Reference Guide

Origin
The origin indicates how the prefix came into BGP. There are three origin codes—IGP, EGP, and INCOMPLETE.
IGP Prefix originated from information learned through an IGP.
EGP Prefix originated from information learned from an EGP, which Next Generation Protocol (NGP) replaced.
INCOMPLETE Prefix originated from an unknown source.
An IGP indicator means that the route was derived inside the originating AS. EGP means that a route was learned from an
external gateway protocol. An INCOMPLETE origin code results from aggregation, redistribution, or other indirect ways of
installing routes into BGP.
The question mark (?) indicates an origin code of INCOMPLETE, and the lower case letter (i) indicates an origin code of IGP.
Origin configuration
OS10# show ip bgp
BGP local RIB : Routes to be Added , Replaced , Withdrawn
BGP local router ID is 30.1.1.1
Status codes: s suppressed, S stale, d dampened, h history, * valid, > best
Path source: I - internal, a - aggregate, c - confed-external, r - redistributed
n - network S - stale
Origin codes: i - IGP, e - EGP, ? - incomplete
Network Next Hop Metric LocPrf Weight Path
*>I 1.1.1.0/24 17.1.1.2 0 0 0 i
*>I 2.2.2.0/24 17.1.1.2 0 0 0 ?
*>I 3.3.3.0/24 17.1.1.2 0 0 0 e
AS path and next-hop
The AS path is the AS list that all the prefixes 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 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 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 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.
Layer 3
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