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Configuring Point-to-Point-over-LAN Circuits

You can deploy IS-IS on broadcast and point-to-point circuits. IS-IS treats these circuits differently in several ways, such as when establishing neighbor adjacencies or flooding link-state information.

Broadcast circuits use designated routers and are represented as virtual nodes in the network topology. They require periodic database synchronization. By default, IS-IS treats the broadcast link as LAN media and tries to bring up the LAN adjacency even when the interface is configured as unnumbered or only a single neighbor exists on that link.

In contrast, point-to-point circuits have less overhead, because they do not use designated routers, the link-state database has no representation of the pseudonode or network LSA, and they do not require periodic database synchronization. However, if more than two routers are connected on the LAN media, routing information in the network is reduced.

Although broadcast circuits are intended to handle more than two devices, in some circumstances you might connect only two routers over the physical or virtual LAN. Even though only two routers are connected, IS-IS treats the circuit as a broadcast circuit that has many more connected routers, with all the associated broadcast overhead but without the benefits of reduced routing information and of optimized flooding that result from having more than two routers on the LAN.

You can use the isis network point-to-point command to configure IS-IS to operate using point-to-point connections on a broadcast circuit when only two routers are on the circuit. This configuration is known as a point-to-point-over-LAN or P2P circuit. This interface configuration tears down the current LAN adjacency that IS-IS has over this interface. IS-IS then reestablishes the adjacency as a point-to-point connection and regenerates the LSPs. The broadcast link is thereafter treated as simple point-to-point interface.

Treating the LAN as a P2P circuit reduces the amount of information that IS-IS has to maintain and manage. For example, there is no need to elect a designated router for the interface. LSP flooding is performed as in P2P links without the need for using periodic CSNPs.

This circuit configuration can be advantageous even when many routers are on the LAN. For example, you might want to organize the routers into multiple smaller VLANs so that you can assign different costs to the IS-IS neighbors. You can apply this configuration to any such VLAN that has only two routers. IS-IS then views the LAN as a mesh of point-to-point connections.

The use of IP unnumbered interfaces makes the most of scarce IP address resources and provides for simpler network management and configuration. This configuration enables IP processing on a point-to-point interface without an explicit IP address. The IP unnumbered interface borrows the IP address of another interface on the node. Point-to-point-over-LAN circuits separate the concept of network type from media type, and enable you to apply unnumbered interface configurations to LANs.

The point-to-point-over-LAN feature requires the following:

The LAN must have only two routers.
Both routers must support the feature.
You must configure the interface at each end as a P2P connection.
If you are using numbered interfaces, both ends must be in same IPv4 subnet.
If you are using unnumbered interfaces, both ends require static ARP entry configuration.

isis network point-to-point

Use to specify that the broadcast circuit is to be treated as a point-to-point circuit.
Issuing this command tears down existing adjacencies, originates or flushes LSPs, and establishes new adjacencies
Example host1(config-intf)#isis network point-to-point
Use the no version to restore the default value, treating the circuit as a broadcast circuit.
See isis network point-to-point

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