BGP Equal cost multipath
By default, BGP does not perform load balancing. BGP will select only a single path for a prefix. You can see this from the very last attribute that is examined. If all other attributes are the same, at the very end, a BGP router will prefer the path with the lowest neighbor IP address. This is the last tiebreaker, and the neighbor IP addresses will never be the same, so one will always win out over the other.
To enable load sharing, you must enable BGP multipath load sharing using the BGP command:
maximum-paths
This will allow a BGP router to select additional paths to a destination as long as the following attributes for those paths are the same:
- Weight
- Local Preference
- AS Path (both AS number and AS path length)
- Origin code
- MED
- IGP metric (this depends upon the implementation)
Take a look at BGP - Multipath Attribute Prerequisites for more information about these prerequisites.
Take a look at the BGP Equal cost multipath configuration syntax for more information about the command.
Links:
Links to this page:
- BGP - ECMP for specific prefixes
- BGP - Multipath Attribute Prerequisites
- BGP - multipath in a DMVPN environment
- BGP Equal cost multipath configuration syntax
- BGP Equal cost multipath
- BGP Load balancing
- BGP advertises only the best path by default
- BGP dmzlink-bw feature
- BGP traffic engineering
- Routing - Using Anycast with BGP Multipath