From: Howard C. Berkowitz (hcb@gettcomm.com)
Date: Sun Dec 05 2004 - 11:54:54 GMT-3
At 10:57 AM +0800 12/5/04, David Duncon wrote:
>Hello Group ,
>
>I got there questions regarding Redistribution and your feed back is
>much appreciated :-)
>
>1) What is the best method to identify that whether or not your
>redistribution is going wrong particularly when you are doing
>multipoint redistribution between same protocols while manipulating
>distances for certain prefixes (owned by specific protocols) so that
>they are moving on desired direction.
There's no one method, but let me offer some tips.
1. There is no such thing as redistribution between routing protocols.
There is, however, a mechanism that allows you to install routes into
one protocol, which have been installed in the routing table by
another routing protocol. If the route isn't in the routing table,
and internally marked as coming from the protocol you are
"redistributing", the second protocol will never know about it.
"Redistribute" is a very misleading term.
router ospf 1
redistribute rip
really means "accept, from the main routing table, all routes that
were installed by RIP, and make them available to the OSPF path
determination process. Using accept/advertise, or import/export,
tends to make things much more clear than redistribute.
2. Understand fully the underlying reasons for split horizon.
Split horizon means that you never export to a source of routing
information something you originally learned from that same source.
The principle is broader than its specific application in distance
vector protocols.
When redistributing, you must ask yourself at every point of
redistribution, "am I protected from advertising back, through this
point, something I originally learned from the source of information
behind this point?" When I say "hehind this point", in a scenario
with multiple points of redistribution between RIP and OSPF, it's not
enough for OSPF to know that it didn't learn a piece of RIP
information at that point. You must know that OSPF doesn't learn
that piece of RIP information from _any_ point before you can
advertise it back into OSPF.
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