The MPLS-OPS Archive

[Rajiv Asati <rajiva@cisco.com>]

Christopher,

In a MPLS network, an ASBR, which is learning ipv4 routes via an eBGP 
neighbor, typically doesn't allocate/advertise any label for those ipv4 
routes that are advertised to its iBGP neighbors.

         <-------AS#1-------->        <-----AS#2.--->
         R1-------R2--------R3--------------P1---------------P2
                 |
                 R4

So in the above topology, R3 may be learning 10,000 ipv4 routes from P1 
(which is an eBGP neighbor) and may advertise those 10,000 ipv4 routes to 
R1 and R4 (R3's iBGP neighbors). From R1 and R4 perspective, R3 is the 
next-hop for all those 10,000 routes. So, R1 will have a label to reach R3 
and it will use that single label to reach all those 10,000 routes.
Similarly, R4 will also have a label to reach R3 and that label will also 
be applied to any of those 10,000 prefixes.

As far as peering to all the internal routes are concerned, MPLS provides 
the ability to make core routers BGP free. In other words, R2 above doesn't 
need to be an iBGP neighbor and so it doesn't need to know about 10,000 
prefixes.

Please let me know if I could be of more help.

Rajiv

At 09:13 PM 5/19/2002, Christopher Poh wrote:
>Hi,
>
>How does an AS border router advertise and make known to all other routers 
>within a domain about label/FEC binding information for every exterior 
>routes it
>carry? Does the router need to peer with all the interior routers in order 
>to distribute this information?
>
>Hope someone could help me with this.
>
>
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