Treating missing meds as the worst meds – Brocade Communications Systems Layer 3 Routing Configuration ICX 6650 User Manual
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Brocade ICX 6650 Layer 3 Routing Configuration Guide
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Optional BGP4 configuration tasks
Brocade(config-bgp-router)#compare-routerid
Syntax: [no] compare-routerid
For more information, refer to
“How BGP4 selects a path for a route”
Configuring the Layer 3 switch to always compare
Multi-Exit Discriminators
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
MED is equivalent to its “metric”:
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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 Layer 3 switch 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.
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The Layer 3 switch compares the MEDs based on one or more of the following conditions. By
default, the Layer 3 switch compares the MEDs of paths only if the first AS in the paths is the
same. (The Layer 3 switch skips over the AS-CONFED-SEQUENCE if present.)
You can enable the Layer 3 switch 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
autonomous systems, 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 Layer 3 switch favoring the route paths that are
missing their MEDs. You can use the med-missing-as-worst command to make the Layer 3 switch
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.
To configure the router to always compare MEDs, enter the following command.
Brocade(config-bgp-router)#always-compare-med
Syntax: [no] always-compare-med
Treating missing MEDs as the worst MEDs
By default, the Layer 3 switch favors a lower MED over a higher MED during MED comparison.
Since the Layer 3 switch assigns the value 0 to a route path MED if the MED value is missing, the
default MED comparison results in the Layer 3 switch favoring the route paths that are missing
their MEDs.
To change this behavior so that the Layer 3 switch favors a route that has a MED over a route that
is missing its MED, enter the following command at the BGP4 configuration level of the CLI.