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RE: ebgp->ebgp->ibgp but what about the locals? posted 10/09/2002
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Richard,

I think what may be confusing you here is your terminology. Basically you
have the following scenarios:

1. A route learnt via an EBGP peer is advertised to all IBGP peers
2. A route learnt via an IBGP peer is not advertised to other IBGP peers but
is advertised to EBGP peers.
3. A route injected locally is advertised to all IBGP peers as IT IS NOT
learnt via IBGP. (this is the reason)
4. The exceptions to this rule are of course route reflectors

When you inject the route locally (redistribute, network statement, etc)
then no other IBGP peer would be advertising this route to the local router,
hence it is OK for the local router to advertise it via IBGP to all other
IBGP peers in that AS. This is why a full mesh is required in IBGP (or route
reflectors or confeds) - take for example the following:

R1		R2
AS100 --------    AS100
\                         /
 \		/
  \	         /
   \	      /
	R3
	AS100

If a locally generated route is configured on R1, R1 will send this to R2
and R3. R2 and R3, however will not send this to each other respectively as
they have learnt about it from an IBGP neighbour. It is OK, however for R1
to send this as R1 has not learnt this via IBGP, but has learnt about it
from another method.

If we remove the link from R1 to R3, R3 will never learn about the locally
generated R1 route unless R2 is configured as a route reflector. This is
because this would break the IBGP split horizon rule. (which states that an
IBGP router may not advertise a route learnt via IBGP to any other IBGP
router)

Let me know if this makes sense, and if not I will try and clarify it some
more.

Jason Sinclair CCIE #9100
Manager, Network Control Centre
POWERTEL
55 Clarence Street, 
SYDNEY NSW 2000 
AUSTRALIA
office: + 61 2 8264 3820
mobile: + 61 416 105 858
email: sinclairj@xxxxxxxxxxxxxxx

 -----Original Message-----
From: 	Richard Davidson [mailto:rich@xxxxxxxxxxxxxx] 
Sent:	Tuesday, 8 October 2002 13:23
To:	groupstudy
Subject:	ebgp->ebgp->ibgp but what about the locals?

ebgp learned routes are sent to ibgp peer but ibgp learned routes are never
sent to ibgp peers.  That makes sense but what happens to locally generated
routes via the network statement.   They seem to be sent to all ibgp peers.
If this is true, can some one point me to some documentation that explains
what happens to the localy generated routes and how it is sent between ibgp
peers?


Rich


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