Configuring route preferences

A route can be learned by the router from different protocols, in which case, the costs are not comparable. When this occurs, the preference value is used to decide which route is installed in the forwarding table if several protocols calculate routes to the same destination. The route with the lowest preference value is selected.

Different protocols should not be configured with the same preference, if this occurs the tiebreaker is per the default preference table as defined in Table: Route preference defaults by route type . If multiple routes are learned with an identical preference using the same protocol, the lowest cost route is used.

If multiple routes are learned with an identical preference using the same protocol and the costs (metrics) are equal, then the decision of what route to use is determined by the configuration of the ecmp in the config>router context.

The following CLI commands are displayed to illustrate route preference features. The command parameters can be defined at the same time you are configuring OSPF. See Configuring OSPF components.

Use the following CLI syntax to configure a route preference.

CLI syntax

ospf ospf-instance
  preference preference
    external-preference preference

Use the following CLI syntax to configure a route preference for the OSPF3.

CLI syntax

ospf ospf-instance
ospf3
   preference preference
   external-preference preference

The following displays a route preference configuration example:

A:ALA-49>config>router>ospf# info
----------------------------------------------
            asbr
            overload
            overload-on-boot timeout 60
            traffic-engineering
            preference 9
            external-preference 140
            export "OSPF-Export"
            exit
            area 0.0.0.0
                virtual-link 10.0.0.1 transit-area 0.0.0.1
                    authentication-type message-digest
                    message-digest-key 2 md5 "Mi6BQAFi3MI" hash
                exit
                virtual-link 1.2.3.4 transit-area 1.2.3.4
                    hello-interval 9
                    dead-interval 40
                exit
                interface "system"
                exit
            exit
            area 0.0.0.1
            exit
            area 0.0.0.20
                stub
                exit
                interface "to-103"
                exit
            exit
            area 0.0.0.25
                nssa
                exit
                interface "if2"
                    priority 100
                exit
            exit
            area 0.0.0.40
                interface "test1"
                    authentication-type password
                    authentication-key "3WErEDozxyQ" hash
                exit
            exit
            area 1.2.3.4
            exit
----------------------------------------------

The following displays a route preference configuration example:

A:ALA-48>config>router>ospf3# info
----------------------------------------------
            asbr
            overload
            timers
                lsa-arrival 50000
            exit
            preference 9
            external-preference 140
            export "OSPF-Export"
            area 0.0.0.0
                virtual-link 4.3.2.1 transit-area 4.3.2.1
                exit
                interface "system"
                exit
            exit
            area 0.0.0.20
                stub
                exit
                interface "SR1-2"
                exit
            exit
            area 0.0.0.25
                nssa
                exit
            exit
            area 4.3.2.1
            exit
----------------------------------------------
A:ALA-48>config>router>ospf3#