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Example: Configuring Node-Link Protection for IS-IS Routes in a Layer 3 VPN

Node-link protection establishes an alternate path through a different routing device. Use node-link protection when you assume that access to a node is lost when a link is no longer available. Junos OS calculates a backup path that avoids the primary next-hop routing device.


This example requires Junos OS Release 9.5 or later.

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


In this example, core-facing interfaces are enabled for IS-IS Level 2, LDP, and RSVP. Node-link protection is enabled on all the core-facing interfaces, which means that if the primary next hop for any destination that traverses the interfaces becomes unavailable, Junos OS uses a backup link that avoids the next-hop router altogether if necessary.

You also need to configure a routing policy that requires all traffic to use per-packet load balancing in order to enable Packet Forwarding Engine local repair. With local repair, the Packet Forwarding Engine can correct a path failure and implement a backup loop-free alternate route before it receives recomputed paths from the Routing Engine.

Figure 1 shows the topology used in this example.

On Device PE1, an RSVP LSP is configured as a backup path for IS-IS. Relying on the shortest-path-first (SPF) calculation of backup paths for one-hop neighbors might result in less than 100 percent backup coverage for a specific network topology. You can enhance coverage of IS-IS and LDP LSPs by configuring RSVP LSPs as backup paths. To configure a specific RSVP LSP as a backup path, include the backup statement at the [edit protocols mpls label-switched-path lsp-name] hierarchy level.

CLI Quick Configuration shows the configuration for all of the devices in Figure 1. The section #configuration364__isis-node-link-protection-step-by-step describes the steps on Device P1.




Step-by-Step Procedure

The following example requires you to 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 multi-level IS-IS:


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.

Checking the MPLS LSP Backup Path


Display information about the MPLS label-switched-paths (LSPs) designated as the backup route for the IS-IS routes.


On Device PE1, from operational mode, enter the show isis backup label-switched-path command.


The output shows that the backup path is up and operational.

Checking Which Next-Hop Neighbors Are Designated as Backup Paths to the Destination Node


Display SPF calculations for each neighbor for a given destination.


On Device PE1, from operational mode, enter the show isis backup spf results command.


The output indicates whether a specific interface or node has been designated as a backup path and why.

Checking the Backup Coverage


Check the percentage of protected nodes and prefixes.


From operational mode, enter the show isis backup coverage command.


The level of backup coverage available through IS-IS routes depends on the actual network topology and is typically less than 100 percent for all destinations on any given routing device. You can extend backup coverage to include RSVP LSPs.

Checking the Type of Protection Configured


On all nodes in the IS-IS domain, check the type and percentage of protected nodes and prefixes.


From operational mode, enter the show isis interface detail command.


The output shows that node-link protection is configured on the interfaces.