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Example: Configuring Layer 2 Mapping for IS-IS

This example shows how to configure Layer 2 mapping for IS-IS, that is, mapping a Layer 2 MAC address to the IPv4 address of the forwarding next hop. Layer 2 mapping minimizes traffic loss, provides better security, and reduces Layer 2 resolution processing on core links while activating an Ethernet link.


This example uses the following hardware and software components:

  • Two MX Series routers

  • Junos OS Release 16.1 or later running on all the devices


Layer 2 mapping ensures that the forwarding next-hop resolution is topology-driven rather than traffic-driven. IS-IS LAN and point-to-point Hellos supply all relevant Layer 2 and Layer 3 binding address information for address resolution. The device at the receiving end can use the information to populate the ARP or neighbor discovery cache of the kernel even before the route installation time. When Layer 2 mapping is enabled, IS-IS installs ARP or neighbor discovery next-hop entries into the forwarding table. Because this provides Layer 2 next-hop bindings ahead of time, IS-IS networks do not experience traffic loss while bringing up a link.


In Figure 1, Router R1 is connected to Router R2. Layer 2 mapping is enabled on Router R1. Router R2 receives the Layer 2 information from Router R1 and updates the forwarding table.

Figure 1: Configuring Layer 2 Mapping for IS-ISConfiguring Layer 2 Mapping for IS-IS


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


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 Junos OS CLI User Guide.

To configure Layer 2 mapping on Router R1:


Repeat this procedure for Router 2 after modifying the appropriate interface names, addresses, and other parameters.

  1. Configure the device interfaces.

  2. Configure the loopback interface.

  3. Configure the router id.

  4. Configure RSVP, MPLS, and LDP on all interfaces excluding the management interface.

  5. Enable Layer 2 mapping.

  6. Disable level 2 IS-IS on interface ge-1/0/3.0.


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, commit the configuration.


Confirm that the configuration is working properly.

Verifying IS-IS Adjacencies


Verify that the expected adjacencies have formed between Router R1 and Router R2.


From operational mode, run the show isis adjacency command on Router R1.


The interface ge-1/0/3.0 on Router R1 has established adjacency with Router R2.

Verifying That Layer 2 Mapping Is Enabled


Verify that Layer 2 mapping is enabled on Router R1.


From operational mode, run the show isis interface detail command on Router R1.


The output confirms that Layer 2 mapping is enabled on Router R1.

Verifying That the Layer 2 Address Is Mapped


Display Layer 3 next hop and the mapped data link address in the kernel for the routing instances.


From operational mode, run the show isis layer2-map command on Router R1.


The Layer 2 MAC address of the next hop is mapped to the IP address of interface ge-1/0/3.0 in the kernel.