bgp route selection

[Balajee Muggalla]

Hi,

   I have a question on bgp route selection. If weight and local pref are same, then bgp would prefer the following as per text book.

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3          The routes are evaluated based on the origination, with preference to the path that was sourced locally on the router. Here is the complete order with decreasing preference: default-originate (configured per neighbor), default-information originate (config- ured per address family), network, redistribute, aggregate-address.

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Let us assume that R1 receives a default route from R2 and R3. R2 generates default route with the command "default-originate"(per-neighbor command) whereas R3 generates using  default-information originate (config- ured per address family). Receiving router R1 how does it differentiate those two routes? Any special attributes are defined?

Thanks,

Balajee

[Giuseppe Larosa · Accepted solution]

Hello Balajee,

in your example R1 cannot know how R2 and R3 have generated a defaul route. It receives a default route from both and it choices one as BGP best path.

The origin code BGP attribute can discriminate between origin IGP ( network command) and origin ? (result  of redistribution) with the first preferred.

But in your example no info about the way R2 or R3 generates a default route can be seen in BGP origin code.

The sentence that you have quoted provides the order of preference among all the possible methods of locally generating a BGP route: for example network command is preferred over redistribution and redistribution is preferred over an aggregate-address.

This is important  to solve cases where configuration leads to competition between different methods of generating a BGP route in local node, for example between a network command and redistribute connected.

Hope to help

Giuseppe