  is the best  match (longest
  match) for that destination address
- use the label  which was assigned to that prefix by  the LSR which routing
  says is the next hop for that prefix. 

It  doesn't make sense  for Rd  to bind  labels to  prefixes which  are more
specific  than  the  one in  the  routing  table,  as the  label  imposition
procedure will never make use of such labels. 

I don't think we want to allow  LDP to put prefixes in the routing table, or
else we'd  have to  treat it as  a second  routing algorithm and  that would
raise all kinds of issues.

It is possible though to imagine a different imposition procedure: 

- get the IP destination address, A

- find the prefix, P, in the routing table which is the best match (longest
  match) for that destination address

- get the next hop, N, for P, as specified by routing

- if N has distributed any label bindings for prefixes which are longer than
  P but begin with the same sequence of  bits as P, and if any of these, say
  Q, is a better match for A than  P, then use the label which N assigned to
  Q. 

This is a complicated procedure, though.   You can't just match A against Q,
because there might be some prefix  R in the routing table, with a different
next hop, which is  a better match for A than P is, but  not as good a match
as Q.  The above imposition procedure  says that the next hop for the packet
is determined  by the  route to  R not  by the route  to P  (or Q).   It's a
complex matter  to keep  the forwarding tables  in proper shape,  given that
prefixes like Q can come and go asynchronously. 

There are other  complications as well.  For example, what do  you do if you
get a packet  which is labeled for Q,  but the next hop has  changed and the
new next hop has not distributed a label for Q?  

I suppose we could figure all this out if there were an actual problem which
didn't have a better solution. 






   

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