Articles

Video 22: BGP Tuning Attributes

In BGP on April 27, 2011 by carlosfvc

What are BGP Attributes?

– The BGP metric is not simple
– Attributes are ways that you can ‘tag’ incoming or outgoing BGP routes

– Some attributes are <well-known> (everyone supports) while others are optinal

– Some attributes are manditory (must be in the update) while others are discretionary

– Some attributes are transitive (travel from router to router) while others are non-transitive)

Understanding the BGP Attribute Lineup

– Well known attributes:

Autonomous system path (as-path – mandatory)
Next hop address (mandatory)
Origin (Mandatory)
Local preference( discretionary)
Atomic aggegate (discretionary)

– Optional Attributes

Aggregator
Multi-exit discriminator (MED/METRIC)

How BGP Finds the best path

0. Ignore routes with an inaccessible next hop address

1. Prefer the path with the highest WEIGHT

2. Prefer the path with the highest LOCAL_PREF.

3. Prefer the path that was locally originated via a network

4. Prefer the path with the shortest AS_PATH.

5. Prefer the path with the lowest origin type.

6. Prefer the path with the lowest multi-exit discriminator (MED).

7. Prefer eBGP over iBGP paths.

8. Prefer the path with the lowest IGP metric to the BGP next hop.

9. Determine if multiple paths require installation in the routing table for BGP Multipath.

10. When both paths are external, prefer the path that was received first (the oldest one).

11. Prefer the route that comes from the BGP router with the lowest router ID.

12. If the originator or router ID is the same for multiple paths, prefer the path with the minimum cluster list length.

13. Prefer the path that comes from the lowest neighbor address.
Tuning BGP Attributes

Weight
As-Path
Next hop address
Origin
Local Preference
Metric

Router 1

Verify sh ip bgp sum
sh ip bgp

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