Brocade Virtual ADX Switch and Router Guide (Supporting ADX v03.1.00) User Manual
Page 226
212
Brocade Virtual ADX Switch and Router Guide
53-1003246-01
Configuring the device to always compare Multi-Exit Discriminators
8
The attribute-map map-name parameter configures the device to set attributes for the aggregate
routes based on the specified route map.
NOTE
For the suppress-map, advertise-map, and attribute-map parameters, the route map must already
be defined. Refer to “
on page 252 for information on defining a route map.
Configuring the device to always compare
Multi-Exit Discriminators
A Multi-Exit Discriminator (MED) is a value that the BGP4 algorithm uses when it compares multiple
paths received from different BGP4 neighbors in the same AS for the same route. In BGP4, a MED
for a route is equivalent to its metric.
BGP4 compares the MEDs of two otherwise equivalent paths if and only if the routes were learned
from the same neighboring AS. This behavior is called deterministic MED. Deterministic MED is
always enabled and cannot be disabled.
You can enable the device to always compare the MEDs, regardless of the AS information in the
paths. For example, if the device receives UPDATES for the same route from neighbors in three ASs,
the device can compare the MEDs of all the paths together instead of comparing the MEDs for the
paths in each AS individually.
To enable this comparison, enter the always-compare-med command at the BGP4 configuration
level of the CLI. This option is disabled by default.
By default, value 0 (most favorable) is used in MED comparison when the MED attribute is not
present. The default MED comparison results in the device favoring route paths that do not have
their MEDs. Use the med-missing-as-worst command to force the device to regard a BGP4 route
with a missing MED attribute as the least favorable route.
NOTE
MED comparison is not performed for internal routes originated within the local AS unless the
compare-med-empty-aspath command is configured.
To configure the device to always compare MEDs, enter the following command.
Virtual ADX(config-bgp)# always-compare-med
Syntax: [no] always-compare-med
The following BGP4 command directs BGP4 to take the MED value into consideration even if the
route has an empty as-path path attribute.
Virtual ADX(config)# router bgp
Virtual ADX(config-bgp-router)# compare-med-empty-aspath
Syntax: [no] compare-med-empty-aspath