From: Howard C. Berkowitz (hcb@xxxxxxxxxxxx)
Date: Wed May 08 2002 - 20:20:21 GMT-3
At 7:25 AM +0900 5/9/02, Jenkins, Buddy wrote:
>Jeff,
>
>Refer to Doyle Volume1 Chapter 11 on redistribution. In the
>questions at the end of the chapter there is a question (I don't
>have the book with me right now so I can't tell you the specific
>question) that has the same scenario that you have. Doyle says that
>the reason for that type of behaviour is because of the rule of
>split horizon. This makes sense for you scanario below because IGRP
>would use split horizon and OSPF does not use split horizon.
>However this is the part that still confuses me. I do not know why
>split horizon is applied to a redistribution process. I always
>though split horizon was implemented to not advertise a route out an
>interface from which that route was learned. Can any of the guru's
>out there explain why split horizon is applied during mutual
>redistribution?
>
>Buddy
I don't blame you for being confused. The idea of using split horizon
in redistribution is obvious--once you understand it!
One of the first things toward understanding is that we are NOT
talking about the split horizon mechanism of any particular routing
protocol. If you look a little more abstractly about the underlying
idea of split horizon, you can describe it as a rule:
"Never announce to another router information that you originally
learned from that router, but you are stating positively to be true."
In other words, the real initiating router is closer to the
information and knows more about it than the receiving router. Think
about Dilbert's Pointy-Haired Boss lecturing him about technology.
In redistribution, the concept of split horizon is an abstraction
between routing domains. Routing domain A should never readvertise
to domain B anything it originally learned from domain B, and vice
versa.
By the act of redistribution, you are defeating the protocol-specific
split horizon mechanisms (when protocols have them), because those
mechanisms are intended to work on updates from the SAME protocol.
Between DIFFERENT protocols, you need manually to impose the LOGIC of
split horizon, using distribute lists, route maps, etc.
-- "What Problem are you trying to solve?" ***send Cisco questions to the list, so all can benefit -- not directly to me*** ******************************************************************************* * Howard C. Berkowitz hcb@gettcomm.com Chief Technology Officer, GettLab/Gett Communications http://www.gettlabs.com Technical Director, CertificationZone.com http://www.certificationzone.com "retired" Certified Cisco Systems Instructor (CID) #93005
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