Information Technology Reference
In-Depth Information
internal (iBGP). The origin attribute is received from the source BGP router. There are
three types:
IGP:
Indicated by an
i
in the BGP table. Present when the route is learned by way of
the
network
statement.
■
EGP:
Indicated by an
e
in the BGP table. Learned from EGP.
■
Incomplete:
Indicated by a question mark (?) in the BGP table. Learned from redis-
tribution of the route.
■
In terms of choosing a route based on origin, BGP prefers routes that have been verified
by an IGP over routes that have been learned from EGP peers, and BGP prefers routes
learned from eBGP peers over incomplete paths.
Autonomous System Path Attribute
The autonomous system path is a well-known mandatory attribute that contains a list of
ASNs in the path to the destination. Each autonomous system prepends its own ASN to
the autonomous system path. The autonomous system path describes all the autonomous
systems a packet would have to travel to reach the destination IP network. It is used to en-
sure that the path is loop free. When the autonomous system path attribute is used to se-
lect a path, the route with the fewest autonomous system hops is preferred. In the case of
a tie, other attributes, such as MED, break the tie. Example 12-1 shows the autonomous
system path for network 200.50.32.0/19. To reach the destination, a packet must pass au-
tonomous systems 3561, 7004, and 7418. The command
show ip bgp 200.50.32.0
dis-
plays the autonomous system path information.
Example 12-1
Autonomous System Path Attribute
Router#
show ip bgp 200.50.32.0
BGP routing table entry for 200.50.32.0/19, version 93313535
Paths: (1 available, best #1)
Not advertised to any peer
3561 7004 7418
206.24.241.181 (metric 490201) from 165.117.1.219 (165.117.1.219)
Origin IGP, metric 4294967294, localpref 100, valid, internal, best
Community: 2548:182 2548:337 2548:666 3706:153
MED Attribute
The MED attribute, also known as a metric, tells an external BGP peer the preferred path
into the autonomous system when multiple paths into the same autonomous system exist.
In other words, MED influences which one of many paths a neighboring autonomous sys-
tem uses to reach destinations within the autonomous system. It is an optional nontransi-
tive attribute carried in eBGP updates. The MED attribute is not used with iBGP peers.
The lowest MED value is preferred, and the default value is 0. Paths received with no
MED are assigned a MED of 0. The MED is carried into an autonomous
not leave the autonomous system.
