BGP Best path Selection Process

Answered Question
Apr 10th, 2009

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

I have this problem too.
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Correct Answer by Giuseppe Larosa about 7 years 8 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

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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