Users Guide

For EBGP neighbors, the next-hop address corresponding to a BGP route does not resolve if the next-hop address is not the
same as the neighbor IP address. The next-hop attribute also serves as a way to direct traffic to another BGP speaker, instead
of waiting for a speaker to advertise. When a next-hop BGP neighbor is unreachable, the connection to that BGP neighbor goes
down after the hold-down timer expires.
When you enable fast-external-fallover and if the router has learned the routes from the BGP neighbor, the BGP
session terminates immediately if the next-hop becomes unreachable, without waiting for the hold-down time.
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
Layer 3
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