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bgp route selection

Balajee Muggalla
Beginner
Beginner

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



1 Accepted Solution

Accepted Solutions

Giuseppe Larosa
Hall of Fame Master Hall of Fame Master
Hall of Fame Master

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

View solution in original post

1 Reply 1

Giuseppe Larosa
Hall of Fame Master Hall of Fame Master
Hall of Fame Master

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

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