Users Guide

Multiexit discriminators
If two autonomous systems connect in more than one place, use a multiexit discriminator (MED) to assign a preference to a
preferred path. MED is one of the criteria used to determine best path—other criteria may also impact selection.
One AS assigns the MED a value. Other AS uses that value to decide the preferred path. Assume that the MED is the only
attribute applied and there are two connections between AS 100 and AS 200. Each connection is a BGP session. AS 200 sets
the MED for its Link 1 exit point to 100 and the MED for its Link 2 exit point to 50. This sets up a path preference through Link
2. The MEDs advertise to AS 100 routers so they know which is the preferred path.
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 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BGP local RIB : Routes to be Added , Replaced , Withdrawn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 that are 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 that is 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.
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Layer 3