Juniper JUNOSE, Configuration Manual

Page: 160 / 738

Router Dublin advertises a MED of 25 for route 192.168.33.0/24, which is
lower—more preferred—than the MED advertised by router Paris or router Nice.
However, the AS path for the route through router Dublin is longer than that through
router Paris. The AS path is the same length for router Paris and router Nice, but the
MED advertised by router Paris is lower than that advertised by router Nice.
Consequently, router London prefers the path through router Paris.
Suppose, however that router Dublin was not configured to set the MED for route
192.168.33.0/24 in its outbound route map 10. Would router London receive a MED
of 50 passed along by router Paris through router Dublin? No, because the MED
attribute is nontransitive. Router Dublin does not transmit any MED that it receives.
A MED is only of value to a direct peer.
bgp always-compare-med
■
Use to enable the comparison of the MED for paths from neighbors in different
ASs.
■
Unless you specify the bgp always-compare-med command, the router compares
MED attributes only for paths from external neighbors that are in the same AS.
■
The BGP path decision algorithm selects a lower MED value over a higher one.
■
Unlike local preferences, the MED attribute is exchanged between ASs, but does
not leave the AS.
■
The value is used for decision making within the AS only.
■
When BGP propagates a route received from outside the AS to another AS, it
removes the MED.
■
Example
host1(config-router)# bgp always-compare-med
■
Changes apply automatically whenever BGP subsequently runs the best-path
decision process for a destination prefix; that is, whenever a best route is picked
for a given prefix.
To force BGP to run the decision process on routes already received, you must
use the
clear ip bgp command to perform an inbound soft clear or hard clear
of the current BGP session.
■
Use the no version to disable the feature.
■
See bgp always-compare-med.
set metric
■
Use to set the metric value—for BGP, the MED—for a route.
■
Sets an absolute metric. You cannot use both an absolute metric and a relative
metric within the same route map sequence. Setting either metric overrides any
previously configured value.
■
Example
host1(config)# route-map nyc1 permit 10
host1(config-route-map)# set metric 10
124 ■
Selecting the Best Path
JUNOSe 11.0.x BGP and MPLS Configuration Guide