Brocade FastIron Ethernet Switch Layer 3 Routing Configuration Guide - Manual (Page 422)

This command disables comparison of the AS-Path lengths of otherwise equal paths. When you
disable AS-Path length comparison, the BGP4 algorithm shown in
How BGP4 selects a path for a
route (BGP best path selection algorithm) on page 386 skips from Step 4 to Step 6.
Enabling or disabling comparison of device IDs
Device ID comparison is Step 10 in the algorithm BGP4 uses to select the next path for a route.
NOTE
Comparison of device IDs is applicable only when BGP4 load sharing is disabled.
When device ID comparison is enabled, the path comparison algorithm compares the device IDs of
the neighbors that sent the otherwise equal paths:
• If BGP4 load sharing is disabled (maximum-paths 1), the instructions in this section selects the path
that came from the neighbor with the lower device ID.
• If BGP4 load sharing is enabled, the device load shares among the remaining paths. In this case,
the device ID is not used to select a path.
NOTE
Device ID comparison is disabled by default.
To enable device ID comparison, enter the compare-routerid command at the BGP4 configuration
level of the CLI.
device(config-bgp-router)# compare-routerid
Syntax: [no] compare-routerid
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
autonomous systems, the device can compare the MEDs of all the paths together instead of
comparing the MEDs for the paths in each autonomous system 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.
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 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, when comparing the MEDs of the routes.
Enabling or disabling comparison of device IDs
422 FastIron Ethernet Switch Layer 3 Routing Configuration Guide
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