Border Gateway Protocol - CCIE Routing and Switching Exam Certification Guide - page 442

The show ip bgp 200.50.32.0 Output Example 9-9

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

The MED attribute, also known as the metric, tells external BGP peers the preferred path into
the AS when there are multiple paths into the AS. In other words, MED influences which one
of many paths a neighboring AS uses to reach destinations within the AS. It is an optional,
nontransitive attribute carried in eBGP updates. You do not use the MED attribute with iBGP
peers. The lowest MED value is preferred and the default value is 0. The MED is not compared
for paths received from two separated autonomous systems.

Consider the diagram in Figure 9-7. With all attributes considered equal, Router C selects
Router A as its best path into AS 100, which is based on Router A's lower RID. The configura-
tions below assign a higher MED to Router A to make Router C select Router B as the best path
to AS 100. No additional configuration is required on Router B because the default MED is 0.

MED Attribute

Figure 9-7

AS 200

Router C
2.2.2.1

Router A
1.1.1.1
MED = 200

Router B
1.2.1.1
MED = 0

AS 100

The configuration of Router A from Figure 9-7 is shown in Example 9-10. A route map is
configured on Router A for neighbor 2.1.1.1 (Router C) to set the metric to 200 for advertised
routes.