Configuring Multiple Instances of IS-IS

You can configure multiple instances of IS-IS for administrative separation.

To configure multiple routing instances, perform the following tasks:

1. Configure the IS-IS default instance at the [edit protocols isis] or [edit logical-systems logical-system-name protocols isis] hierarchy levels with the statements needed for your network so that routes are installed in inet.0 and in the forwarding table. Make sure to include the routing table group.
2. Configure an IS-IS routing instance for each additional IS-IS routing entity, configuring the following items:
   • Interfaces
   • Routing options
   • IS-IS protocol statements belonging to that entity
   • Routing table group
3. Configure a routing table group to install routes from the routing instance into the inet.0 routing table. You can do this in two ways:
   • Create a common routing table group so that either one of two conditions is configured:
     • Routes from the routing instances are installed in inet.0 and therefore installed in the forwarding table.
     • Routes from one router in a routing instance are forwarded to another router in the same routing instance.
   • Create a routing table group with just the routing table from one instance and inet.0 to keep the routes from going to other instances.
4. Create an export policy to export routes with a specific tag and to use that tag to export routes back into the instances. For more information, see the Junos Policy Framework Configuration Guide.

Example: Configuring Multiple Routing Instances of IS-IS

Figure 4 shows how you can use multiple instances of IS-IS to segregate traffic within a large network. The network consists of three administrative entities: voice_policy, other_policy, and the backbone or core. Each entity is composed of several geographically separate sites that are connected by the backbone and managed by the backbone entity.