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Example: Configuring a Routing Policy to Prioritize IS-IS Routes

In a network with a large number of IS-IS routes, it can be useful to control the order in which routes are updated in response to a network topology change. This example shows how to define a routing policy to prioritize some IS-IS routes over others. In the event of an IS-IS topology change, high priority prefixes are updated in the routing table first, followed by medium and then low priority prefixes. Internet Service Providers (ISP) can use this feature to ensure faster convergence for important customers.


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

This example uses the following hardware and software components:

  • Three routers that can be a combination of M Series, MX Series, or T Series routers

  • Junos OS Release 17.1 or later on the device


Beginning with Junos OS Release 17.1, you can prioritize or reject IS-IS routes that are installed in the routing table. Use the reject policy option to reject routes from a specific prefix or routes marked with a particular tag.

You can prioritize IS-IS routes for better convergence and to provide differentiated services. In a network with a large number of IGP prefixes with BGP Layer 3 VPN or label-based psuedowire service established on top of some IGP prefixes, it is important to control the order in which routes get updated in the forwarding table. You can configure an import policy and use a route tag or filter the routes based on their prefix before setting a priority of high, medium, or low as per your network requirements. The IS-IS protocol downloads routes to the rpd routing table based on the configured priority. If you do not configure an import policy, all routes are set to a medium priority by default.

An IS-IS import policy can be used to set priority or to filter IS-IS external routes based on the following criteria:


Use route-filter policy option to filter known prefixes.

Route Tag

Use tag policy option to assign a specific priority for prefixes that contain a particular tag.


If an IS-IS import policy is applied that results in a reject terminating action for a non-external 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. However, you can use the the reject policy option to reject routes based on the prefix or the configured tag.


You might see an increase in micro loop traffic as order of route download changes.


In Figure 1, Router R1 is connected to Router R3 via Router R2. We need to set a high priority to a route to Router R3 to ensure quicker convergence. An import routing policy is configured on Router R1, which sets a high priority to routes connecting to Router R3. Routes matching are installed first because they have a priority of high. LDP imports routes and their configure priority from IS-IS. This route is restored first in the event of a network topology change.

Figure 1: Example: Configuring a Routing Policy to prioritize IS-IS RoutesExample: Configuring a Routing Policy to prioritize IS-IS Routes


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, copy and paste the commands into the CLI at the [edit] hierarchy level, and then enter commit from configuration mode.

Router R1

Router R2

Router R3

Configuring Router R1

Step-by-Step Procedure

The following example requires that you navigate various levels in the configuration hierarchy. For information about navigating the CLI, see Using the CLI Editor in Configuration Mode in the CLI User Guide.

To configure Router R1:


Repeat this procedure for other routers after modifying the appropriate interface names, addresses, and other parameters.

  1. Configure the interfaces with IPv4 and IPv6 addresses.

  2. Configure the loopback address.

  3. Configure MPLS.

  4. Enable IS-IS protocol on the interfaces.

  5. Configure LDP protocol on the interfaces.

  6. Define a policy to prioritize IS-IS routes to Router R3. .

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


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.


Verifying the Priority for LDP Routes


Verify that LDP has inherits route from IS-IS protocol.


From operational mode, run the show route extensive command on Router R1.


The output shows that LDP inherits the route, with priority high from IS-IS.

Verifying the Priority of IS-IS Routes


Verify that the priority is set for route in IS-IS.



The routes are displayed in the order of the set priorities. Route, which is set with high priority is displayed at the very top followed by routes with medium or low priority.