Example: Configuring a Route Filter Policy to Specify Priority for Prefixes Learned Through OSPF

Requirements

Before you begin:

• Configure the device interfaces. See the Junos OS Interfaces Library for Security Devices.
• Configure the router identifiers for the devices in your OSPF network. See Example: Configuring an OSPF Router Identifier.
• Control OSPF designated router election See Example: Controlling OSPF Designated Router Election
• Configure a single-area OSPF network. See Example: Configuring a Single-Area OSPF Network .
• Configure a multiarea OSPF network. See Example: Configuring a Multiarea OSPF Network.

Overview

In a network with a large number of OSPF routes, it can be useful to control the order in which routes are updated in response to a network topology change. In Junos OS Release 9.3 and later, you can specify a priority of high, medium, or low for prefixes included in an OSPF import policy. In the event of an OSPF topology change, high priority prefixes are updated in the routing table first, followed by medium and then low priority prefixes.

OSPF import policy can only be used to set priority or to filter OSPF external routes. If an OSPF import policy is applied that results in a reject terminating action for a nonexternal route, then the reject action is ignored and the route is accepted anyway. By default, such a route is now installed in the routing table with a priority of low. This behavior prevents traffic black holes, that is, silently discarded traffic, by ensuring consistent routing within the OSPF domain.

• Summary discard routes have a default priority of low.
• Local routes that are not added to the routing table are assigned a priority of low.
• External routes that are rejected by import policy and thus not added to the routing table are assigned a priority of low.

Any available match criteria applicable to OSPF routes can be used to determine the priority. Two of the most commonly used match criteria for OSPF are the route-filter and tag statements.

In this example, the routing device is in area 0.0.0.0, with interfaces fe-0/1/0 and fe-1/1/0 connecting to neighboring devices. You configure an import routing policy named ospf-import to specify a priority for prefixes learned through OSPF. Routes associated with these prefixes are installed in the routing table in the order of the prefixes’ specified priority. Routes matching 200.3.0.0/16 orlonger are installed first because they have a priority of high. Routes matching 200.2.0.0/16 orlonger are installed next because they have a priority of medium. Routes matching 200.1.0.0/16 orlonger are installed last because they have a priority of low. You then apply the import policy to OSPF.

Note: The priority value takes effect when a new route is installed, or when there is a change to an existing route.

Configuration

CLI Quick Configuration

To quickly configure an OSPF import policy that prioritizes specific prefixes learned through OSPF, copy the following commands, removing any line breaks, and then paste the commands into the CLI.

Step-by-Step Procedure

1. Configure the interfaces.
   [edit]user@host# set interfaces fe-0/1/0 unit 0 family inet address 192.168.8.4/30user@host# set interfaces fe-0/2/0 unit 0 family inet address 192.168.8.5/30
2. Enable OSPF on the interfaces.

   Note: For OSPFv3, include the ospf3 statement at the [edit protocols] hierarchy level.

   [edit]user@host# set protocols ospf area 0.0.0.0 interface fe-0/1/0user@host# set protocols ospf area 0.0.0.0 interface fe-0/2/0
3. Configure the policy to specify the priority for prefixes learned through OSPF.
   [edit ]user@host# set policy-options policy-statement ospf-import term t1 from route-filter 200.1.0.0/16 orlongeruser@host# set policy-options policy-statement ospf-import term t1 then priority lowuser@host# set policy-options policy-statement ospf-import term t1 then acceptuser@host# set policy-options policy-statement ospf-import term t2 from route-filter 200.2.0.0/16 orlongeruser@host# set policy-options policy-statement ospf-import term t2 then priority mediumuser@host# set policy-options policy-statement ospf-import term t2 then acceptuser@host# set policy-options policy-statement ospf-import term t3 from route-filter 200.3.0.0/16 orlongeruser@host# set policy-options policy-statement ospf-import term t3 then priority highuser@host# set policy-options policy-statement ospf-import term t3 then accept
4. Apply the policy to OSPF.
   [edit]user@host# set protocols ospf import ospf-import
5. If you are done configuring the device, commit the configuration.
   [edit]user@host# commit

Results

Confirm your configuration by entering the show interfaces, show policy-options, and the show protocols ospf commands. If the output does not display the intended configuration, repeat the instructions in this example to correct the configuration.