connect to routers in any other area. It is appropriate to configure this router as IS type Level 1. R2 connects to the internal area routers and also to router R3 in a different area. R2 must do both Level 1 and Level 2 routing, so it is left at the default setting. However, there is no need for R2 to send Level 2 hellos out the interface connected to R1. It is appropriate to set the IS-IS circuit type of the FastEthernet 0/0 of R2 to Level 1. Similarly, because the Serial 0/0/1 interface of R2 connects only to a Level 2 router, the IS-IS circuit type should be set to Level 2. In the topology shown, the serial link is slower than the Fast Ethernet link. Using the default metric of 10 does not give the routers a true picture of the value of each link. As a result, the routers cannot make the best routing decisions. As shown in the sample configuration, you should change the IS-IS metric at each serial interface to reflect your preference for a link.
Content 4.5 Optimizing IS-IS 4.5.5 Configuring Route Summarization in IS-IS Routing protocol scalability is a function of the appropriate use of route summarization. An IS can be configured to aggregate a range of IP addresses into a summary address, using the summary-address command as shown in Figure . This command can be used on any router in an IS-IS network. The router summarizes IP routes into Level 1, Level 2, or both. The optional tag number is used to tag the summary route. The optional metric value is applied to the summary route. The benefits of summarization are as follows: To remove route summarization, use the no form of the command.
Content 4.6 Verifying and Troubleshooting IS-IS 4.6.1 IS-IS Lab To verify and troubleshoot the IS-IS configuration and IP functionality of the Integrated IS-IS network, use the following commands: The sample output from the show ip protocols command in Figure shows information about IP routing being performed using Integrated IS-IS. In this example, IS-IS is running, it is not redistributing any other protocols, and address summarization has not been configured. This example also illustrates the following: The sample output from the show ip route isis command in Figure shows the IS-IS routes only. One route is from Level 1, as indicated by the i L1 tag, and the other is from Level 2, as indicated by the i L2 tag. Integrated IS-IS uses, by default, an administrative distance of 115. The metric shown for each route is taken from the IS-IS cost to the destination. In the figure, for the value of [115/20], 115 is the Integrated IS-IS administrative distance, and 20 is the IS-IS metric.
Content 4.6 Verifying and Troubleshooting IS-IS 4.6.2 Verifying CLSN IS-IS Structures You can use the following show clns commands to verify the router configuration and to troubleshoot the Integrated IS-IS network: You can reduce the list of neighbors displayed to those across a particular interface if you specify the interface type and number in the command. The optional keyword detail displays the area addresses advertised by the neighbor in the hello messages.
Content 4.6 Verifying and Troubleshooting IS-IS 4.6.3 Troubleshooting Integrated IS-IS Configurations You can use the following show commands to verify the router configuration and to troubleshoot the Integrated IS-IS network:
Content 4.6 Verifying and Troubleshooting IS-IS 4.6.4 Troubleshooting Integrated IS-IS Configuration Example Figure shows three routers in two areas: R1 and R2 belong to area 49.0001; R3 belongs to area 49.0002. R1 is a Level 1 router doing only Level 1 routing. R2 is a Level 1–2 router doing both Level 1 and Level 2 routing. R3 is a Level 2 router doing only Level 2 routing. The topology in the figure forms the basis for the following show command examples. The show isis topology command displays the topology databases with the least-cost paths to destination ISs. Notice that the output for R1 (a Level 1 router) shows the topology database for Level 1 only, and the output for R2 (a Level 1–2 router) shows that separate topology databases exist for Level 1 and Level 2. The fields in the topology database are common for both levels of routing. They are as follows: The topology database on R1 (a Level 1 router) shows only routers within the local area. R1 is doing only Level 1 routing, and therefore does not know of any routers outside its area. Traffic bound for other areas would be forwarded to the nearest router doing Level 2 routing, in this case, R2. R2 is doing both levels of routing. It therefore maintains two topology databases. The Level 1 database looks very much like the R1 database; only routers within the local area are listed. The Level 2 database is where the external router, R3, shows up. In Figure , the example output from the show clns protocol command shows this information: