Brocade BigIron RX Series Configuration Guide - Manual (Page 913)

BigIron RX Series Configuration Guide 835
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Configuring the device to always compare MEDs
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The advertise-map <map-name> parameter configures the router to advertise the more specific
routes in the specified route map.
The attribute-map <map-name> parameter configures the router 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 “
“Defining route maps” on page 876 for information on defining a route map.
Configuring the device to always compare MEDs
A Multi-Exit Discriminator (MED) is a value that the BGP4 algorithm uses when comparing multiple
paths received from different BGP4 neighbors in the same AS for the same route. In BGP4, a
route’s MED 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.
In addition, you can enable the device to always compare the MEDs, regardless of the AS
information in the paths. To enable this comparison, enter the always-compare-med command at
the BGP4 configuration level of the CLI. This option is disabled by default.
You can enable the device to always compare the MEDs, regardless of the AS information in the
paths. For example, if the router receives UPDATES for the same route from neighbors in three ASs,
the router would compare the MEDs of all the paths together, rather than comparing the MEDs for
the paths in each AS individually.
NOTE
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 the route paths that are missing
their MEDs. You can use the med-missing-as-worst command to make the device regard a BGP route
with a missing MED attribute as the least favorable route, when comparing the MEDs of the routes.
NOTE
MED comparison is not performed for internal routes originated within the local AS or confederation
unless the compare-med-empty-aspath command is configured.
NOTE
The AS-path is empty when routes are redistributed from other protocols (e.g. redistribute static,
redistribute connected, or redistribute OSPF).
To configure the router to always compare MEDs, enter the following command.
BigIron RX(config-bgp)# always-compare-med
Syntax: [no] always-compare-med
The following BGP command directs BGP to take the MED value into consideration even if the route
has an empty as-path path attribute.
BigIron RX(config) router bgp
BigIron RX(config-bgp-router)# compare-med-empty-aspath