OS10 Enterprise Edition User Guide Release 10.4.1.0

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Best path selection

Best path selection selects the best route out of all paths available for each destination, and records each selected route in the IP routing table for traffic forwarding. Only valid routes are considered for best path selection. BGP compares all paths, in the order in which they arrive, and selects the best paths. Paths for active routes are grouped in ascending order according to their neighboring external AS number.

OS10 follows deterministic MED to select different best paths from a set of paths. This may depend on the order the different best paths are received from the neighbors — MED may or may not get compared between adjacent paths. BGP best path selection is deterministic by default.

The best path in each group is selected based on specific criteria—only one best path is selected at a time. If BGP receives more than one best path, it moves on to the next list of valid paths in the list, and continues until it reaches the end of the list.

When you configure the non-deterministic-med command, paths are compared in the order they arrive. OS10 follows this method to select different best paths from a set of paths, depending on the order they were received from the neighbors—MED may or may not get compared between the adjacent paths.

By default, the bestpath as-path multipath-relax command is disabled. This prevents BGP from load-balancing a learned route across two or more EBGP peers. To enable load-balancing across different EBGP peers, enter the bestpath as-path multipath-relax command.

If you configure the bgp bestpath as-path ignore command and the bestpath as-path multipath-relax command at the same time, an error message displays—only enable one command at a time.

Best path selection


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