BGP Best path Selection Process

Answered Question
Apr 10th, 2009
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Hi


I have some confusion in BGP Best path selection process. As per order,


1. WEIGHT

2. LOC Pref

3. Locally generated path via Network/redistribution command

4. AS-PATH

5. Lowest Origin


Here i am confused between 3rd and 5th Point. For me Both seems to be same.


Because Point 3 states that, perfer Network or redistribution commands ( ORIGIN=i for network ORIGIN=? for redistribution)


Again, 5th point states the lowest origin for path (ie IGP<EGP<incomplete)


Actually where i am going wrong. Please explain


R.B.Kumar



Correct Answer by Giuseppe Larosa about 8 years 3 months ago

Hello R.B.Kumar,


the origin code is attached to the advertisement and travels with it.


So even if it is true that a network statement means origin code i and a redistribution means origin code ? also non locally generated routes have the origin code.


So point 3 says: if there is a BGP path locally generated prefer it over the other possible choices.

Cisco implementation does this by providing a Cisco weight of 32,768 to locally generated routes.

The BGP next-hop is set to 0.0.0.0 as seen by the local node.


Point 5 says: if multiple paths with same parameters for points 1-4 exist choice the one with the lowest origin code i < e < ?


So they are different.


Edit:

see the BGP selection process as a sequence of checks with a strict hierarchy :

if at step N there is a single winner path there is no need to go on.


Hope to help

Giuseppe


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Correct Answer
Giuseppe Larosa Fri, 04/10/2009 - 08:00
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Hello R.B.Kumar,


the origin code is attached to the advertisement and travels with it.


So even if it is true that a network statement means origin code i and a redistribution means origin code ? also non locally generated routes have the origin code.


So point 3 says: if there is a BGP path locally generated prefer it over the other possible choices.

Cisco implementation does this by providing a Cisco weight of 32,768 to locally generated routes.

The BGP next-hop is set to 0.0.0.0 as seen by the local node.


Point 5 says: if multiple paths with same parameters for points 1-4 exist choice the one with the lowest origin code i < e < ?


So they are different.


Edit:

see the BGP selection process as a sequence of checks with a strict hierarchy :

if at step N there is a single winner path there is no need to go on.


Hope to help

Giuseppe


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