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| 7.5 |  | Configuration of Integrated IS-IS |
| 7.5.2 |  | Multiarea Integrated IS-IS configuration |

This section explores a simple multi-area Integrated IS-IS scenario. The IS-IS configuration commands successfully route IP by way of IS-IS. For example, the routing tables of both Rtr-A and Rtr-D are automatically populated with a default route. The following is the correct behavior for IS-IS:

> Rtr-A#show ip route

> Gateway of last resort is 192.168.120.10 to network 0.0.0.0

> C 192.168.120.0/24 is directly connected, FastEthernet0/0
> 192.168.1.0/32 is subnetted, 2 subnets
> i L1 192.168.1.1 [115/10] via 192.168.120.10, FastEthernet0/0
> C 192.168.1.5 is directly connected, Loopback0
> i L1 192.168.222.0/24 [115/20] via 192.168.120.10, FastEthernet0/0
> i*L1 0.0.0.0/0 [115/10] via 192.168.120.10, FastEthernet0/0

> Rtr-D#show ip route

> Gateway of last resort is 192.168.111.2 to network 0.0.0.0

> C 192.168.111.0/24 is directly connected, FastEthernet0/0
> 192.168.2.0/32 is subnetted, 2 subnets
> i L1 192.168.2.2 [115/10] via 192.168.111.2, FastEthernet0/0
> C 192.168.2.4 is directly connected, Loopback0
> i*L1 0.0.0.0/0 [115/10] via 192.168.111.2, FastEthernet0/0

By default, Cisco IOS enables both L1 and L2 operations on IS-IS routers. If a router is to operate only as an area router, or only as a backbone router, this can be specified by entering the is-type command in IS-IS router configuration mode. To specify that the router will act only as an area or L1 router, specify is-type level-1. To specify that the router will act only as a backbone or L2 router, specify is-type level-2-only. Rtr-A and Rtr-D use the is-type level-1 command.

Although the router may be an L1L2 router, it may be required to only establish L1 adjacencies over certain interfaces and only L2 adjacencies over other interfaces. The interface command isis circuit-type can specify either level-1 or level-2-only. If this is not specified, the IOS will attempt to establish both types of adjacencies over the interface. The Rtr-C configuration uses the isis circuit-type command. Notice that L1 adjacencies cannot form between areas. No isis circuit-type command was applied to interface Fa0/0 on Rtr-B. The optimal configuration would include the isis circuit-type level-1 command on this interface so as to avoid an attempt by Rtr-B to form an L2 adjacency with Rtr-A. This would fail anyway because Rtr-A is configured with the IS-IS router mode command is-type level-1.

Unlike some other IP protocols, IS-IS takes no account of line speed or bandwidth when setting its link metrics. All interfaces are assigned a metric of ten (10). To change this value, use the interface command isis metric <value> level-1|level-2. The metric can have different values for Level 1 and Level 2 over the same interface. The isis metric interface command is not used in the scenario presented in this section.

The show clns command indicates Rtr-B is running IS-IS in IP only mode. This is because none of the interfaces are configured with the clns router isis command. This would force the router to begin forwarding CLNP packets. Despite the fact that CLNS is not being routed, the show protocol output appears to contradict this. The output says, "CLNS routing is enabled" and it says, "CLNS enabled" on each of the FastEthernet0/0 and Serial0/1 interfaces. However, CLNP packets are not being routed, as verified by the show clns traffic output.

The show isis database output demonstrates the automatic setting of the attached bit (ATT) by the L1L2 router, Rtr-B. This indicates to the L1 routers that it is a potential exit point for the area.

The show clns traffic output allows CLNS traffic statistics to be analyzed. For this command output, keep the following in mind:

- LSPs sourced indicates stability of IS
- LSP retransmissions should stay low
- Partial route calculations (PRCs) cannot be checked elsewhere
- LSP checksum errors are a bad sign
- Update queue should not stay full
- Update queue should not drop much

The debug isis spf-triggers command is useful for determining the cause, or trigger, for an SPF calculation.

Figure gives a quick summary of some useful Integrated IS-IS troubleshooting show commands. Figures , , , , , , and provide some imscellaneous IS-IS show and debug output for this scenario.

| Lab Activity e-Lab Activity: Multiarea Integrated IS-IS Configuration This lab is to a configure a multiarea IS-IS network.