Case Study | Cisco IP Routing Protocols: Trouble Shooting Techniques (Charles River Media Networking/Security)

Consider an IS-IS network in which there are three ISs: R1, R2, and R3. This scenario is depicted in Figure 10.6

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Figure 10.6: An IS-IS network scenario.

In Figure 10.6, R1 is connected to R2 using fast Ethernet, fe1. R2 is connected to R3 using interface pos3. The IP addresses of the interfaces are shown in the figure. The IP addresses of the three routers are listed in Table 10.5.

Table 10.5: IP Addresses of IS-IS Routers
Router	IP Address
R1	172.32.1.1
R2	172.32.2.1
R3	172.32.3.1

The following problems need to be solved during configuration of individual routers:

Redistribution of routing information among R1, R2, and R3.
R1 and R2 are configured with the same System ID.

Redistribution of Routing Information

In the configuration of the routers, the IP-redistribution policy needs to be established so that redistribution of routing information takes place correctly within the same routing domain. The command for redistribution configuration in R1 is shown in Listing 10.31.

Listing 10.31 Redistribution Configuration in R1

interface create ip fe1/1 address-netmask 14.2.2.8/24 port et.1.1 interface create ip fe2/1 address-netmask 172.32.1.1/24 port gi.3.1 ! isis add area 49.0001 isis add interface fe1/1 isis add interface fe2/1 isis set system-id 0000.0000.0001 isis start ! ip-router policy redistribute from-proto direct to-proto isis-level-1 ! system set name R1

In Listing 10.31, for R1, fe1 and fe2 represent the fast Ethernet interfaces that connect R1 to R2, or to any other router in the network. To configure R1 in the IS-IS network:

Add the R1 area to the IS-IS network after creating interfaces and assigning IP addresses.
Add interfaces to the area, which is added currently. In R1, interfaces fe1 and fe2 are added to Area 49.001.
Configure the System ID of R1 as 0000.0000.0001.
Enable the IS-IS routing protocol in R1 by using the isis start command.
Set R1 for redistribution by using the ip-router policy redistribute command.
Set the name R1 by using the system set name command.

The configuration of R1 is complete. R2 and R3 follow similar steps for the configuration in the IS-IS network. The command for redistribution configuration in R2 is shown in Listing 10.32.

Listing 10.32 Redistribution Configuration in R2

interface create ip pos3/1 address-netmask 15.1.1.1/30 port so.3.1 interface create ip fe1/1 address-netmask 14.2.2.2/24 port et.1.1 interface create ip gi2/1 address-netmask 172.32.2.1/24 port gi.2.1 ! isis add area 49.0002 isis add interface pos3/1 isis add interface fe1/1 isis add interface gi2/1 isis set system-id 0000.0000.0002 isis start ! ip-router policy redistribute from-proto direct to-proto isis-level-1 ! system set name R2

The command for redistribution configuration in R3 is shown in Listing 10.33.

Listing 10.33 Redistribution Configuration in R3

interface create ip pos3/1 address-netmask 15.1.1.2/30 port so.3.1 interface create ip fe1/1 address-netmask 172.32.3.1/24 port et.1.1 ! isis add area 49.0003 isis add interface pos3/1 isis set system-id 0000.0000.0003 isis start ! ip-router policy redistribute from-proto direct to-proto isis-level-1 ! system set name R3

R1 and R2 Configured with Same System ID

Consider that in Figure 10.6, the configuration of R1 is as shown in Listing 10.34.

Listing 10.34 R1 Configuration

interface create ip fe1/1 address-netmask 14.2.2.8/24 port et.1.1 interface create ip fe2/1 address-netmask 172.32.1.1/24 port gi.3.1 ! isis add area 49.0001 isis add interface fe1/1 isis add interface fe2/1 isis set system-id 1720.3200.1001 isis start ! ip-router policy redistribute from-proto direct to-proto isis-level-1 ! system set name R1

Consider that in Figure 10.6, the configuration of R2 is as shown in Listing 10.35.

Listing 10.35 R2 Configuration

interface create ip pos3/1 address-netmask 15.1.1.1/30 port so.3.1 interface create ip fe1/1 address-netmask 14.2.2.2/24 port et.1.1 interface create ip gi2/1 address-netmask 172.32.2.1/24 port gi.2.1 ! isis add area 49.0002 isis add interface pos3/1 isis add interface fe1/1 isis add interface gi2/1 isis set system-id 1720.3200.1001 isis start ! ip-router policy redistribute from-proto direct to-proto isis-level-1 ! system set name R2

The System ID for both routers is 1720.3200.1001. R1 and R2 will send LSPs to each other with the same System ID. While trying to establish adjacency, both R1 and R2 will not be able to distinguish each other’s LSPs. No adjacency will be established as a result of this misconfiguration.

To troubleshoot this problem, configure R1, R2, and R3 with three distinct System IDs. The new configuration of R1 is as shown in Listing 10.36.

Listing 10.36 New Configuration of R1

interface create ip fe1/1 address-netmask 14.2.2.8/24 port et.1.1 interface create ip fe2/1 address-netmask 172.32.1.1/24 port gi.3.1 ! isis add area 49.0001 isis add interface fe1/1 isis add interface fe2/1 isis set system-id 0000.0000.0001 isis start ! ip-router policy redistribute from-proto direct to-proto isis-level-1 ! system set name R1

The new configuration of R2 is as shown in Listing 10.37.

Listing 10.37 New Configuration of R2

interface create ip pos3/1 address-netmask 15.1.1.1/30 port so.3.1 interface create ip fe1/1 address-netmask 14.2.2.2/24 port et.1.1 interface create ip gi2/1 address-netmask 172.32.2.1/24 port gi.2.1 ! isis add area 49.0002 isis add interface pos3/1 isis add interface fe1/1 isis add interface gi2/1 isis set system-id 0000.0000.0002 isis start ! ip-router policy redistribute from-proto direct to-proto isis-level-1 ! system set name R2

The new configuration of R3 is as shown in Listing 10.38.

Listing 10.38 New Configuration of R3

interface create ip pos3/1 address-netmask 15.1.1.2/30 port so.3.1 interface create ip fe1/1 address-netmask 172.32.3.1/24 port et.1.1 ! isis add area 49.0003 isis add interface pos3/1 isis set system-id 0000.0000.0003 isis start ! ip-router policy redistribute from-proto direct to-proto isis-level-1 ! system set name R3