Users Guide

MEDs are nontransitive attributes. If AS 100 sends the MED to AS 200, AS 200 does not pass it on to AS 300 or AS 400. The MED is a
locally relevant attribute to the two participating AS — AS 100 and AS 200. The MEDs advertise across both links—if a link goes down, AS
100 has connectivity to AS 300 and AS 400.
Origin
The origin indicates how the prex came into BGP. There are three origin codes—IGP, EGP, and INCOMPLETE.
IGP Prex originated from information learned through an IGP.
EGP Prex originated from information learned from an EGP, which Next Generation Protocol (NGP) replaced.
INCOMPLETE Prex 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 conguration
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BGP local RIB : Routes to be Added , Replaced , Withdrawn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 prexes 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 trac 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 trac forwarding. Only valid routes are considered for best path selection. BGP compares all paths, in the order in which they
Layer 3
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