IBGP Question --- Router Reflector or iBGP Mesh

Iljitsch van Beijnum iljitsch at muada.com
Wed Jan 12 11:20:22 UTC 2005

On 12-jan-05, at 9:06, Alexei Roudnev wrote:

> Are you sure? RR should just distribute routes.

> RR do not make any route decisions, and (btw) iBGP do not make route
> decisions - they are mostly based on IGP routing.

Route reflectors only propagate their idea of the best route for a 
destination. If this decision is based on eBGP-learned information such 
as the AS path this doesn't matter because all routers in the AS would 
make the same decision anyway, but if the IGP metric comes into play 
(which invariably happens in large networks) then all the reflector 
clients only see the route that is best based on the IGP metrics the 
reflector sees.

(Obviously the IGP metric will be different at the client, but the 
client doesn't see the other routes, so it can't make a different 
decision. The real fun starts when the next (intra-AS) hop isn't a 
reflector client and the packet now takes a different path than the 
reflector client thought it would take.)

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