Misconfiguration Problems in IS-IS


Misconfiguration can be caused by manual or system errors. You need to take extreme care when configuring an IS-IS routing protocol in a router, because even a small configuration error can affect all network traffic.

Misconfiguration can occur if the NET configuration is inaccurate. As a result, the IS-IS network may not route as expected.

To discuss the misconfiguration problem, consider the example shown in Figure 10.2. In the figure, router R1 with System ID 190.65.4.1 is connected to router R2 with System ID 190.65.14.2 via an Ethernet interface, e1. R2 is connected to a third router, R3, with System ID 190.65.28.4 via a serial interface, s1.

click to expand
Figure 10.2: An IS-IS network depicting a NET misconfiguration scenario.

The configuration of R1 is shown in Listing 10.17.

Listing 10.17 Configuration of R1

start example
! hostname R1 ! interface loopback0    ip address 190.65.4.1  255.255.255.255 ! interface Ethernet1     ip address 190.65.24.1 255.255.255.0     ip router isis ! router isis   passive-interface Loopback0   net 49.001.1900.6500.4001.01 !
end example

In Listing 10.17, NET is configured incorrectly for R1. This is the misconfiguration. The last field of NET is 01; whereas, it should be 00 for an IS-IS router. The configuration of R2 is shown in Listing 10.18.

Listing 10.18 Configuration of R2

start example
! hostname R2 ! interface Loopback0     ip address 190.65.14.2 255.255.255.255 ! interface Ethernet1    ip address 190.65.24.2 255.255.255.0    ip router isis ! interface Serial1   ip address 190.65.28.4 255.255.255.253   ip router isis ! router isis    passive-interface Loopback0   net 49.001.1906.5001.4002.00 !
end example

The show commands are used to troubleshoot NET for R1 and R2. The output of the show isis command in R1 is shown in Listing 10.19.

Listing 10.19 Output of show isis Command at R1

start example
R1#show isis Global ISIS information ISIS process tag: R1 System ID: 1900.6500.4001 NET: 49.001.1900.6500.4001.01 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 : 250 Maximum of 1 path per route Number of SPF runs, L1: 14, L2: 0
end example

The output of the show isis command in R2 is shown in Listing 10.20.

Listing 10.20 Output of show isis Command at R2

start example
R2#show isis Global ISIS information ISIS process tag: R2 System ID: 1906.5001.4002 NET: 49.001.1906.5001.4002.00 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 : 250 Maximum of 1 path per route Number of SPF runs, L1: 14, L2: 0
end example

To troubleshoot the NET misconfiguration in an IS-IS network, check the NET field and calculate the NET address. For example, consider an IS with System ID 192.36.15.4 and area ID 49.002. The NET is calculated as shown in Figure 10.3.

click to expand
Figure 10.3: Calculation of NET value in IS-IS networks.

In Figure 10.2, the NET for R1 is calculated as shown:

  • Area ID: 49.001

  • IP address to be configured for R1 is 190.65.4.1

  • System ID => 190.065.004.001=>1900.6500.4001

  • NSEL = 00 for an IS-IS router

  • NET = AreaID.SystemID.NSEL

The NET for R2 is calculated as shown:

  • Area ID is 49.001

  • IP address to be configured for R2 is 190.65.14.2

  • System ID => 190.065.014.002=>1900.6501.4002

  • NSEL = 00 for an IS-IS router

  • NET = AreaID.SystemID.NSEL

From the calculations, it is clear that the System ID and NSEL are configured incorrectly for R1, and the System ID is configured incorrectly for R2. The NET for R1 and R2 should be configured correctly. The correct configuration for R1 is shown in Listing 10.21.

Listing 10.21 Correct Configuration for R1

start example
! hostname R1 ! 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. 1900.6500.4001.00 !
end example

In Listing 10.21, the change in configuration is shown in bold. The correct configuration for R2 is shown in Listing 10.22.

Listing 10.22 Correct Configuration of R2

start example
! hostname R2 ! interface Loopback0     ip address 190.65.14.2 255.255.255.255 ! interface e1    ip address 190.65.24.2 255.255.255.0    ip router isis ! interface s1   ip address 190.65.28.4 255.255.255.253   ip router isis ! router isis    passive-interface Loopback0   net 49.001.1900.6500.4001.00 !
end example

In Listing 10.22, the change in configuration is shown in bold.




Cisco IP Routing Protocols(c) Trouble Shooting Techniques
Cisco IP Routing Protocols: Trouble Shooting Techniques (Charles River Media Networking/Security)
ISBN: 1584503416
EAN: 2147483647
Year: 2006
Pages: 130

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