BGP Best Path Selection & Manipulation

I’ll start with manipulation. These are the most common ways based on the direction of traffic flow.

Influence Inbound Traffic Flow based on updates sent to peers:
-AS-Path Prepending
-MED
-Communities

Influence Outbound Traffic Flow based on LOCAL config:
-Weight
-Local Preference

Below is BGP’s best path selection algorithm. This is the order used to select the paths.

1. WEIGHT – path with HIGHEST weight (Cisco Proprietary)
2. LOCAL_PREF – path with the HIGHEST LOCAL_PREF # (Default = 100)
3. Locally originated via NETWORK or AGGREGATE subcommands or through REDISTRIBUTION from IGP
4. AS_PATH – path with SHORTEST AS_PATH
5. Path with the LOWEST origin type
NOTE: IGP is lower than EGP and EGP is lower than INCOMPLETE
6. MED – path the the LOWEST MED
7. Prefer eBGP over iBGP paths
8. Path with the LOWEST IGP metric to the BGP next hop

That is not all of them but you’ll never get that far hopefully.

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