Users Guide
AS path and next-hop
The AS path is the AS list that all the prexes listed in the update have passed through. The BGP speaker adds the local AS number when
advertising to an EBGP neighbor. Any update that contains the AS path number 0 is valid.
The next-hop is the IP address used to reach the advertising router:
• For EBGP neighbors, the next-hop address is the IP address of the connection between neighbors.
• For IBGP neighbors, the EBGP next-hop address is carried into the local AS. A next hop attribute sets when a BGP speaker advertises
itself to another BGP speaker outside the local AS and when advertising routes within an AS.
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 trac 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
expiry.
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 trac 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 dierent best paths from a set of paths. This may depend on the order the dierent 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 specic 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 congure the non-deterministic-med command, paths are compared in the order they arrive. OS10 follows this method
to select dierent 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 dierent EBGP peers, enter the bestpath as-path multipath-
relax
command.
If you congure 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.
278
Layer 3