BGP decision process - Local origination

Unanswered Question
Feb 12th, 2010

Hello

I'm trying to figure out the preference of locally generated routes. In BGP Design and Implementation by Randy Zhang pg25 says:


"Here is the complete order with decreasing preference: default-originate (configured per neighbor) default-information originate (configured per address family), network, redistribute, aggregate-address."

I understand how a router injected by network is more preferred over redistribute command, but have a hard time understanding the preference of other injection methods. Can somebody give me an example or clarify?

Thank you in advance.

Tom

I have this problem too.
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Giuseppe Larosa Fri, 02/12/2010 - 13:07

Hello Tom,

this is somewhat similar to what happens with origin code.

the network command provides routes with an origin code of i internal

redistributed routes provide an origin  code of ?.

Aggregate-address origine code depends from configuration and from codes of component routes.

Going back to your question:

Both redistributing and using aggregate-address creates locally originated routes but with some loss of details.

for aggregate-address the original AS path of each single component route is lost so it is information that lacks some information in comparison to original data structures.

This may explain also the hierarchy in locally generated routes.

To be noted that first two neigh default-originate and default-information originate comes into play only when it is the case of deciding what default route to pick up to send to a neighbor:

first command neigh .. default-originate wins for being more specific then second

default-information originate should win over using network 0.0.0.0  (that is a possible way to generate a BGP default route if a default route exists in the local node IP routing table).

Hope to help

Giuseppe

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