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Example: Advertising Label-Switched Paths into IS-IS

This example shows how to advertise label-switched paths (LSPs) into IS-IS as point-to-point links (sometimes referred to as forwarding adjacencies) so that the LSPs can be used in SPF calculations. The advertisement contains a local address (the from address of the LSP), a remote address (the to address of the LSP), and a metric.


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


Figure 1 shows the topology used in this example.

Figure 1: IS-IS Advertising a Label-Switched Path TopologyIS-IS Advertising a Label-Switched Path Topology

The example shows how to configure the LSP from Device E to Device D and then advertise this path through IS-IS. The configuration is verified by performing a traceroute operation from Device A to Device D and making sure that the LSP is used for forwarding.

CLI Quick Configuration shows the configuration for all of the devices in Figure 1. The section #configuration442__isis-lsp-advertise-step-by-step describes the steps on Device E.



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 A

Device B

Device C

Device D

Device E

Device F

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 advertise LSPs into IS-IS:

  1. Configure the interfaces.

  2. Enable a signaling protocol on the interfaces.

  3. Enable MPLS on the interfaces.

  4. Configure the LSP.

    Make sure that you configure the reverse LSP on the endpoint, in this case on Device D.

  5. Configure internal BGP (IBGP) peering among the devices that must run MPLS.

  6. Enable IS-IS on the interfaces, and set the link metric.

    IS-IS Level 1 and Level 2 are enabled when you include the interface at [edit protocols isis]. By disabling Level 1, you are in effect creating a Level 2 IS-IS interface.

  7. Advertise the LSP through IS-IS.

    Make sure that you advertise the LSP on the endpoint, in this case on Device D.

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


From configuration mode, confirm your configuration by entering the show interfaces, show protocols, 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 IS-IS Neighbor


Verify that another neighbor is listed and is reachable over the LSP. The interface field indicates the name of the LSP.


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


As expected, Interface: E-D is shown in the output, and the state is shown as One-way.

Checking the IS-IS SPF Calculations


Verify that the LSP is being used in the SPF calculations.


From operational mode, enter the show isis spf brief command.


As expected, the SPF results include the LSP, E-D.

Checking the Forwarding Path


Verify that a traceroute operation from Device A to Device D uses the LSP.



The output shows that the LSP is used.