Best Path Selection Criteria - Dell Z9500 Configuration Manual

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Best Path Selection Criteria

Paths for active routes are grouped in ascending order according to their neighboring external AS number (BGP best path

selection is deterministic by default, which means the bgp non-deterministic-med command is NOT applied).

The best path in each group is selected based on specific criteria. Only one "best path" is selected at a time. If any of the criteria

results in more than one path, BGP moves on to the next option in the list. For example, two paths may have the same weights,

but different local preferences. BGP sees that the Weight criteria results in two potential "best paths" and moves to local

preference to reduce the options. If a number of best paths is determined, this selection criteria is applied to group's best to

determine the ultimate best path.

In non-deterministic mode (the bgp non-deterministic-med command is applied), paths are compared in the order in

which they arrive. This method can lead to Dell Networking OS choosing different best paths from a set of paths, depending on

the order in which they were received from the neighbors because MED may or may not get compared between the adjacent

paths. In deterministic mode, Dell Networking OS compares MED between the adjacent paths within an AS group because all

paths in the AS group are from the same AS.

The bgp bestpath as-path multipath-relax command is disabled by default, preventing BGP from load-

balancing a learned route across two or more eBGP peers. To enable load-balancing across different eBGP peers, enable

the bgp bestpath as-path multipath-relax command. A system error results if you configure the bgp bestpath

as-path ignore command and the bgp bestpath as-path multipath-relax command at the same time. Only

enable one command at a time.

The following illustration shows that the decisions BGP goes through to select the best path. The list following the illustration

details the path selection criteria.

Figure 20. BGP Best Path Selection

Border Gateway Protocol IPv4 (BGPv4)