Disabling Or Re-Enabling Comparison Of The As-Path Length; Redistributing Ibgp Routes - Dell PowerConnect B-RX Configuration Manual

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Redistributing IBGP routes

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
Syntax: [no] compare-med-empty-aspath

Disabling or re-enabling comparison of the AS-path length

AS-Path comparison is Step 5 in the algorithm BGP4 uses to select the next path for a route.
Comparison of the AS-Path length is enabled by default. To disable it, enter the following command
at the BGP configuration level of the CLI.
BigIron RX(config-bgp)# as-path-ignore
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" on page 26-3 skips from Step 4 to Step 6.
Syntax: [no] as-path-ignore
Redistributing IBGP routes
By default, the device does not redistribute IBGP routes from BGP4 into RIP, OSPF, or ISIS. This
behavior helps eliminate routing loops. However, if your network can benefit from redistributing the
IBGP routes from BGP4 into OSPF, ISIS or RIP, you can enable the device to redistribute the routes.
To enable the device to redistribute BGP4 routes into OSPF, RIP, or ISIS, enter the following
command.
760 BigIron RX Series Configuration Guide
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