Configuring Cisco Routers for Appropriate Integrated IS-IS Operation

To enable the IS-IS process, simply perform the following tasks while in global configuration mode:

 Router50B(config)# router isis [tag] 

This will start IS-IS routing and specify an IS-IS process for IP, placing you in router configuration mode. The [tag] is optional and is used to supply some unique and descriptive name for the routing process if more than one IS-IS process is configured on the router.

Your next step is to configure the NET for the routing process. The exact syntax for the net command is net network-entity-title . There is no network command as there is in IP. If you need to route CLNS packets as well, you also use the clns router isis [ tag ] interface configuration command. The parameter network-entity-title designates the area address and SysID for the IS-IS routing process. You can designate a name for a NET, as well as an address, with the following command:

 Router50B(config-router)#  net   00.000x.xyxy.xyxy.xyxy.00  

The next step in the process is to go into interface configuration mode by using the following:

 Router50B(config-router)#  interface   type number  

For example:

 Router50B(config-router)# int S1 

Finally, you specify which interfaces will actively route IS-IS by using the following command in interface configuration mode:

 Router50B(config-if)# ip router isis [tag] 

Cisco enables both the Level 1 and Level 2 routing operations by default on routers running IS-IS. You can designate a router to run only Level 1 or Level 2 operations with the following router configuration commands:

 Router50B(config-router)#  is-type  {  level-1level-1-2level-2-only  } 

If you use the level-1 parameter, the IS functions solely as a "station" router for intra-area routing. It uses the nearest L1L2 router for inter-area routing. The is-type parameter level-1-2 is the default setting and tells the IS to operate as both a Level 1 station and an area router. As you have learned, this IS runs two instances of the link-state algorithm and database. The third option is to make the router an area router only as a part of the IS-IS backbone. It does not communicate with station routers within its own area. This is how you could configure the L2 router in Area-52, as shown in Figure 6.5.

The Cisco IS-IS implementation enables you to modify several interface-specific IS-IS parameters. Most interface configuration commands can be configured separately from other attached routers. You can perform the following tasks, described in the accompanying sections:

• Configure the IS-IS circuit type

• Configure the IS-IS link-state metrics

• Set the advertised Hello Interval

• Set the Retransmission Interval

• Set the LSP Transmissions Interval

• Set the Hello Multiplier

• Specify designated router election

• Assign a password for an interface

• Generate a default route

• Summarize address ranges

Configure the IS-IS Circuit Type

Even though your IS-IS router is functioning as a L1L2 router, you might need to control the type of adjacencies that are established on a per-interface basis. Remember that the default Cisco behavior is to function as an L1L2 router and attempt to generate both types of adjacencies, if possible. Here is the syntax to use in this situation:

 Router50B(config-if)# isis circuit-type {level-1level-1-2level-2-only} 

Configure the IS-IS Link-State Metrics

You can configure a cost for a specified interface by setting the default-metric for Level 1 or Level 2 routing. To configure the metric (or cost) for the specified interface, use the following command in interface configuration mode:

 Router50B(config-if)# isis metric default-metric {level-1  level-2} 

Set the Advertised Hello Interval

You can designate the amount of time (in seconds) between hello packets that are sent on the interface. The hello interval can be configured independently for Level 1 and Level 2, except on serial point-to-point interfaces. (Because there is only a single type of hello packet sent on serial links, it is independent of Level 1 or Level 2.)

Remember that Cisco recommends that your frame relay networks be configured with point-to-point subinterfaces as a best practice.

To specify the length of time, in seconds, between the hello packets that are sent on the specified interface, enter the following command in interface configuration mode:

 Router50B(config-if)# isis hello-interval seconds {level-1  level-2} 

Set the Retransmission Interval

You can configure the number of seconds between retransmission of IS-IS link-state PDUs (LSPs) for point-to-point links. The value you choose should be an integer greater than the expected round-trip delay between any two ISs on the attached network. This parameter setting should be conservative, or unnecessary retransmissions will occur. The value should also be larger for serial lines. To configure the number of seconds between retransmission of IS-IS LSPs for point-to-point links, perform the following task in interface configuration mode:

 Router50B(config-if)# isis retransmit-interval seconds 

Set the LSP Transmissions Interval

To configure the delay between successive IS-IS link-state packet transmissions, you can perform the following task in interface configuration mode:

 Router50B(config-if)# isis lsp-interval milliseconds 

Set the Hello Multiplier

To designate the number of IS-IS hello packets a neighbor must miss before the router should declare the adjacency as "down," use the following command in interface configuration mode (the default value is 3):

 Router50B(config-if)# isis hello-multiplier multiplier {level-1  level-2} 

Specify Designated Router Election

You can configure the priority used for designated router (DIS) election. Priorities can be configured separately for Level 1 and Level 2. To specify the designated router election, perform the following task in interface configuration mode:

 Router50B(config-if)# isis priority value {level-1  level-2} 

Assign a Password for an Interface

You can assign different passwords for different routing levels as well. Specifying level-1 or level-2 configures the password for only Level 1 or Level 2 routing, respectively. If you do not designate a particular level, then the default is level-1 . Authentication is automatically disabled. To configure a password for the specified level, perform the following task in interface configuration mode:

 Router50B(config-if)# isis password password {level-1  level-2} 

Generate a Default Route

You can force a default route into an IS-IS routing domain. Whenever you specifically configure redistribution of routes into an IS-IS routing domain, the Cisco IOS software does not, by default, redistribute the default route into the IS-IS routing domain. The following feature enables you to force the boundary router to redistribute the default route or generate a default route into its L2 LSP. You can also use a route-map to advertise the default route conditionally, depending on the existence of another route in the router's routing table. To generate a default route, perform the following task in router configuration mode:

 Router50B(config-router)# default-information originate [route-map map-name] 

Summarize Address Ranges

You can create aggregate addresses that are represented in the routing table by a summary address. As you are aware, this process is called route summarization (or aggregation). One summary address can include multiple groups of addresses for a given level. Routes learned from other routing protocols can also be summarized. The metric used to advertise the summary is the smallest metric of all the more specific routes. To create a summary of addresses for a given level, perform the following task in router configuration mode:

 Router50B(config-router)# summary-address address mask {level-1  level-1-2  level-2} 

The Cisco IOS will try to summarize the area ID as much as and whenever it is feasible . If an IS-IS network was broken up into several subdivisions of a major area, then the area ID assignments reflect this design. For example, between the sub-areas the router routes according to the entire area ID, and between the major areas, the router summarizes the area ID up to the major boundary.

A Sample Dual Area Integrated IS-IS Environment

For a simple real-world configuration example, you can look at a subset of the internetwork shown in Figure 6.5. In Figure 6.6, we have an Integrated IS-IS scenario with an IS-IS process and NET configuration on a router serving as an IP-only L1L2 router.

Figure 6.6. A simple Integrated IS-IS scenario between two areas.

Listing 6.1 shows a possible Integrated IS-IS configuration for Router50-2. It is a part of the area 49.0050, but it also links to the area 49.0051 through neighbor Router51-1 on an Ethernet connection. In this example, both Router50-2 and Router51-1 are L1L2 ISs, which is the default setting for the is-type command, so we do not need to include this in the router configurations.

Listing 6.1 The Configuration of Router50-2 As an L1l2 IS in Area 49.0050

 Router(config)# hostname Router50-2 Router50-2(config)# int e0 Router50-2(config-if)# ip address 192.168.100.2 255.255.255.0 Router50-2(config-if)# ip router isis Router50-2(config-if)# isis circuit-type level-2-only ! Router50-2(config)# int S1 Router50-2(config-if)# ip address 192.168.200.2 255.255.255.0 Router50-2(config-if)# ip router isis Router50-2(config-if)# isis circuit-type level-1 ! Router50-2(config)# router isis Router50-2(config-router)# net 49.0050.1921.6820.0002.00 

In Listing 6.1, we have configured IP for the Ethernet and Serial interfaces and enabled the Integrated IS-IS process. On the Ethernet 0 interface link with the neighboring L1L2 router, we also used the isis circuit-type command to designate that only Level 2 adjacency should be established with Router51-1. On the Serial 1 interface, we are using the isis circuit-type command to specify that a Level 1 adjacency is generated only with Router50-1 (see Figure 6.6). We could have issued the isis metric command as well to configure different metric values for the Ethernet and Serial interfaces, if necessary.

We don't want to forget the Level 1 router configuration for Router50-1. Listing 6.2 includes a sample configuration for that station router in Area 49.0050, where the is-type parameter is set to level-1 :

Listing 6.2 The Configuration of Router50-1 As an L1 IS in Area 49.0050

 Router(config)# hostname Router50-1 Router50-1(config)# int s1 Router50-1(config-if)# ip address 192.168.200.1 255.255.255.0 Router50-1(config-if)# ip router isis ! Router50-1(config)# router isis Router50-1(config-router)# net 49.0050.1921.6820.0001.00 Router50-1(config-router)# is-type level-1 

Verifying and Troubleshooting IS-IS

Many commands are available for troubleshooting and verifying Integrated IS-IS. As you might have guessed, even if the environment is IP-only, you have to inspect CLNS information such as IS-IS neighbor adjacencies. The show clns command displays general information concerning your CLNS network, and the show clns protocol command can be issued at the EXEC-mode prompt, as well, to find out information about specific IS-IS processes on your router. Listing 6.3 shows the output for Router50-2 in Figure 6.6.

Listing 6.3 The show clns protocol Command

 Router50-2# show clns protocol IS-IS Router: <Null Tag>   System Id: 1921.6820.0002.00  IS-Type: level-1-2   Manual area address(es):         49.0050   Routing for area address(es):         49.0050   Interfaces supported by IS-IS:          Serial1  IP          Ethernet0  IP   Redistributing:     Static   Distance: 110 

The show clns interface command can be utilized to show individualized information on an interface-by-interface basis. For example, Listing 6.4 represents the output of this command on the Serial 1 interface of Router50-2.

Listing 6.4 The show clns interface Command Output

 Router50-2# show clns interface s1 Serial1 is up, line protocol is up   Checksums enabled, MTU 1500, Encapsulation HDLC   ERPDUs enabled, min. interval 10 msec.   RDPDUs enabled, min. interval 100 msec., Addr Mask enabled   Congestion Experienced bit set at 4 packets   CLNS fast switching enabled   CLNS SSE switching disabled   DEC compatibility mode OFF for this interface   Next ESH/ISH in 18 seconds     Routing Protocol: IS-IS       Circuit Type: level-1       Interface number 0x1, local circuit ID 0x100       Level-1 Metric: 10, Priority: 64, Circuit ID: 1921.6820.0002.00       Number of active level-1 adjacencies: 1       Next IS-IS Hello in 3 seconds 

You can also use the show ip route isis command to see just the IS-IS routes from the IP routing table. For instance, the output for Router50-2 would look like so:

 Router50-2# show ip route isis 1*L1 0.0.0.0/0 [115/10] via 192.168.100.2, Serial1 

Notice that the IP Administrative Distance for IS-IS routes is a default value of 115. Another EXEC-mode command that can come in handy is the show clns command. This command shows you how many interfaces have CLNS enabled, as well as the router IS level and area for which it is routing. The show isis route EXEC-mode command displays IS-IS Level 1 routes to all the other intra-area systems, and the show clns route command shows the IS-IS Level 2 routing table. If you wanted to get some quick neighbor information about your neighbors (similar to the show cdp nei command), you could issue the show clns neighbors or the show clns is-neighbors commands to discover information such as the number of neighbors, neighbor SNPA, state, hold time data, and neighbor level types. You can view the contents of the IS-IS link-state database with the show isis database EXEC-mode command. The show isis spf-log command shows you how often as well as the reasons that a router has run a full SPF calculation session.

Finally, you are likely to use a few of the debug commands on a very limited basis because of the router overhead and effect on other processes. However, the debug isis adj-packets verification command is used to print out IS-IS adjacency- related packets to your console. In addition, use the debug isis update-packets command to output any IS-IS update packets to your console session.