Re: BGP4 questions

[John Krawczyk <[e-mail]>]

   On Thu, 25 July, Cristina Radulescu-Banu ([e-mail]) wrote:

   
   	Hi. I have 2 questions:
   
   	1. What does this means:
   
   	A BGP speaker needs to be able to support disabling
   advertisement of external border routers. (RFC 1771, page 22)
   


   	2. Did someone actually implement this?

Cristina,

I had to look at the spec to put this into context.  This is in the
section talking about the NEXT_HOP path attribute, and concerns what is
sometimes referred to as "third party" advertisement.  That is, if
routers A, B, and C are all on the same multi-access network, A can
advertise B's NLRI to C using "B" as the NEXT_HOP, so as to avoid
sending packets out onto the same subnet twice (C->B instead of
C->A->B).  This is the default behavior in our implementation.

I can think of two interpretations for "disabling" this:

1) Have A use its own address as NEXT_HOP.  This would be necessary over
an NBMA network where there is no direct connectivity between B and C.
I can't think of any other good reason for doing it (that doesn't mean
that there aren't other good reasons... :).

2) Prevent A from advertising B's routes to C.  You might want to do
this to save memory, but you are also sacrificing a backup mechanism for
forwarding from C to B.

You can do all of this in our implementation through BGP policies.  It's
not as easy as just turning a knob to say "don't do that".

-jj