L1 Router Problem

L1 routers perform routing based on the area ID of the IS-IS packet. If the destination address specified in the packet is not within the same area as the L1 router, the L1 router sends these packets to the nearest L2 router.

Configuring two L1 routers within the same IS-IS area with the same System ID can cause misconfiguration problems in IS-IS. A warning “Possible Duplicate System IDs” is issued and the L1 routers treat each other’s LSPs as their own. Each misconfigured L1 router with the same System ID will generate a new LSP with a higher sequence number. This process continues infinitely, resulting in a major performance bottleneck in the L1 routers.

Consider the scenario depicted in Figure 10.1. In this IS-IS network, the same System ID (1900.6500.4001) is configured for both R1 and R2. R1 and R4 function as L1 routers and R2 and R4 are L1-L2 routers. Each router will send LSPs to every other router within Area 49.001 while flooding Hello packets for establishing adjacency relationships.

Both the routers cannot distinguish from which router the current LSP has arrived. R1 and R2 will consider each other’s LSPs as their own, because the routers have the same System ID. R1 and R2 increment the sequence numbers infinitely. As a result, the adjacency relationship is never established between R1 and R2. To troubleshoot this adjacency problem in L1 routers:

1. Use the command show isis in R1 and R2 and verify the NET. The output of show isis command in R1 is as shown in Listing 10.23.

Listing 10.23 Verification of System ID on R1

R1#show isis Global ISIS information ISIS process tag: R1 System ID: 1900.6500.4001 NET: 49.001.1900.6500.4001.00 Maximum number of areas: 2 There is 1 manual area address 49 There is 1 active area address 49 ISIS level-1 ISIS is enabled on 2 interfaces Distance : 50 Maximum of 1 path per route Number of SPF runs, L1: 4, L2: 0

The output of show isis command in R2 is as shown in Listing 10.24.

Listing 10.24 Verification of System ID on R2

R2#show isis Global ISIS information ISIS process tag: R2 System ID: 1900.6500.4001 NET: 49.001.1900.6500.4001.00 Maximum number of areas: 2 There is 1 manual area address 49 There is 1 active area address 49 ISIS level-1 ISIS is enabled on 2 interfaces Distance : 150 Maximum of 1 path per route Number of SPF runs, L1: 14, L2: 10

If the middle fields in the NET are same, the System IDs are identical to 1900.6500.4001.

1. Reconfigure one of the routers with a different System ID using the net command and enable IS-IS protocol in the newly reconfigured router. Let us reconfigure R2 as shown in Listing 10.25.

Listing 10.25 Reconfiguration of R2

hostname R2 ! interface loopback0    ip address 190.65.4.1  255.255.255.255 ! interface e1     ip address 190.65.24.1 255.255.255.0     ip router isis ! router isis   passive-interface Loopback0   net 49.001.1900.6500.4002.00 !

In Listing 10.25, the code in bold shows the reconfigured System ID as 1900.6500.4002. The NET for R1 remains unchanged.

1. Verify the router configuration by using the show isis command to check whether both the routers have the same System ID. The output for the show isis command at R1 is shown in Listing 10.26.

Listing 10.26 Output for show isis Command at R1

R1#show isis Global ISIS information ISIS process tag: R1 System ID: 1900.6500.4001 NET: 49.001.1900.6500.4001.00 Maximum number of areas: 2 There is 1 manual area address 49 There is 1 active area address 49 ISIS level-1 ISIS is enabled on 2 interfaces Distance : 50 Maximum of 1 path per route Number of SPF runs, L1: 4, L2: 0

The output for show isis command at R2 is shown in Listing 10.27.

Listing 10.27 Output for show isis Command at R2

R2#show isis Global ISIS information ISIS process tag: R2 System ID: 1900.6500.4002 NET: 49.001.1900.6500.4002.00 Maximum number of areas: 2 There is 1 manual area address 49 There is 1 active area address 49 ISIS level-1 ISIS is enabled on 2 interfaces Distance : 150 Maximum of 1 path per route Number of SPF runs, L1: 14, L2: 10