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Example: Configuring Routing Policy Prefix Lists

In Junos OS, prefix lists provide one method of defining a set of routes. Junos OS provides other methods of accomplishing the same task, such as route filters. A prefix list is a listing of IP prefixes that represent a set of routes that are used as match criteria in an applied policy. Such a list might be useful for representing a list of customer routes in your autonomous system (AS). A prefix list is given a name and is configured within the [edit policy-options] configuration hierarchy.


No special configuration beyond device initialization is required before configuring this example.

Updated and re-validated using Junos OS Release 22.1R1.


Prefix lists are similar to a list of route filters. The functional difference between route filters and prefix lists is that you cannot specify a range using a prefix list. You can simulate a range using a prefix list by including additional prefixes in the list, or by using two prefix lists, one shorter and one longer, setting one to accept and the other to reject. You can also filter a prefix list using the prefix-list-filter match condition. Your choices are exact, longer, and orlonger.

The benefit of a prefix list over a list of route filters is seen when the prefixes are referenced in several different locations. For instance, a prefix list can be referenced in a BGP import policy, an export policy, an RPF policy, in firewall filters, in loopback filters, in setting a multicast scope, and so on.

When your list of prefixes changes, rather than trying to remember the many different locations prefixes are configured, you can instead update the prefix list, changing the prefix one time instead of multiple times. This helps to reduce the likelihood of configuration errors, such as mistyping the address in a location or forgetting to update one or more locations.

Prefix lists also help when managing a large number of devices. You can write the various filters and policies as generically as possible, referencing prefix lists instead of specific IP addresses. The more complex logic in the filters and policies has to be written only one time, with minimal per-device and per-site customizations.

As shown in Figure 1, each router in AS 65000 has customer routes. Device R1 assigns customer routes within the subnet. Device R2 and Device R3 assign customer routes within the and subnets, respectively. Device R1 has been designated the central point in AS 65000 to maintain a complete list of customer routes. Device R1 has a prefix list called customers, as follows:

As you can see, the prefix list does not contain a match type for each route (as you would see with a route filter). This is an important point when using a prefix list in a policy. Routes match only if they exactly match one of the prefixes in the list. In other words, each route in the list must appear in the routing table exactly as it is configured in the prefix list.

You reference the prefix list as a match criterion within a policy like this:

In this example, all the routes in the customers prefix list appear in the routing table on Device R1. Device R2 and Device R3 export to Device R1 static routes to their customers.

As previously mentioned, you can use the prefix-list-filter match condition with the exact, longer, or orlonger match type. This provides a way to avoid the prefix list exact-match limitation of prefix lists. For example:

The example demonstrates the effects of both the prefix-list match condition and the prefix-list-filter match condition.


Figure 1 shows the sample network.

Figure 1: BGP Topology for Policy Prefix Lists BGP Topology for Policy Prefix Lists

CLI Quick Configuration shows the configuration for all of the devices in Figure 1.

The section #configuration449__policy-prefix-list-st describes the steps on Device R1.


CLI Quick Configuration

To quickly configure this example, copy the following commands, paste them into a text file, remove any line breaks, change any details necessary to match your network configuration, and then copy and paste the commands into the CLI at the [edit] hierarchy level.

Device R1

Device R2

Device R3

Device R4


Step-by-Step Procedure

We are showing the step-by-step procedure to configure R1. The other routers have a similar step-by-step process. The following example requires that you navigate various levels in the configuration hierarchy. For information about navigating the CLI, see Use the CLI Editor in Configuration Mode in the Junos OS CLI User Guide.

To configure R1:

  1. Configure the interfaces.

  2. Configure the internal BGP (IBGP) peerings to R2 and R3.

  3. Configure the external BGP (EBGP) peering to R4. The export policy configuration is shown in a later step.

  4. Configure the OSPF peerings to R2 and R3. The OSPF protocol provides the learning of the loopback address for each device which allows the IBGP peerings to establish.

  5. Configure the prefix list.

  6. Configure the routing policy that references the prefix list as a match criterion.

  7. Configure the static routes for the network. We are using static routes to emulate the customer routes.

  8. Configure the autonomous system (AS) number and router ID.


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

If you are done configuring the device, enter commit from configuration mode.


Confirm that the configuration is working properly.

Verifying the Routes on R1


On R1, check the routes in the routing table.



Device R1 has learned its own static routes (S) and the BGP routes from Devices R2 and R3 (B).

Verifying the Route Advertisement to R4


On R1, make sure that the customer routes are advertised to R4.



As expected, only the routes from the customer prefix list are advertised to R4.

Experimenting with the prefix-list-filter Statement


See what can happen when you use prefix-list-filter instead of prefix-list.


  1. On R3, add a static route that is longer than one of the existing static routes.

  2. On R1, deactivate the prefix list and configure a prefix list filter with the orlonger match type.

  3. On R1, check which routes are advertised to R4.


As expected, R1 is now advertising the route to R4, even though is not in the prefix list.