the Router The router isis global configuration command enables Integrated IS-IS on the router. Optionally, you can apply a tag to identify multiple IS-IS processes. Just as multiple OSPF processes can be present on the same router, it is possible to have multiple IS-IS processes. The area-tag option is significant only to the local router. If it is omitted, the Cisco IOS software assumes a tag of 0. If more than one IS-IS process is used, the network plan should indicate which interfaces will participate in which IS-IS process. By default, Cisco IOS software makes the router a Level 1–2 router. Note
When configuring ISO CLNS (and not Integrated IS-IS), ensure that CLNS routing is enabled by entering the clns routing global configuration command. CLNS routing can also be configured at each interface. It is a good practice to have CLNS routing enabled on the router and the appropriate interfaces.
Content 4.4 Configuring Basic Integrated IS-IS 4.4.6 Step 3: Configure the NET After the Integrated IS-IS process is enabled, the router must be identified for IS-IS by assigning a NET to it with the net router configuration command. The NET is a combination of area number, a unique system ID number for each particular router, and the NSEL of 00 at the end. The area number must be at least 1 byte in length and can be as long as 13 bytes. The system ID has a fixed length of 6 bytes in Cisco routers and must be unique throughout each area (Level 1) and throughout the backbone (Level 2).
Content 4.4 Configuring Basic Integrated IS-IS 4.4.7 Step 4: Enable Integrated IS-IS on the Proper Interface The final step is to select which interfaces participate in IS-IS routing. Interfaces that use IS-IS to route IP (and therefore must establish IS-IS adjacencies) must be configured using the ip router isis [area-tag] interface configuration command. Enable Integrated IS-IS on the appropriate interfaces. Do not forget interfaces to stub IP networks, such as loopback interfaces (even though there are no CLNS neighbors on those interfaces). If there is more than one IS-IS process, the IS-IS process to which the interface belongs must be specified using the appropriate process name in the optional area tag field. If no area tag is listed, Cisco IOS software assumes an area-tag value of 0. If there is only one IS-IS process active on the router, no area-tag value is needed. Use the clns router isis [area-tag] interface command to enable the IS-IS routing process on an interface to support CLNS routing.
Content 4.4 Configuring Basic Integrated IS-IS 4.4.8 Simple Integrated IS-IS Example Figure displays an example of a simple Integrated IS-IS configuration for IP routing only. CLNS routing is not enabled. This configuration specifies only one IS-IS process, so the optional tag is not needed. The net command configures the router to be in area 49.0001 and assigns a system ID of 0000.0000.0002. IS-IS has been enabled on the FastEthernet 0/0 and Serial 0/0/1 interfaces. Because no level has been configured under the IS-IS routing process, this router acts as a Level 1–2 router by default. Interactive Media Activity Drag and Drop: Basic IS-IS Configuration Upon completion of this activity, the student will be able to perform a basic IS-IS configuration on a Cisco router.
Content 4.5 Optimizing IS-IS 4.5.1 Changing the IS-IS Router Type Optimizing IS-IS facilitates its smooth functioning and maximizes its efficiency. The default configuration of IS-IS results in the router having an IS type of Level 1–2. Although this configuration has the advantage of allowing all routers to learn of each other and pass routes without too much administrative oversight, it is not the most efficient way to build an IS-IS network. Routers with the default configuration send out both Level 1 and Level 2 hellos and maintain both Level 1 and Level 2 LSDBs. Each router should be configured to support the minimum level of routing required, which does the following: If a router is to operate only as an internal area router or a backbone router, specify this configuration with the is-type router configuration command. To specify that the router act as an internal area (Level 1)-only router, use the is-type level-1 command. To specify that the router act as a backbone (Level 2)-only router, use the is-type level-2-only command. If the level type has been changed from the default, and the router needs to return to acting as a Level 1–2 router, use the is-type level-1-2 command. Figure provides an example of configuring the IS-IS router type.
Content 4.5 Optimizing IS-IS 4.5.2 Changing the IS-IS Interface Level Although the router can be a Level 1–2 router, establishing both types of adjacencies over all interfaces may not be required. If a particular interface has only Level 1 routers connected to it, there is no need for the router to send Level 2 hellos out that interface. Similarly, if an interface has only Level 2 routers connected to it, there is no need for the router to send Level 1 hellos. Trying to establish adjacencies that do not exist wastes bandwidth and router resources. To make IS-IS more efficient in these types of situations, configure the interface to send only the needed type of hellos by using the isis circuit-type interface command and specifying either the level-1 or level-2-only keyword. If the circuit type is not configured, Cisco IOS software attempts to establish both types of adjacencies over the interface (Level 1–2). Figure provides an example of configuring the IS-IS interface level.
Content 4.5 Optimizing IS-IS 4.5.3 Changing the IS-IS Metric Unlike some other IP routing protocols, a Cisco router running IS-IS does not take into account line speed or bandwidth when it sets its link metrics. All interfaces are assigned a metric value of 10 by default. In a network with links of varying types and speeds, this assignment can result in suboptimal routing. To change the metric value, use the isis metric metric {level-1 | level-2} interface command. The metric can have different values for Level 1 and Level 2 over the same interface. Note
The IS-IS specification defines four types of metrics. Cost is the default metric and is supported by all routers. Delay, expense, and error are optional metrics. The delay metric measures transit delay, the expense metric measures the monetary cost of link utilization, and the error metric measures the residual error probability associated with a link. The default Cisco implementation uses cost only. However, you can set all four metrics using the isis metric command. If the metric value for all interfaces needs to be changed, each IS-IS interface needs to be changed individually. This can be time-consuming and error-prone, especially for routers with many IS-IS interfaces. The metric command changes the metric value for all IS-IS interfaces. If the keyword level-1 or level-2 is not entered, the metric is applied to both Level 1 and Level 2 IS-IS interfaces. This command is only available in Cisco IOS Software Release 12.3(4)T and later, and it supports the cost metric only. Figure provides an example of configuring the IS-IS metric.
Content 4.5 Optimizing IS-IS 4.5.4 Tuning IS-IS Configuration Example In Figure , there are two different areas. Area 49.0002 contains only one router (R3) and needs to do only Level 2 routing. It is appropriate to change the IS type of router R3 to Level 2.Area 49.0001 has two routers. R1 is strictly an internal area router; it does not