Table Of Contents
Enabling IS-IS and Assigning Areas
Enabling IP Routing for an Area on an Interface
Configuring IS-IS Interface Parameters
Configuring IS-IS Link-State Metrics
Setting the Advertised Hello Interval
Setting the Advertised CSNP Interval
Setting the Retransmission Interval
Setting the LSP Transmissions Interval
Setting the Retransmission Throttle Interval
Specifying Designated Router Election
Specifying the Interface Circuit Type
Assigning a Password for an Interface
Blocking Flooding on Specific Interfaces
Configuring Miscellaneous IS-IS Parameters
Configuring IS-IS Authentication Passwords
Changing the Routing Level for an Area
Tuning LSP Interval and Lifetime
Customizing IS-IS Throttling of LSP Generation, SPF Calculation, and PRC
Partial Route Computation (PRC)
Benefits of Throttling IS-IS LSP Generation, SPF Calculation, and PRC
How Throttling of IS-IS LSP Generation, SPF Calculation, and PRC Works
Modifying the Output of show Commands
IS-IS Multiarea Configuration for CLNS Network Example
Configuring Integrated IS-IS
This chapter describes how to configure Integrated Intermediate System-to-Intermediate System (IS-IS). For a complete description of the integrated IS-IS commands listed in this chapter, refer to the "Integrated IS-IS Commands" chapter of the Cisco IOS IP and IP Routing Command Reference publication. To locate documentation of other commands that appear in this chapter, use the command reference master index or search online.
IS-IS is an International Organization for Standardization (ISO) dynamic routing specification. IS-IS is described in ISO 10589. The Cisco implementation of IS-IS allows you to configure IS-IS as an IP routing protocol.
IS-IS Configuration Task List
To configure IS-IS, perform the tasks in the following sections. Enabling IS-IS is required; the remainder of the tasks are optional, although you might need to perform them, depending upon your specific application.
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Enabling IS-IS and Assigning Areas (Required)
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In addition, you can filter routing information and specify route redistribution. For more information about these features, see the "Filter Routing Information" and "Redistribute Routing Information" sections, respectively, in the "Configuring IP Routing Protocol-Independent Features" chapter of this document.
Enabling IS-IS and Assigning Areas
Unlike other routing protocols, enabling IS-IS requires that you create an IS-IS routing process and assign it to a specific interface, rather than to a network. You can specify more than one IS-IS routing process per Cisco router, using the multiarea IS-IS configuration syntax. You then configure the parameters for each instance of the IS-IS routing process.
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Small IS-IS networks are built as a single area that includes all the routers in the network. As the network grows larger, it is usually reorganized into a backbone area made up of the connected set of all Level 2 routers from all areas, which is in turn connected to local areas. Within a local area, routers know how to reach all system IDs. Between areas, routers know how to reach the backbone, and the backbone routers know how to reach other areas.
Routers establish Level 1 adjacencies to perform routing within a local area (intra-area routing). Routers establish Level 2 adjacencies to perform routing between Level 1 areas (interarea routing).
Some networks use legacy equipment that supports only Level 1 routing. These devices are typically organized into many small areas that cannot be aggregated due to performance limitations. Cisco routers are used to interconnect each area to the Level 2 backbone.
A single Cisco router can participate in routing in up to 29 areas, as well as perform Level 2 routing in the backbone. In general, each routing process corresponds to an area. By default, the first instance of the routing process configured performs both Level 1 and Level 2 routing. You can configure additional router instances, which are automatically treated as Level 1 areas. You must configure the parameters for each instance of the IS-IS routing process individually.
For IS-IS multiarea routing, you can configure only one process to perform Level 2 routing, although you can define up to 29 Level 1 areas for each Cisco router. If Level 2 routing is configured on any process, all additional processes are automatically configured as Level 1. You can configure this process to perform Level 1 routing at the same time. If Level 2 routing is not desired for a router instance, remove the Level 2 capability using the is-type command. Use the is-type command also to configure a different router instance as a Level 2 router.
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Network entity titles (NETs) define the area addresses for the IS-IS area and the system ID of the router. Refer to the "Configuring ISO CLNS" chapter in the Cisco IOS Apollo Domain, Banyan VINES, ISO CLNS, and XNS Configuration Guide for a more detailed discussion of NETs.
To enable IS-IS and specify the area for each instance of the IS-IS routing process, use the following commands beginning in global configuration mode:
See the "IS-IS Configuration Examples" section at the end of this chapter for examples of configuring IS-IS as an IP routing protocol.
Enabling IP Routing for an Area on an Interface
To enable IP routing and specify the area for each instance of the IS-IS routing process, use the following commands beginning in global configuration mode:
See the "IS-IS Configuration Examples" section at the end of this chapter for examples of configuring IS-IS as an IP routing protocol.
Configuring IS-IS Interface Parameters
The Cisco IS-IS implementation allows you to alter certain interface-specific IS-IS parameters. Most interface configuration commands can be configured independently from other attached routers. The isis password command should configure the same password on all routers on a network. The settings of other commands (isis hello-interval, isis hello-multiplier, isis retransmit-interval, isis retransmit-throttle-interval, isis csnp-interval, and so on) can be different on different routers or interfaces. However, if you decide to change certain values from the defaults, it makes sense to configure them on multiple routers and interfaces.
You can perform these tasks, described in the following sections:
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Configuring IS-IS Link-State Metrics
You can configure a cost for a specified interface. You can configure the default-metric for Level 1 or Level 2 routing. To configure the metric for the specified interface, use the following command in interface configuration mode:
isis metric default-metric {level-1 | level-2}
Configure the metric (or cost) for the specified interface.
Setting the Advertised Hello Interval
You can specify the length of time (in seconds) between hello packets that the Cisco IOS software sends on the interface. The hello interval multiplied by the hello multiplier equals the hold time.
To specify the length of time between hello packets for the specified interface, use the following command in interface configuration mode:
isis hello-interval {seconds | minimal} {level-1 | level-2}
Specify the length of time, in seconds, between hello packets the Cisco IOS software sends on the specified interface.
If the minimal keyword is specified, the hold time is 1 second and the system computes the hello interval based on the hello multiplier.
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.) Specify an optional level for X.25, Switched Multimegabit Data Service (SMDS), and Frame Relay multiaccess networks. X25, SMDS, ATM, and Frame Relay networks should be configured with point-to-point subinterfaces.Setting the Advertised CSNP Interval
Complete sequence number PDUs (CSNPs) are sent by the designated router to maintain database synchronization. You can configure the IS-IS CSNP interval for the interface.
To configure the CSNP interval for the specified interface, use the following command in interface configuration mode:
isis csnp-interval seconds {level-1 | level-2}
Configure the IS-IS CSNP interval for the specified interface.
This feature does not apply to serial point-to-point interfaces. It applies to WAN connections if the WAN is viewed as a multiaccess meshed network.
Setting the Retransmission Interval
You can configure the number of seconds between retransmission of IS-IS link-state packets (LSPs) for point-to-point links. To set the retransmission level, use the following command in interface configuration mode:
isis retransmit-interval seconds
Configure the number of seconds between retransmission of IS-IS LSPs for point-to-point links.
The value you specify should be an integer greater than the expected round-trip delay between any two routers on the attached network. The setting of this parameter should be conservative, or needless retransmission will result. The value should be larger for serial lines.
Setting the LSP Transmissions Interval
To configure the delay between successive IS-IS LSP transmissions, use the following command in interface configuration mode:
isis lsp-interval milliseconds
Configure the delay between successive IS-IS LSP transmissions.
Setting the Retransmission Throttle Interval
You can configure the maximum rate at which IS-IS LSPs will be re-sent on point-to-point links, in terms of the number of milliseconds between packets. This configuration is different from the retransmission interval, which is the amount of time between successive retransmissions of the same LSP.
The retransmission throttle interval is typically not necessary, except in cases of very large networks with high point-to-point neighbor counts. To set the retransmission throttle interval, use the following command in interface configuration mode:
isis retransmit-throttle-interval milliseconds
Configure the IS-IS LSP retransmission throttle interval.
Setting the Hello Multiplier
To specify 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 command. The default value is 3.
Specifying Designated Router Election
You can configure the priority to use for designated router election. Priorities can be configured for Level 1 and Level 2 individually.
To specify the designated router election, use the following command in interface configuration mode:
isis priority value {level-1 | level-2}
Configure the priority to use for designated router election.
Specifying the Interface Circuit Type
You can specify adjacency levels on a specified interface. This parameter is also referred to as the interface circuit type.
To specify the interface circuit type, use the following command in interface configuration mode:
isis circuit-type {level-1 | level-1-2 | level-2-only}
Configure the type of adjacency desired for neighbors on the specified interface (the interface circuit type).
Assigning a Password for an Interface
You can assign different passwords for different routing levels. Specifying Level 1 or Level 2 configures the password for only Level 1 or Level 2 routing, respectively. If you do not specify a level, the default is Level 1. By default, authentication is disabled.
To configure a password for the specified level, use the following command in interface configuration mode:
isis password password {level-1 | level-2}
Configure the authentication password for a specified interface.
Limiting LSP Flooding
Limiting LSP flooding is important to IS-IS networks in general, and is not limited to configuring multiarea IS-IS networks. In a network with a high degree of redundancy, such as a fully meshed set of point-to-point links over a nonbroadcast multiaccess (NBMA) transport, flooding of LSPs can limit network scalability. You can reduce LSP flooding in two ways:
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The advantage of full blocking over mesh groups is that it is easier to configure and understand, and fewer LSPs are flooded. Blocking flooding on all links permits the best scaling performance, but results in a less robust network structure. Permitting flooding on all links results in poor scaling performance.
The advantage of mesh groups over full blocking is that mesh groups allow LSPs to be flooded over one hop to all routers on the mesh, while full blocking allows some routers to receive LSPs over multiple hops. This relatively small delay in flooding can have an impact on convergence times, but the delay is negligible compared to overall convergence times.
Blocking Flooding on Specific Interfaces
You can completely block flooding (full blocking) on specific interfaces, so that new LSPs will not be flooded out over those interfaces. However, if flooding is blocked on a large number of links, and all remaining links go down, routers cannot synchronize their link-state databases even though there is connectivity to the rest of the network. When the link-state database is no longer updated, routing loops usually result.
To use CSNPs on selected point-to-point links to synchronize the link-state database, configure a CSNP interval using the isis csnp-interval command on selected point-to-point links over which normal flooding is blocked. You should use CSNPs for this purpose only as a last resort.
Configuring Mesh Groups
Configuring mesh groups (a set of interfaces on a router) can help to limit redundant flooding. All routers reachable over the interfaces in a particular mesh group are assumed to be densely connected (each router has many links to other routers), where many links can fail without isolating one or more routers from the network.
Normally, when a new LSP is received on an interface, it is flooded out over all other interfaces on the router. When the new LSP is received over an interface that is part of a mesh group, the new LSP will not be flooded out over the other interfaces that are part of that same mesh group.
Mesh groups rely on a full mesh of links between a group of routers. If one or more links in the full mesh goes down, the full mesh is broken, and some routers might miss new LSPs, even though there is connectivity to the rest of the network. When you configure mesh groups to optimize or limit LSP flooding, be sure to select alternative paths over which to flood in case interfaces in the mesh group go down.
To minimize the possibility of incomplete flooding, you should allow unrestricted flooding over at least a minimal set of links in the mesh. Selecting the smallest set of logical links that covers all physical paths results in very low flooding, but less robustness. Ideally you should select only enough links to ensure that LSP flooding is not detrimental to scaling performance, but enough links to ensure that under most failure scenarios no router will be logically disconnected from the rest of the network.
Configuring Miscellaneous IS-IS Parameters
The following tasks differ from the preceding interface-specific IS-IS tasks because they configure IS-IS itself, rather than the interface.
You can configure optional IS-IS parameters as described in the following sections:
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Generating 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 command generates a default route into IS-IS, which can be controlled by a route map. You can use the route map to identify the level into which the default route is to be announced, and you can specify other filtering options configurable under a route map. You can use a route map to conditionally advertise the default route, depending on the existence of another route in the routing table of the router.
To generate a default route, use the following command in router configuration mode:
default-information originate [route-map map-name]
Force a default route into the IS-IS routing domain.
See also the discussion of redistribution of routes in the "Configuring IP Routing Protocol-Independent Features" chapter of this document.
Specifying the System Type
You can configure the router to act as a Level 1 (intra-area) router, as both a Level 1 router and a Level 2 (interarea) router, or as an interarea router only.
To specify router level support, use the following command in router configuration mode:
is-type {level-1 | level-1-2 | level-2-only}
Configure the system type (area or backbone router).
Configuring IS-IS Authentication Passwords
You can assign passwords to areas and domains.
The area authentication password is inserted in Level 1 (station router level) LSPs, and the routing domain authentication password is inserted in Level 2 (area router level) LSPs.
To configure either area or domain authentication passwords, use the following commands in router configuration mode:
area-password password
Configure the area authentication password.
domain-password password
Configure the routing domain authentication password.
Summarizing Address Ranges
You can create aggregate addresses that are represented in the routing table by a summary address. This process is called route summarization. One summary address can include multiple groups of addresses for a given level. Routes learned from other routing protocols also can 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, use the following command in router configuration mode:
summary-address address mask {level-1 | level-1-2 | level-2}
Create a summary of addresses for a given level.
Setting the Overload Bit
You can configure the router to set the overload bit (also known as the hippity bit) in its nonpseudonode LSPs. Normally the setting of the overload bit is allowed only when a router runs into problems. For example, when a router is experiencing a memory shortage, it might be that the link-state database is not complete, resulting in an incomplete or inaccurate routing table. By setting the overload bit in its LSPs, other routers can ignore the unreliable router in their shortest path first (SPF) calculations until the router has recovered from its problems.
The result will be that no paths through this router are seen by other routers in the IS-IS area. However, IP and CLNS prefixes directly connected to this router will be still be reachable.
This command can be useful when you want to connect a router to an IS-IS network, but do not want real traffic flowing through it under any circumstances. Examples are:
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Unless you specify the on-startup keyword, this command sets the overload bit immediately and it remains set until the no set-overload-bit command is specified. If you specify the on-startup keyword, you must indicate whether it is set for a specified number of seconds or until BGP has converged. If BGP does not signal IS-IS that it is converged, IS-IS will turn off the overload bit after 10 minutes.
To set the overload bit, use the following command in router configuration mode:
Changing the Routing Level for an Area
You can change the routing level configured for an area using the is-type command. If the router instance has been configured for Level 1-2 area (the default for the first instance of the IS-IS routing process in a Cisco router), you can remove Level 2 (interarea) routing for the area using the is-type command and change the routing level to Level 1 (intra-area). You can also configure Level 2 routing for an area using the is-type command, but the instance of the IS-IS router configured for Level 2 on the Cisco router must be the only instance configured for Level 2.
To change the routing level for an IS-IS routing process in a given area, use the following command in router configuration mode:
Router(config-router)# is-type {level-1 | level-1-2 | level-2-only}
Configures the routing level for an instance of the IS-IS routing process.
Tuning LSP Interval and Lifetime
By default, the router sends a periodic LSP refresh every 15 minutes. LSPs remain in a database for 20 minutes by default. If they are not refreshed by that time, they are deleted. You can change the LSP refresh interval or the LSP lifetime. The LSP interval should be less than the LSP lifetime or else LSPs will time out before they are refreshed. If you misconfigure the LSP lifetime to be too low compared to the LSP refresh interval, the software will reduce the LSP refresh interval to prevent the LSPs from timing out.
To change the LSP refresh interval or lifetime, use the appropriate command in router configuration mode:
Customizing IS-IS Throttling of LSP Generation, SPF Calculation, and PRC
Partial Route Computation (PRC)
PRC is the software's process of calculating routes without performing an SPF calculation. This is possible when the topology of the routing system itself has not changed, but a change is detected in the information announced by a particular IS or when it is necessary to attempt to reinstall such routes in the RIB.
Benefits of Throttling IS-IS LSP Generation, SPF Calculation, and PRC
IS-IS throttles three main events: link-state PDU (LSP) generation, Shortest Part First (SPF) computation, and partial route computation (PRC). Throttling slows down the frequency of these events during times of network instability. Although throttling these events slows down network convergence, not throttling could result in a network not functioning. If network topology is unstable, throttling slows down the scheduling of these intervals until the topology becomes stable.
The throttling of LSP generation prevents flapping links from cause many LSPs to be flooded through the network. The throttling of SPF computation and PRC prevents the router from crashing from the demand of too many calculations.
How Throttling of IS-IS LSP Generation, SPF Calculation, and PRC Works
IS-IS throttling of LSP generation, SPF calculations, and PRC occurs by default. You can customize the throttling of these events with the lsp-gen-interval, spf-interval, and prc-interval commands, respectively.
The arguments in each command behave similarly. For each command:
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Each subsequent wait interval is twice as long as the previous one until the wait interval reaches the maximum wait time specified, upon which the wait interval remains constant. After the network calms down and there are no triggers for 2 times the maximum interval, fast behavior is restored (the initial wait time).
Other commands are available to control the delay between successive LSPs, the retransmission of the same LSA, and the retransmission of LSPs on a point-to-point interface.
Perform this task to customize throttling of LSP generation, SPF calculation, PRC, or any combination of the three, beginning in router configuration mode:
Modifying the Output of show Commands
To customize display output when the IS-IS multiarea feature is used, making the display easier to read, use the following command in EXEC mode:
Router# isis display delimiter [return cnt | char cnt]
Specifies the delimiter to be used to separate displays of information about individual IS-IS areas.
For example, the following command causes information about individual areas to be separated by 14 dashes (-) in the display:
isis display delimiter - 14The output for a configuration with two Level 1 areas and one Level 2 area configured is as follows:
dtp-5#show clns neighbors--------------Area L2BB:System Id Interface SNPA State Holdtime Type Protocol0000.0000.0009 Tu529 172.21.39.9 Up 25 L1L2 IS-IS--------------Area A3253-01:System Id Interface SNPA State Holdtime Type Protocol0000.0000.0053 Et1 0060.3e58.ccdb Up 22 L1 IS-IS0000.0000.0003 Et1 0000.0c03.6944 Up 20 L1 IS-IS--------------Area A3253-02:System Id Interface SNPA State Holdtime Type Protocol0000.0000.0002 Et2 0000.0c03.6bc5 Up 27 L1 IS-IS0000.0000.0053 Et2 0060.3e58.ccde Up 24 L1 IS-ISMonitoring IS-IS
To monitor the IS-IS tables and databases, use the following commands in EXEC mode:
IS-IS Configuration Examples
The section contains the following examples:
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Enabling IS-IS Example
The following example shows how to configure three routers to run IS-IS as an IP routing protocol. Figure 37 illustrates the example configuration.
Router A
router isisnet 49.0001.0000.0000.000a.00interface ethernet 0ip router isisinterface serial 0ip router isisRouter B
router isisnet 49.0001.0000.0000.000b.00interface ethernet 0ip router isisinterface ethernet 1ip router isisinterface serial 0ip router isisRouter C
router isisnet 49.0001.0000.0000.000c.00interface ethernet 1ip router isisinterface ethernet 2ip router isisFigure 37 IS-IS Routing
IS-IS Multiarea Configuration for CLNS Network Example
The following example shows a mulitarea IS-IS configuration with two Level 1 areas and one Level 1-2 area. Figure 38 illustrates this configuration.
clns routing...interface Tunnel529ip address 10.0.0.5 255.255.255.0clns router isis BBinterface Ethernet1ip address 10.1.1.5 255.255.255.0clns router isis A3253-01!interface Ethernet2ip address 10.2.2.5 255.255.255.0clns router isis A3253-02...router isis BB ! Defaults to "is-type level-1-2"net 49.2222.0000.0000.0005.00!router isis A3253-01net 49.0553.0001.0000.0000.0005.00is-type level-1!router isis A3253-02net 49.0553.0002.0000.0000.0005.00is-type level-1Figure 38 Multiarea IS-IS Configuration with Three Level 1 Areas and One Level 2 Area
IS-IS Throttle Timers Example
This example shows a system configured with IS-IS throttling of LSP generations, SPF calculations and PRC:
router isisspf-interval 5 10 20prc-interval 5 10 20lsp-gen-interval 2 50 100
