750 BigIron RX Series Configuration Guide53-1001986-01Redistributing IBGP routes26You can enable the device to always compare the MEDs, regardless of the AS information in thepaths. 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 forthe paths in each AS individually.NOTEBy default, value 0 (most favorable) is used in MED comparison when the MED attribute is notpresent. The default MED comparison results in the device favoring the route paths that are missingtheir MEDs. You can use the med-missing-as-worst command to make the device regard a BGP routewith a missing MED attribute as the least favorable route, when comparing the MEDs of the routes.NOTEMED comparison is not performed for internal routes originated within the local AS or confederationunless the compare-med-empty-aspath command is configured.NOTEThe 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-medSyntax: [no] always-compare-medRelease 02.4.01 of the Multi-Service IronWare software initiated support for the following new BGPcommand that directs BGP to take the MED value into consideration even if the route has an emptyas-path path attribute.BigIron RX(config) router bgpBigIron RX(config-bgp-router)# compare-med-empty-aspathSyntax: [no] compare-med-empty-aspathDisabling or re-enabling comparison of the AS-path lengthAS-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 commandat the BGP configuration level of the CLI.BigIron RX(config-bgp)# as-path-ignoreThis command disables comparison of the AS-Path lengths of otherwise equal paths. When youdisable AS-Path length comparison, the BGP4 algorithm shown in “How BGP4 Selects a Path for aRoute” on page 26-3 skips from Step 4 to Step 6.Syntax: [no] as-path-ignoreRedistributing IBGP routesBy default, the device does not redistribute IBGP routes from BGP4 into RIP, OSPF, or ISIS. Thisbehavior helps eliminate routing loops. However, if your network can benefit from redistributing theIBGP routes from BGP4 into OSPF, ISIS or RIP, you can enable the device to redistribute the routes.
