The MPLS-OPS Archive

[Umit.Inan@alcatel.com.tr]

Hello :

"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"

 RFC 1771 & 2858 states that when a BGP speaker advertises the route to an
internal peer, the
 advertising speaker should not modify the next hop information
associated with the route.

As far as I understand from this statement  for all routes learned from P1
next-hop should be P1 instead of R3
from R1 and R4 point of view,as long as next-hop self command is not
practiced at R3 .

Please let me know  if I am wrong.

Best Regards ,





Rajiv Asati <rajiva@cisco.com> on 05/20/2002 08:01:47 AM
                                                              
                                                              
                                                              
  To:          Christopher Poh <tvpoh@essex.ac.uk>            
                                                              
  cc:          mpls-ops@mplsrc.com(bcc: Umit INAN/TR/ALCATEL) 
                                                              
                                                              
                                                              
  Subject      Re: [MPLS-OPS]: AS Border Router               
  :                                                           
                                                              





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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