IS-IS Multiarea Support [IP Routing]

Table Of Contents

IS-IS Multiarea Support

Feature Overview

Redistribution

TARP

Benefits

Restrictions

Related Features and Technologies

Related Documents

Supported Platforms

Supported MIBs and RFCs

Configuration Tasks

Configuration Tasks for Multiarea IS-IS

Verifying Multiarea IS-IS Configuration

Monitoring and Maintaining Multiarea IS-IS

Configuration Example

Multiarea IS-IS Configuration

Command Reference

clns router isis

ip router isis

isis display delimiter

isis mesh-group

is-type

net

partition avoidance

router isis

Show Commands

show clns neighbor areas

show clns route

show isis topology

Glossary

IS-IS Multiarea Support

Feature History

Release

Modification

12.0(5)T

This feature was introduced.

Feature Overview

This feature module describes multiarea Intermediate System-to-Intermediate System (IS-IS) support for ISO Connectionless Network Service (CLNS). It also describes how to use the modified isis mesh-group command to limit link-state packet (LSP) flooding, a technique that is related to IS-IS configuration in general and is not limited to configuring multiarea IS-IS networks. 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.

Previously, each Cisco router could participate in only one area, performing Level 1 (intra-area) routing locally, and Level 2 (interarea) routing to other areas in the network. This limitation meant that when the network was divided into a large number of Level 1 areas, a correspondingly large number of Level 1/2 routers was required to route between all areas, as shown in the example in Figure 1. The need for redundancy increases the number of routers needed.

With this feature release, Cisco IOS software supports configuration of multiple Level 1 areas within a single router.

Figure 1 represents a telco network used to monitor the status of the switching equipment in multiple remote central offices (where the telco equipment resides) from a central monitoring point.

In this example, the CLNS network of the telco is being used to monitor the status of the switching equipment. The CLNS network is not used to perform actual circuit switching (end users making voice or data calls).

Figure 1

IS-IS Network for Monitoring Remote Equipment Status

Figure 2 shows the same network reconfigured using multiarea IS-IS. The number of local access routers has been reduced. Each router continues to provide access to the backbone, but also participates in multiple Level 1 areas.

A more powerful area router platform may be required to handle the increased load, but fewer routers are required at the access level. In this example a 3:1 reduction in the number of routers required is shown. However, if you configure more Level 1 areas on each multiarea router, further reduction in the number of routers required is possible.

Figure 2

Multiarea IS-IS Network for Monitoring Remote Equipment Status

Redistribution

IP routes learned via Level 1 routing are advertised by default into Level 2. Even when multiple Level 1 processes are configured on the same unit, this fact is still true. No additional configuration is required to redistribute all Level 1 IP routes into the Level 2 process.

In CLNS routing there is no redistribution of Level 1 host routes into Level 2. Only Level 1 addresses are advertised into Level 2. Redistribution of all area addresses of all Level 1 areas into Level 2 is implicit in IS-IS, and no additional configuration is required for this redistribution.

Redistribution of other protocols will continue to work with conventional IS-IS configurations as it does in Release 12.0.

TARP

For a conventional IS-IS configuration with a single Level 1 and a Level 2 area (or configuration with a single Level 1 area or a Level 2 area), Target Address Resolution Protocol (TARP) continues to work without any change in previous behavior.

•If multiple Level 1 areas are defined, the router resolves addresses using TARP in the following way:

•The the router obtains the Network Service Access Point (NSAP) of the Level 2 area, if present, from the locally assigned target identifier.

•If only Level 1 areas are configured, the router uses the NSAP of the first active Level 1 area as shown in the configuration at the time of TARP configuration ("tarp run"). (Level 1 areas are sorted alphanumerically by tag name, with capital letters coming before lowercase letters. For example, AREA-1 precedes AREA-2, which precedes area-1.) Note that the target identifier NSAP could change following a reload if a new Level 1 area is added to the configuration after TARP is running.

•The router continues to process all Type 1 and 2 protocol data units (PDUs) that are for this router. Type 1 PDUs are processed locally if the target identifier is in the local target identifier cache. If not, they are "propagated" (routed) to all interfaces in the same Level 1 area. (The same area is defined as the area configured on the input interface.)

•Type 2 PDUs are processed locally if the specified target identifier is in the local target identifier cache. If not, they are propagated via all interfaces (all Level 1 or Level 2 areas) with TARP enabled. If the source of the PDU is from a different area, the information is also added to the local target identifier cache. Type 2 PDUs are propagated via all static adjacencies.

•Type 4 PDUs (for changes originated locally) are propagated to all Level 1 and Level 2 areas (because internally they are treated as "Level 1-2").

•Type 3 and 5 PDUs continue to be routed.

•Type 1 PDUs are only "propagated" (routed) via Level 1 static adjacencies if the static NSAP is in one of the Level 1 areas in this router.

Benefits

Fewer Routers Required to Support the Same Number of Areas

The IS-IS Multiarea Support feature makes it possible for one Cisco router to support multiple Level 1 areas, as opposed to the single Level 1 area previously supported by each router.

Network Scaling

A single Cisco router can now participate in routing in up to 29 areas, as well as perform Level 2 (interarea) routing in the backbone. Expansion of an IS-IS network consisting of many small areas is simpler that previously, because you can configure multiple Level 1 areas in the same Cisco unit, without needing to add and configure physical units for each additional local area.

Connectivity for Local Level 1 Areas on the Same Router

•This feature also provides connectivity between Level 1 areas local to the router. Previously Level 1 areas could only to be connected using the Level 2 backbone.

Restrictions

Configuration Limitations

•This feature supports routing for at most one Level 2 area per router, although you can define up to 29 Level 1 areas for each Cisco unit.

•An interface cannot be part of more than one Level 1 or one Level 2 area per router. You can, however, configure subinterfaces as Level 1 areas when subinterfaces can be used, such as with WAN media.

•The IS-IS Multiarea Support feature is supported only for ISO CLNS.

You can configure only one process to perform Level 2 (interarea) routing. 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 intra-area (Level 1) routing at the same time. You can configure up to 29 additional processes as Level 1-only processes.

Redistribution

Explicit redistribution between IS-IS areas cannot be configured. Redistribution from any other routing protocol into a particular area is possible, and is configured per router instance, as in Release 12.0 of the Cisco IOS software, using the redistribute and route map commands. By default, redistribution is into Level 2.

External Area Merge

The IS-IS protocol allows you to merge areas by configuring multiple network entity titles (NETs) on a router. If these NETs define a router to be in both Area A and Area B, for example, the router can potentially merge areas A and B. The result of the merge will be one Level 1 area with two area addresses: A and B. All routers in this merged area must have NSAPs with unique system IDs.

Distinct areas defined in a multiarea router cannot share a common area address. This means that all routers in a single area must have unique system IDs. However, it is possible for two areas to be "merged" external to the router, such as when a common area number is introduced by other routers in the two areas. When this happens, the areas that now have a common area number are said to be "merged" into a single area.

If this happens, the multiarea router will see its own area data from two different areas on a single interface, and the router will display the CLNS-DUPSYSTEM warning message.

The error is a configuration error and must be corrected manually.

Related Features and Technologies

The IS-IS multiarea feature is an extension of integrated IS-IS and ISO CLNS. Integrated IS-IS is documented in Network Protocols Configuration Guide, Part 1, and the Network Protocols Command Reference, Part 1. ISO CLNS is documented in Cisco IOS Release 12.0 Network Protocols Configuration Guide, Part 3, and the Cisco IOS Release 12.0 Network Protocols Command Reference, Part 3.

Related Documents

For related information on this feature, refer to the following documents:

•Cisco IOS Release 12.0 Network Protocols Configuration Guide, Part 1

•Cisco IOS Release 12.0 Network Protocols Command Reference, Part 1

•Cisco IOS Release 12.0 Network Protocols Configuration Guide, Part 3

•Cisco IOS Release 12.0 Network Protocols Command Reference, Part 3

Supported Platforms

•Cisco 2500 series

•Cisco 2600 series

•Cisco 3600 series

•Cisco 4500 series

•Cisco 7200 series

•Cisco 7500 series

Supported MIBs and RFCs

None.

Configuration Tasks

There is no change to the configuration process for configuring conventional IS-IS with ISO CLNS:

•The configuration procedure for configuring conventional IS-IS with ISO CLNS is described in the "Configuring ISO CLNS" chapter of the Cisco IOS Release 12.0 Network Protocols Configuration Guide, Part 3.

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 unit, using the multiarea IS-IS configuration syntax. You then configure the parameters for each instance of the IS-IS routing process.

In general, each routing process corresponds to an area. By default, the first instance of the routing process configured performs both intra-area (Level 1) and Level 2 (interarea) 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.

You can configure at most only one IS-IS routing process to perform Level 2 (interarea) routing. 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 to configure a different router instance as a Level 2 router.

Configuration Tasks for Multiarea IS-IS

To configure multiarea IS-IS in integrated mode, or ISO CLNS, complete the tasks in the following sections. Enabling IS-IS and ISO CLNS is required; the remainder of the tasks in the following list are optional (although you might need to perform them, depending upon your specific application):

• Enabling IS-IS and Assigning Areas (Required)

• Enabling CLNS Routing for an Area on an Interface (Required for CLNS)

• Modifying the Output of show Commands (Optional)

• Enabling Partitioning Avoidance (Optional, but recommended)

• Changing the Routing Level for an Area (Optional)

• Limiting LSP Flooding (Optional)

• Configuring Other IS-IS Parameters (Optional)

Limiting LSP flooding is described in this document. The other tasks are documented in Cisco IOS Release 12.0 Network Protocols Configuration Guide, Part 1 and Cisco IOS Release 12.0 Network Protocols Configuration Guide, Part 3.

Enabling IS-IS and Assigning Areas

To enable IS-IS and specify the area for each instance of the IS-IS routing process, use the following commands, starting in global configuration mode:
Step

Command

Purpose
Step 1
Router# router isis [area tag]
Enables IS-IS routing for the specified routing process, and places you in router configuration mode.

Use the area tag argument to identify the area to which this IS-IS router instance is assigned. A value for tag is required if you are configuring multiple IS-IS areas.

The first IS-IS instance configured is Level 1-2 by default. Later instances are automatically Level 1. You can change the level of routing to be performed by a particular routing process using the is-type command.
Step 2
Router(config)# net network-entity-title
Configures NETs for the routing process. Specify an NET for each routing process if you are configuring multiarea IS-IS. You can specify a name for a NET and for an address.
Enabling CLNS Routing for an Area on an Interface

To enable CLNS routing and specify the area for each instance of the IS-IS routing process, use the following commands, starting in global configuration mode:
Step

Command

Purpose
Step 1
Router# interface type number
Enters interface configuration mode.
Step 2
Router(config-if)# clns router isis [area 
tag]
Specifies that the interface actively routing IS-IS when the network protocol is ISO-CLNS, and identifies the area associated with this routing process on this interface.
Step 3
Router(config-if)# ip address ip-address-mask
Defines the IP address for the interface.

An IP address is required on all interfaces in an area enabled for IS-IS if any one interface is configured for IS-IS routing.
Modifying the Output of show Commands

To customize display output when the multiarea feature is used, making the display easier to read, use the following EXEC command:
Command

Purpose
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 - 14
The 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 Protocol
0000.0000.0009 Tu529       172.21.39.9         Up     25        L1L2 IS-IS
--------------
Area A3253-01:
System Id      Interface   SNPA                State  Holdtime  Type Protocol
0000.0000.0053 Et1         0060.3e58.ccdb      Up     22        L1   IS-IS
0000.0000.0003 Et1         0000.0c03.6944      Up     20        L1   IS-IS
--------------
Area A3253-02:
System Id      Interface   SNPA                State  Holdtime  Type Protocol
0000.0000.0002 Et2         0000.0c03.6bc5      Up     27        L1   IS-IS
0000.0000.0053 Et2         0060.3e58.ccde      Up     24        L1   IS-IS
Enabling Partitioning Avoidance

In ISO CLNS networks using a redundant topology, it is possible for an area to become "partitioned" when full connectivity is lost between a Level 1-2 border router, all adjacent Level 1 routers, and end hosts. In such a case, multiple Level 1-2 border routers advertise the Level 1 area prefix into the backbone area, even though any one router can reach only a subset of the end hosts in the Level 1 area.

When enabled, the partition avoidance command prevents this partitioning by causing the border router to stop advertising the Level 1 area prefix into the Level 2 backbone.

Other cases of connectivity loss within the Level 1 area itself are not detected or corrected by the border router, and this command has no effect.

To enable partitioning avoidance, use the following router configuration command:
Command

Purpose
Router(config)# partition avoidance
•Causes an IS-IS Level 1-2 border router to stop advertising the Level 1 area prefix into the Level 2 backbone when full connectivity is lost between the border router, all adjacent Level 1 routers, and end hosts.
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 unit), 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 unit 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 router configuration command:
Command

Purpose
Router (config)# is-type {level-1 | 
level-1-2 | level-2-only}
•Configures the routing level for an instance of the IS-IS routing process.
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 non-broadcast multiAccess (NBMA) transport, flooding of LSPs can limit network scalability. You can reduce LSP flooding in two ways:

Blocking Flooding on Specific Interfaces

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.

Configuring Mesh Groups

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 Complete Sequence Number PDUs (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 Other IS-IS Parameters

You can now configure other IS-IS parameters for each area. The procedures and commands are described in the "Configuring Integrated IS-IS" chapter of the Cisco IOS Release 12.0 Network Protocols Configuration Guide, Part 1, or the "Configuring ISO CLNS" chapter of the Cisco IOS Release 12.0 Network Protocols Configuration Guide, Part 3.

Verifying Multiarea IS-IS Configuration

Output for all of the show commands used to verify IS-IS configuration is "per area" when multiarea IS-IS is configured.

The following example shows how to use the show commands to verify a dual CLNS-IP multiarea network.

1. Enter the show clns EXEC command to verify that all desired areas have been configured. Also check that NETs are well formed, and specify the correct area addresses for each configured area.

The following is sample output from the show clns command in a dual CLNS-IP network for Area 49:
dtp-5#show clns 
Global CLNS Information:
  3 Interfaces Enabled for CLNS
  NET: 49.2222.0000.0000.0005.00
  NET: 49.0553.0001.0000.0000.0005.00
  NET: 49.0553.0002.0000.0000.0005.00
  Configuration Timer: 60, Default Holding Timer: 300, Packet Lifetime 64
  ERPDU's requested on locally generated packets
  Intermediate system operation enabled (CLNS forwarding allowed)
  IS-IS level-1-2 Router: L2BB
    Routing for Area: 49.2222
  IS-IS level-1 Router: A3253-01
    Routing for Area: 49.0553.0001
  IS-IS level-1 Router: A3253-02
    Routing for Area: 49.0553.0002
2. Enter the show clns protocol EXEC command to verify that all interfaces are configured in the correct areas for the desired protocols.

The following is sample output from the show clns protocol command:
dtp-5# show clns protocol
IS-IS Router: L2BB
  System Id: 0000.0000.0005.00  IS-Type: level-1-2
  Manual area address(es): 
        49.2222
  Routing for area address(es): 
        49.2222
  Interfaces supported by IS-IS:
        Tunnel529 - OSI - IP
  Redistribute:
    static (on by default)
  Distance for L2 CLNS routes: 110
          
IS-IS Router: A3253-01
  System Id: 0000.0000.0005.00  IS-Type: level-1
  Manual area address(es): 
        49.0553.0001
  Routing for area address(es): 
        49.0553.0001
  Interfaces supported by IS-IS:
        Ethernet1 - OSI - IP
  Redistribute:
    static (on by default)
  Distance for L2 CLNS routes: 110
          
IS-IS Router: A3253-02
  System Id: 0000.0000.0005.00  IS-Type: level-1
  Manual area address(es): 
        49.0553.0002
  Routing for area address(es): 
        49.0553.0002
  Interfaces supported by IS-IS:
        Ethernet2 - OSI - IP
  Redistribute:
    static (on by default)
  Distance for L2 CLNS routes: 110
3. Enter the show clns neighbors command (for a CLNS connection) to verify that all expected adjacencies are up with all neighbors. If not, recheck the area addresses specified in both routers. If the router is running in dual CLNS-IP mode in an area, verify that a valid IP address is configured on each interface in the area. Consider using debug isis adjacency to gather additional information.

The following is sample output from the show clns neighbors command in a dual CLNS-IP network:
dtp-5# show clns neighbors
 
Area L2BB:
System Id      Interface   SNPA                State  Holdtime  Type Protocol
0000.0000.0009 Tu529       172.21.39.9         Up     27        L1L2 IS-IS
 
Area A3253-01:
System Id      Interface   SNPA                State  Holdtime  Type Protocol
0000.0000.0053 Et1         0060.3e58.ccdb      Up     24        L1   IS-IS
0000.0000.0003 Et1         0000.0c03.6944      Up     27        L1   IS-IS
 
Area A3253-02:
System Id      Interface   SNPA                State  Holdtime  Type Protocol
0000.0000.0053 Et2         0060.3e58.ccde      Up     25        L1   IS-IS
0000.0000.0002 Et2         0000.0c03.6bc5      Up     27        L1   IS-IS
4. Enter the show isis topology EXEC command to verify the presence and connectivity between all routers in all areas.

The following is sample output from the show isis topology command in a dual CLNS-IP network:
dtp-5# show isis topology
 
Area L2BB:
IS-IS paths to level-1 routers
System Id       Metric  Next-Hop        Interface       SNPA
0000.0000.0005  --
0000.0000.0009  10      0000.0000.0009  Tu529           *Tunnel*        
0000.0000.0017  20      0000.0000.0009  Tu529           *Tunnel*        
0000.0000.0053  30      0000.0000.0009  Tu529           *Tunnel*        
0000.0000.0068  20      0000.0000.0009  Tu529           *Tunnel*        
 
IS-IS paths to level-2 routers
System Id       Metric  Next-Hop        Interface       SNPA
0000.0000.0005  --
0000.0000.0009  10      0000.0000.0009  Tu529           *Tunnel*        
0000.0000.0017  20      0000.0000.0009  Tu529           *Tunnel*        
0000.0000.0053  30      0000.0000.0009  Tu529           *Tunnel*        
0000.0000.0068  20      0000.0000.0009  Tu529           *Tunnel* 
Area A3253-01:
IS-IS paths to level-1 routers
System Id       Metric  Next-Hop        Interface       SNPA
0000.0000.0003  10      0000.0000.0003  Et1             0000.0c03.6944  
0000.0000.0005  --
0000.0000.0053  10      0000.0000.0053  Et1             0060.3e58.ccdb  
 
Area A3253-02:
IS-IS paths to level-1 routers
System Id       Metric  Next-Hop        Interface       SNPA
0000.0000.0002  10      0000.0000.0002  Et2             0000.0c03.6bc5  
0000.0000.0005  --
0000.0000.0053  10      0000.0000.0053  Et2             0060.3e58.ccde 
5. (For CLNS applications only) Enter either the show clns route or the show ip route isis EXEC command to display all of the destinations to which this router knows how to route packets. The show clns route command shows the IS-IS Level 2 routing table in addition to static and ISO Interior Gateway Routing Protocol (IGRP) learned prefix routes. This table stores IS-IS area addresses and prefix routes. Destinations are sorted by category.

These commands, used with ping, verify connectivity to all desired end hosts in the network.

As the display shows, neighbors are not included in the show clns route output.

The following is sample output from the show clns route command:
dtp-5# show clns route
Codes: C - connected, S - static, d - DecnetIV
       I - ISO-IGRP,  i - IS-IS,  e - ES-IS
 
C  49.0553.0002.0000.0000.0005.00 [1/0], Local IS-IS NET
C  49.0553.0001.0000.0000.0005.00 [1/0], Local IS-IS NET
C  49.2222.0000.0000.0005.00 [1/0], Local IS-IS NET
C  49.0553.0002 [2/0], Local IS-IS Area
C  49.0553.0001 [2/0], Local IS-IS Area
C  49.2222 [2/0], Local IS-IS Area
The following is sample output from the show ip route isis command:
dtp-5# show ip route isis
     11.0.0.0/24 is subnetted, 1 subnets
i L1    11.0.0.0 [115/20] via 10.0.0.9, Tunnel529, L2BB, from LSP 3
i*L1 0.0.0.0/0 [115/10] via 10.0.0.9, Tunnel529, L2BB, from LSP 0
               [115/10] via 10.2.2.53, Ethernet2, A3253-02, from LSP 0
               [115/10] via 10.1.1.53, Ethernet1, A3253-01, from LSP 0
6. Use either the ping clns or the ping ip EXEC command to send packets to test whether a remote host can be reached over an ISO CLNS or an IP network. If there is a response from the remote system, the remote host can be reached. Note that the Cisco implementation of the ping command supports PDU types that have been proposed but not yet standardized. The remote host may not recognize these packets, but will typically generate an error packet (ERPDU) as a response. This ERPDU confirms that the remote host can be reached. If the remote host is reachable, multiarea IS-IS is probably configured correctly.

Monitoring and Maintaining Multiarea IS-IS

Command

Purpose

Router# show clns es-neighbors

Displays end system (ES) neighbor adjacencies.

Router# show clns is-neighbors

Displays internal system (IS) neighbor adjacencies.

Router# show clns neighbor areas

Displays ISIS neighbors and the areas to which they belong.

Router# show clns neighbors

Displays CLNS neighbor adjacencies.

Router# show clns protocol

Displays CLNS routing protocol process information.

Router# show clns route

Displays one or all of the destinations to which the router knows how to route CLNS packets.

Router# show clns traffic

Displays CLNS protocol activity statistics.

Router# show isis database

Displays IS-IS link-state database.

Router# show isis routes

Displays IS-IS Level 1 routing table.

Router# show isis spf-log

Displays IS-IS shortest path first (SPF) log.

Router# show isis topology

Displays IS-IS paths to intermediate systems.

Configuration Example

This section provides the following configuration example:

• Multiarea IS-IS Configuration

Multiarea IS-IS Configuration

The following output shows an IS-IS configuration with two Level 1 areas and one Level 1-2 area. Figure 3 illustrates this configuration.
clns routing
...
interface Tunnel529
 ip address 10.0.0.5 255.255.255.0
 ip router isis BB
 clns router isis BB
interface Ethernet1
 ip address 10.1.1.5 255.255.255.0
 ip router isis A3253-01
 clns router isis A3253-01
!
interface Ethernet2
 ip address 10.2.2.5 255.255.255.0
 ip router isis A3253-02
 clns router isis A3253-02
...
router isis BB                          ! Defaults to "is-type level-1-2"
 net 49.2222.0000.0000.0005.00
!
router isis A3253-01
 net 49.0553.0001.0000.0000.0005.00
 is-type level-1
!
router isis A3253-02
 net 49.0553.0002.0000.0000.0005.00
 is-type level-1
Figure 3 Multiarea IS-IS Configuration with Three Level 1 Areas and One Level 2 Area

Command Reference

This section documents new or modified commands. All other commands used with this feature are documented in the Cisco IOS Release 12.0 command reference publications.

• clns router isis

• ip router isis

• isis display delimiter

• isis mesh-group

• is-type

• net

• partition avoidance

• router isis

• show clns neighbor areas

• show clns route

• show isis topology

clns router isis

To configure an Intermediate System-to-Intermediate System (IS-IS) routing process for International Standards Organization (ISO) Connectionless Network Service Protocol (CLNS) on an interface and to attach an area designator to the routing process, use the clns router isis interface configuration command. Use the no form of the command to disable IS-IS for ISO.

clns router isis [area-tag]
no clns router isis [area-tag]
Syntax Description

area-tag

Optional for conventional IS-IS configuration. Required for multiarea IS-IS configuration.

Defines a meaningful name for an area routing process. If not specified, a null tag is assumed. It must be unique among all CLNS router processes for a given router. The area-tag argument is used later as a reference to this area routing process.

Each area in a multiarea configuration should have a nonnull area tag to facilitate identification of the area.

DefaultsDefault

No routing processes are specified.

Command Modes

Interface configuration

Command History

Release

Modification

10.0

This command was introduced.

12.0(5)T

Multiarea functionality was added, changing the way the tag argument (now area-tag) is used.

Usage Guidelines

Before the IS-IS router process is useful, a network entity title (NET) must be assigned with the net command and some interfaces must be enabled with IS-IS.

If you have IS-IS running and at least one ISO-IGRP process, the IS-IS process and the ISO-IGRP process cannot both be configured without an area tag. The null tag can be used by only one process. If you run ISO-IGRP and IS-IS, a null tag can be used for IS-IS, but not for ISO-Interior Gateway Routing Protocol (IGRP) at the same time. However, each area in an IS-IS multiarea configuration should have a nonnull area tag to facilitate identification of the area.

You can configure only one process to perform Level 2 (interarea) routing. 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 intra-area (Level 1) routing at the same time. You can configure up to 29 additional processes as Level 1-only processes. Use the is-type command to remove Level 2 routing from a router instance. You can then use the is-type command to enable Level 2 routing on some other IS-IS router instance.

An interface cannot be part of more than one area, except in the case where the associated routing process is performing both Level 1 and Level 2 routing. On media (such as WAN media, for example) where subinterfaces are supported, different subinterfaces could be configured for different areas.

Examples

The following example enables IS-IS routing for ISO CLNS on Ethernet interface 0:
router isis cisco
 net 39.0001.0000.0c00.1111.00
interface ethernet 0
 clns router isis cisco
The following example shows an IS-IS configuration with two Level 1 areas and one Level 1-2 area:
clns routing
...
interface Tunnel529
 ip address 10.0.0.5 255.255.255.0
 ip router isis BB
 clns router isis BB
interface Ethernet1
 ip address 10.1.1.5 255.255.255.0
 ip router isis A3253-01
 clns router isis A3253-01
!
interface Ethernet2
 ip address 10.2.2.5 255.255.255.0
 ip router isis A3253-02
 clns router isis A3253-02
...
router isis BB                          ! Defaults to "is-type level-1-2"
 net 49.2222.0000.0000.0005.00
!
router isis A3253-01
 net 49.0553.0001.0000.0000.0005.00
 is-type level-1
!
router isis A3253-02
 net 49.0553.0002.0000.0000.0005.00
 is-type level-1
Related Commands

Command

Description

router isis

Enables the IS-IS routing protocol and specifies an IS-IS process. When multiarea IS-IS is configured, this command specifies the area to which the routing process belongs.

ip router isis

To configure an Intermediate System-to-Intermediate System (IS-IS) routing process for IP on an interface and to attach an area designator to the routing process, use the ip router isis interface configuration command. Use the no form of the command to disable IS-IS for IP.

ip router isis [area-tag]
no ip router isis [area-tag]
Syntax Description

area-tag

Optional for conventional IS-IS configuration. Required for multiarea IS-IS configuration for ISO CLNS.

Defines a meaningful name for an area routing process. If not specified, a null tag is assumed. It must be unique among all IP router processes for a given router. The area-tag argument is used later as a reference to this area routing process.

Each area in a multiarea configuration should have a nonnull area tag to facilitate identification of the area.

Defaults

No routing processes are specified.

Command Modes

Interface configuration

Command History

Release

Modification

10.0

This command introduced.

12.0(5)T

Multiarea functionality was added, changing the way the tag argument (now area-tag) is used.

Usage Guidelines

Before the IS-IS routing process is useful, a network entity title (NET) must be assigned with the net command and some interfaces must have IS-IS enabled.

If you have IS-IS running and at least one ISO-Interior Gateway Routing Protocol (IGRP) process, the IS-IS process and the ISO-IGRP process cannot both be configured without an area tag. The null tag can be used by only one process. If you run ISO-IGRP and IS-IS, a null tag can be used for IS-IS, but not for ISO-IGRP at the same time. However, each area in an IS-IS multiarea configuration should have a nonnull area tag to facilitate identification of the area.

You can configure only one process to perform Level 2 (interarea) routing. 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 intra-area (Level 1) routing at the same time. You can configure up to 29 additional processes as Level 1-only processes. Use the is-type command to remove Level 2 routing from a router instance. You can then use the is-type command to enable Level 2 routing on some other IS-IS router instance.

An interface cannot be part of more than one area, except in the case where the associated routing process is performing both Level 1 and Level 2 routing. On media (such as WAN media, for example) where subinterfaces are supported, different subinterfaces could be configured for different areas.

Examples

The following example specifies IS-IS as an IP routing protocol for a process named Finance, and specifies that the Finance process will be routed on interfaces Ethernet 0 and serial 0:
router isis Finance
 net 49.0001.aaaa.aaaa.aaaa.00
interface Ethernet 0
 ip router isis Finance
interface serial 0
 ip router isis Finance
The following example shows an IS-IS configuration with two Level 1 areas and one Level 1-2 area:
ip routing
...
interface Tunnel529
 ip address 10.0.0.5 255.255.255.0
 ip router isis BB
interface Ethernet1
 ip address 10.1.1.5 255.255.255.0
 ip router isis A3253-01
1
!
interface Ethernet2
 ip address 10.2.2.5 255.255.255.0
 ip router isis A3253-02
...
router isis BB                          ! Defaults to "is-type level-1-2"
 net 49.2222.0000.0000.0005.00
!
router isis A3253-01
 net 49.0553.0001.0000.0000.0005.00
 is-type level-1
!
router isis A3253-02
 net 49.0553.0002.0000.0000.0005.00
 is-type level-1
Related Commands

Command

Description

net

Configures a NET for the IS-IS routing process by specifying an area address and system ID for the process.

router isis

Enables the IS-IS routing protocol and specifies an IS-IS process. When multiarea IS-IS is configured, this command specifies the area to which the routing process belongs.

isis display delimiter

To make output from multiarea displays easier to read by specifying the delimiter to use to separate displays of information, use the isis display delimiter global configuration command. This command displays the output from different areas as a string or additional white space. Use the no form of the command to disable this output format.

isis display delimiter [return cnt | char cnt]
no isis display delimiter [return cnt | char cnt]
Syntax Description

return

(Optional) Delimit with carriage returns.

char

(Optional) Character to use for delimiter string.

cnt

(Optional) Number of carriage returns or length of string to use for delimiter.

Defaults

By default the isis display delimiter command is disabled.

Command Modes

Global configuration

Command History

Release

Modification

12.0(5)T

This command was introduced.

Usage Guidelines

Use this command to customize display output when the IS-IS multiarea feature is used.

Examples

The following command causes different areas in multiarea displays (such as show command output) to be delimited by a string of dashes (-):
isis display delimiter - 14
With three IS-IS neighbors configured, this command displays the following output from the show clns neighbors command:
dtp-5# show clns neighbors
--------------
Area L2BB:
System Id      Interface   SNPA                State  Holdtime  Type Protocol
0000.0000.0009 Tu529       172.21.39.9         Up     25        L1L2 IS-IS
--------------
Area A3253-01:
System Id      Interface   SNPA                State  Holdtime  Type Protocol
0000.0000.0053 Et1         0060.3e58.ccdb      Up     22        L1   IS-IS
0000.0000.0003 Et1         0000.0c03.6944      Up     20        L1   IS-IS
--------------
Area A3253-02:
System Id      Interface   SNPA                State  Holdtime  Type Protocol
0000.0000.0002 Et2         0000.0c03.6bc5      Up     27        L1   IS-IS
0000.0000.0053 Et2         0060.3e58.ccde      Up     24        L1   IS-IS
Related Commands

Command

Description

show clns es-neighbors

Displays IS-IS information for ES neighbors that this router knows about.

show clns is-neighbors

Displays IS-IS information for IS neighbors that this router knows about.

show clns neighbors

Displays IS-IS information for both ES and IS neighbors that this router knows about.

show clns protocol

Displays a list of the protocol-specific information for each ISO IGRP routing process in the router.

show clns traffic

Displays a list of the CLNS packets this router has seen.

show isis database

Displays the IS-IS link-state database. A summary display is provided if no options are specified.

show isis mesh-groups

Optimizes LSP flooding in NBMA networks using highly meshed (having many links), point-to-point topologies.

show isis routes

Displays the IS-IS Level 1 forwarding table for IS-IS learned routes.

show isis spf-log

Displays a history of the SPF calculations for IS-IS.

show isis topology

Displays a list of all connected routers in all areas.

isis mesh-group

To optimize LSP flooding in nonbroadcast multiaccess (NBMA) networks with highly meshed, point-to-point topologies, use the isis mesh-group interface configuration command. To remove a (sub)interface from a mesh group, use the no form of this command.

isis mesh-group [num | blocked]
no isis mesh-group [num | blocked]
Syntax Description

standard-list-number

A standard extended community list number from 1 to 99 that identifies one or more permit or deny groups of extended community attributes.

expanded-list-number

An expanded extended community list number from 100 to 500 that identifies one or more permit or deny groups of extended community attributes.

Defaults

The interface performs normal flooding.

Command Modes

Interface configuration

Command History

Release

Modification

12.0

This command was introduced.

12.0(5)T

Command was modified to include multiarea IS-IS routing.

Usage Guidelinescr

LSPs that are first received on (sub)interfaces that are not part of a mesh group are flooded to all other (sub)interfaces in the usual way.

LSPs that are first received on (sub)interfaces that are part of a mesh group are flooded to all interfaces except those in the same mesh-group. If the "blocked" keyword is configured on a (sub)interface, then a newly received LSP is not flooded out over that interface.

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. In other words, blocking flooding on all links permits the best scaling performance, but there is no flooding. Permitting flooding on all links results in very poor scaling performance.

Examples

In the following example six interfaces are configured in three mesh groups. LSPs received are handled as follows:

•LSPs received first via atm 1/0.1 are flooded to all interfaces except atm 1/0.2 (which is part of the same mesh group) and atm 1/2.1, which is blocked.

•LSPs received first via atm 1/1.2 are flooded to all interfaces except atm 1/1.1 (which is part of the same mesh group) and atm 1/2.1, which is blocked.

•LSPs received first via atm 1/2.1 are not ignored, but flooded as usual to all interfaces. LSPs received first via atm 1/2.2 are flooded to all interfaces, except atm 1/2.1, which is blocked.
interface atm 1/0.1

ip router isis

isis mesh-group 10
interface atm 1/0.2
ip router isis

isis mesh-group 10
interface atm 1/1.1

ip router isis

isis mesh-group 11
interface atm 1/1.2

ip router isis

isis mesh-group 11
interface atm 1/2.1

ip router isis
isis mesh-group blocked
interface atm 1/2.2

ip router isis
Related Commands

Command

Description

router isis

Enables the IS-IS routing protocol and specifies an IS-IS process. When multiarea IS-IS is configured, this command specifies the area to which the routing process belongs.

is-type

To configure the routing level for an instance of the Intermediate System-to-Intermediate System (IS-IS) routing process, use the is-type router configuration command. To reset the default value, use the no form of this command.

is-type {level-1 | level-1-2 | level-2-only}
no is-type {level-1 | level-1-2 | level-2-only}
Syntax Description

level-1

Router performs only Level 1 (intra-area) routing. This router only learns about destinations inside its area. Level 2 (interarea) routing is performed by the closest Level 1-2 router.

level-1-2

Router performs both Level 1 and Level 2 routing. This router runs two instances of the routing process. It has one link-state packet database (LSDB) for destinations inside the area (Level 1 routing) and runs an shortest path first (SPF) calculation to discover the area topology. It also has another link-state packet database (LSDB) with link-state packets (LSPs) of all other backbone (Level 2) routers, and runs another SPF calculation to discover the topology of the backbone, and the existence of all other areas.

level-2-only

Routing process acts as a Level 2 (interarea) router only. This router is part of the backbone, and does not communicate with Level 1-only routers in its own area.

Defaults

In conventional IS-IS configurations, the router acts as both a Level 1 (intra-area) and a Level 2 (interarea) router.

In multiarea IS-IS configurations, the first instance of the IS-IS routing process configured is by default a Level 1-2 (intra-area and interarea) router. The remaining instances of the IS-IS process configured by default are Level 1 routers.

Command Modes

Router configuration

Command History

Release

Modification

10.3

This command was introduced.

12.0(5)T

Multiarea IS-IS routing was included.

Usage Guidelines

It is highly recommended that you configure the type of an IS-IS routing process. If you are configuring multiarea IS-IS, you must configure the type of the router, or allow it to be configured by default. By default, the first instance of the IS-IS routing process that you configure using the router isis command is a Level 1-2 router.

If there is only one area in the network, there is no need to run both Level 1 and Level 2 routing algorithms. If IS-IS is used for Connectionless Network Service (CLNS) routing (and there is only one area), Level 1-only must be used everywhere. If IS-IS is used for IP routing only (and there is only one area), you can run Level 2-only everywhere. Areas you add after the Level 1-2 area exists are by default Level 1 areas.

If the router instance has been configured for Level 1-2 (the default for the first instance of the IS-IS routing process in a Cisco unit), you can remove Level 2 (interarea) routing for the area using the is-type command. You can also use is-type to configure Level 2 routing for an area, but it must be the only instance of the IS-IS routing process configured for Level 2 on the Cisco unit.

Examples

The following example specifies an area router:
router isis 
 is-type level-2-only
Related Commands

Command

Description

router isis

Enables the IS-IS routing protocol and specifies an IS-IS process. When multiarea IS-IS is configured, this command specifies the area to which the routing process belongs.

show clns neighbor areas

Displays information about IS-IS neighbors and the areas to which they belong.

net

To configure an Intermediate System-to-Intermediate System (IS-IS) network entity title (NET) for the routing process, use the net router configuration command. To remove a NET, use the no form of this command.

net network-entity-title
no net network-entity-title
Syntax Description

network-entity-title

NET that specifies the area address and the system ID for an IS-IS routing process. This argument can be either an address or a name.

Defaults

Hierarchical Navigation

IP Routing

IS-IS Multiarea Support

Downloads

Table Of Contents

IS-IS Multiarea Support

Feature Overview

Redistribution

TARP

Benefits

Restrictions

Related Features and Technologies

Related Documents

Supported Platforms

Supported MIBs and RFCs

Configuration Tasks

Configuration Tasks for Multiarea IS-IS

Enabling IS-IS and Assigning Areas

Enabling CLNS Routing for an Area on an Interface

Modifying the Output of show Commands

Enabling Partitioning Avoidance

Changing the Routing Level for an Area

Limiting LSP Flooding

Blocking Flooding on Specific Interfaces

Configuring Mesh Groups

Configuring Other IS-IS Parameters

Verifying Multiarea IS-IS Configuration

Monitoring and Maintaining Multiarea IS-IS

Configuration Example

Multiarea IS-IS Configuration

Command Reference

clns router isis

Syntax Description

DefaultsDefault

Command Modes

Command History

Usage Guidelines

Examples

Related Commands

ip router isis

Syntax Description

Defaults

Command Modes

Command History

Usage Guidelines

Examples

Related Commands

isis display delimiter

Syntax Description

Defaults

Command Modes

Command History

Usage Guidelines

Examples

Related Commands

isis mesh-group

Syntax Description

Defaults

Command Modes

Command History

Usage Guidelinescr

Examples

Related Commands

is-type

Syntax Description

Defaults

Command Modes

Command History

Usage Guidelines

Examples

Related Commands

net

Syntax Description

Defaults