Dell MX9116n EMC SmartFabric OS10 User Guide Release 10.5.0 - Page 558

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, 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 simultaneously, an error message displays-only enable one command at a time. More path support More path (Add-Path) reduces convergence times by advertising multiple paths to its peers for the same address prefix without replacing existing paths with new ones. By default, a BGP speaker advertises only the best path to its peers for a given address prefix. If the best path becomes unavailable, the BGP speaker withdraws its path from its local router information base (RIB) and recalculates a new best path. This situation requires both IGP and BGP convergence and is a lengthy process. BGP add-path also helps switch over to the next new best path when the current best path is unavailable. The Add-Path capability to advertise more paths is supported only on IBGP peers-it is not supported on EBGP peers or BGP peer groups. Ignore router ID calculations Avoid unnecessary BGP best path transitions between external paths under certain conditions. The bestpath router-id ignore command reduces network disruption that is caused by routing and forwarding plane changes and allows for faster convergence. Advertise cost As the default process for redistributed routes, OS10 supports IGP cost as MED. Both autosummarization and synchronization are disabled by default. BGPv4 and BGPv6 support 558 Layer 3