Table Of Contents
Configuring a Basic IS-IS Network
First Published: November 30, 2007Last Updated: May 5, 2008
This module describes the tasks to configure and monitor a basic Intermediate System-to-Intermediate System (IS-IS) network. The IS-IS process and adjacency formation is also explained. IS-IS is link-state protocol that allows the network designer to organize the network into a group of flooding domains. Often deployed as the Interior Gateway Protocol (IGP) for an ISP network backbone, IS-IS is capable of handling large topologies and large numbers of routing changes.
Finding Feature Information in This Module
Your Cisco IOS software release may not support all of the features documented in this module. For the latest feature information and caveats, see the release notes for your platform and software release. To reach links to specific feature documentation in this module and to see a list of the releases in which each feature is supported, use the "Feature Information for Configuring a Basic IS-IS Network" section.
Finding Support Information for Platforms and Cisco IOS and Catalyst OS Software Images
Use Cisco Feature Navigator to find information about platform support and Cisco IOS and Catalyst OS software image support. To access Cisco Feature Navigator, go to http://www.cisco.com/go/cfn. An account on Cisco.com is not required.
Prerequisites for Configuring a Basic IS-IS Network
•Before performing the tasks in this module, you should be familiar with the concepts described in the "Integrated IS-IS Routing Protocol Overview" module.
•You should know your network design and how you want traffic to flow through it before configuring IS-IS. Define areas, prepare an addressing plan for the routers (including defining the NETs), and determine the interfaces that will run Integrated IS-IS. To facilitate verification, a matrix of adjacencies should be prepared before you configure your routers, showing what neighbors should be expected in the adjacencies table. For more information on verifying IS-IS configuration and formed adjacencies, see the "Monitoring IS-IS" section.
Information About the IS-IS Routing Protocol
IS-IS Process and Adjacencies
IS-IS requires some configuration on both the router and the interface. An IS-IS process is created when you enable IS-IS on a router and define a specific tag to identify that routing process. Interfaces configured with a specific tag will be part of the corresponding router process. More than one IS-IS process can run on a router for Connectionless Network Service (CLNS), but only one IS-IS process can run for IP.
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. The areas are 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 (inter-area routing).
If the network administrator does not specify Level 1 or Level 2 routing for the routing process being configured, the default routing behavior for the routing process will be Level 1-2.
If Level 2 routing is configured on any process, additional processes are automatically configured as Level 1, with the exception of previously configured Level 2 process, which will remain Level 2. You can have only one Level-2 process. You can configure the Level-2 process to perform Level-1 routing at the same time. If Level-2 routing is not desired for a router instance, use the is-type command in router configuration mode to remove the Level-2 capability. You can also use the is-type command to configure a different router instance as a Level-2 router.
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.
Network entity titles (NETs) define the area addresses and the system ID of the router. See the "Configuring ISO CLNS" module in the Cisco IOS ISO CLNS Configuration Guide for a more detailed discussion of NETs.
PDU Packet Types in IS-IS Routing
The OSI stack defines a unit of data as a protocol data unit (PDU). A frame therefore is regarded by OSI as a data-link PDU, and a packet is regarded as a network PDU. There are four types of PDU packets, and each type can be Level 1 or Level 2:
•LSP—Link-state PDU. Used to distribute link-state information.
•IIH PDU—For IS-IS this is called the IS-IS Hello PDU. Used to establish and maintain adjacencies.
Note On point-to-point links, IIH PDUs will be the same for Level 1 and Level 2. Both Level-1 and Level-2 IIH use the same type of PDU, but they carry different circuit types.
•PSNP—partial sequence numbers protocol data unit (PDU). Used to acknowledge and request link-state information.
•CSNP—complete sequence number protocol data unit (PDU). Used to distribute the complete link-state database of a router.
IS-IS LSPs include specific information about the router's attachments. The following information is included in multiple TLV fields in the main body of the LSP:
•The links to neighbor router intermediate systems (ISs), including the metrics of those interfaces
•The links to the neighbor end systems (ESs)
How to Create, Monitor, and Make Changes to a Basic IS-IS Network
Enabling IS-IS as an IP Routing Protocol on the Router
2. configure terminal
3. router isis [area-tag]
4. net network-entity-title
Enabling IS-IS as an IP Routing Protocol on the Interface
2. configure terminal
3. interface type number
4. ip address ip-address mask [secondary]
5. ip router isis [area-tag]
2. configure terminal
3. isis display delimiter [return count | character count]
5. show ip protocols
6. show clns is area-tag neighbors [type number] [detail]
7. show clns interface [type number]
8. show clns is area-tag neighbors [type number] [detail]
9. show clns area-tag traffic
10. show ip route [ip-address [mask]] [[longer-prefixes] | protocol [process-id] | list [access-list-number | access-list-name] | static download]]
11. show isis [process-tag] database [level-1] [level-2] [l1] [l2] [detail] [lspid]
12. show isis database verbose
13. show isis lsp-log
14. show isis [area-tag] [ipv6 | *] spf-log
15. show isis [process-tag] [ipv6 | *] topology
16. show isis area-tag neighbors [detail]
When the show isis neighbors command is entered with the detail keyword, the output provides information on the IS-IS adjacencies that have formed.Router1# show isis neighbors detailSystem Id Type Interface IP Address State Holdtime Circuit IdRouter2 L2 Et1/0 10.1.1.0 UP 255 Circuit3.01Area Address(es): 32SNPA: aabb.cc00.2001State Changed: 00:00:14LAN Priority: 64Format: Phase V
You can use the following two system debugging commands to check your IS-IS IPv4 implementation.
•If adjacencies are not coming up properly, use the debug isis adj-packets command.
•To display a log of significant events during an IS-IS SPF calculation, use the debug isis spf-events command.
Shutting Down IS-IS to Make Changes to Your IS-IS Network
You can shut down IS-IS (placing it in an administrative down state) to make changes to the IS-IS protocol configuration, without losing your configuration parameters. You can shut down IS-IS at the interface level or at the global IS-IS process level. If the router was rebooted when the protocol was turned off, the protocol would be expected to come back up in the disabled state. When the protocol is set to the administrative down state, network administrators are allowed to administratively turn off the operation of the IS-IS protocol without losing the protocol configuration, to make a series of changes to the protocol configuration without having the operation of the protocol transition through intermediate—and perhaps undesirable—states, and to then reenable the protocol at a suitable time.
Before the introduction of the Integrated IS-IS Protocol Shutdown Support Maintaining Configuration Parameters feature, there was no nondestructive way to disable IS-IS operation. The only way to disable IS-IS at the router level was to issue the no router isis command, which removes the IS-IS configuration. At the interface level there are two ways to disable IS-IS operation. You can enter the no ip router isis command to remove IS-IS from the specified interface, or you can put the interface into passive mode such that the IP address of the specified interface will still be advertised. In either case, the current IS-IS configuration will be removed.
Shutting Down IS-IS in Interface Mode
2. configure terminal
3. interface type number
4. isis protocol shutdown
Shutting Down IS-IS in Router Mode
2. configure terminal
3. router isis area-tag
4. protocol shutdown
Configuration Examples for a Basic IS-IS Network
Example: Configuring a Basic IS-IS Network
The following example shows how to configure three routers to run IS-IS as an IP routing protocol. Figure 1 illustrates the sample configuration.
Router A Configurationrouter isisnet 49.0001.0000.0000.000a.00interface ethernet0/0ip address 10.1.1.1 255.255.255.0ip router isisinterface serial 2/0ip router isisip address 192.168.1.2 255.255.255.0
Router B Configurationrouter isisnet 49.0001.0000.0000.000b.00interface ethernet0/0ip router isisip address 172.17.1.1 255.255.255.0interface serial2/0ip router isisip address 192.168.1.1 255.255.255.0interface serial5/0ip router isisip address 172.21.1.1 255.255.255.0
Router C Configurationrouter isisnet 49.0001.0000.0000.000c.00interface ethernet2/0ip router isisip address 172.21.1.2 255.255.255.0interface serial5/0ip router isisip address 172.22.1.1 255.255.255.0
The show isis topology command displays the following information about how the routers are connected within the IS-IS network:RouterB# show isis topologyIS-IS paths to level-1 routersSystem Id Metric Next-Hop Interface SNPARouterA 10 RouterA Se2/0 *HDLC*RouterB --RouterC 10 RouterC Se5/0 *HDLC*IS-IS paths to level-2 routersSystem Id Metric Next-Hop Interface SNPARouterA 10 RouterA Se2/0 *HDLC*RouterB --RouterC 10 RouterC Se5/0 *HDLC*
The show isis database command displays following information for the Level 1 and Level 2 LSPs for each router in the IS-IS network.RouterB# show isis databaseIS-IS Level-1 Link State Database:LSPID LSP Seq Num LSP Checksum LSP Holdtime ATT/P/OLRouterA.00-00 0x00000005 0x1A1D 1063 0/0/0RouterB.00-00 * 0x00000006 0xD15B 1118 0/0/0RouterC.00-00 0x00000004 0x3196 1133 1/0/0IS-IS Level-2 Link State Database:LSPID LSP Seq Num LSP Checksum LSP Holdtime ATT/P/OLRouterA.00-00 0x00000008 0x0BF4 1136 0/0/0RouterB.00-00 * 0x00000008 0x1701 1137 0/0/0RouterC.00-00 0x00000004 0x3624 1133 0/0/0
The show ip route command displays information about the interfaces of each router, including their IP addresses and how they are connected to Router B:RouterB# show ip routeCodes: C - connected, S - static, R - RIP, M - mobile, B - BGPD - EIGRP, EX - EIGRP external, O - OSPF, IA - OSPF inter areaN1 - OSPF NSSA external type 1, N2 - OSPF NSSA external type 2E1 - OSPF external type 1, E2 - OSPF external type 2i - IS-IS, su - IS-IS summary, L1 - IS-IS level-1, L2 - IS-IS level-2ia - IS-IS inter area, * - candidate default, U - per-user static routeo - ODR, P - periodic downloaded static routeGateway of last resort is not set172.17.0.0/24 is subnetted, 1 subnetsC 172.17.1.0 is directly connected, Ethernet0/0172.16.0.0/24 is subnetted, 1 subnetsC 172.16.1.0 is directly connected, Serial4/0172.21.0.0/24 is subnetted, 1 subnetsC 172.21.1.0 is directly connected, Serial5/0172.22.0.0/24 is subnetted, 1 subnetsi L1 172.22.1.0 [115/20] via 172.21.1.2, Serial5/010.0.0.0/24 is subnetted, 1 subnetsi L1 10.1.1.0 [115/20] via 192.168.1.2, Serial2/0C 192.168.1.0/24 is directly connected, Serial2/0C 192.168.3.0/24 is directly connected, Serial3/0
The show isis spf-log command displays logs of Level 1 and Level 2 LSPs including time of occurrence, duration, count, and the event that triggered the LSP.RouterB## show isis spf-loglevel 1 SPF logWhen Duration Nodes Count First trigger LSP Triggers00:01:30 0 3 7 RouterB.00-00 PERIODIC NEWADJ NEWLSP TLVTlevel 2 SPF logWhen Duration Nodes Count First trigger LSP Triggers00:01:31 0 3 7 RouterB.00-00 PERIODIC NEWADJ NEWLSP TLVT
Figure 1 IS-IS Routing
Example: Shutting Down IS-IS in Interface Mode
The following router output shows that the router has two IS-IS adjacencies:Router# show clns neighborsSystem Id Interface SNPA State Holdtime Type Protocolfirst Et3/1 0002.7dd6.1c21 Up 25 L1L2 IS-ISsecond Et3/2 0004.6d25.c056 Up 29 L1L2 IS-IS
When the isis protocol shutdown command is entered for Ethernet interface 3/1, the IS-IS protocol will be disabled for the specified interface:Router# configure terminalEnter configuration commands, one per line. End with CNTL/Z.Router(config)# interface ethernet3/1Router(config-if)# isis protocol shutdownRouter(config-if)# end
The following router output shows that the adjacency for Ethernet interface 3/1 has not formed:Router# show clns neighborsSystem Id Interface SNPA State Holdtime Type Protocolsecond Et3/2 0004.6d25.c056 Up 27 L1L2 IS-IS
Example: Shutting Down IS-IS in Router Mode
The following router output shows that the router has two IS-IS adjacencies:Router# show clns neighborsSystem Id Interface SNPA State Holdtime Type Protocolsouth Et3/1 0002.7dd6.1c21 Up 29 L1L2 IS-ISnorth Et3/2 0004.6d25.c056 Up 28 L1L2 IS-IS
The protocol shutdown command is entered so that IS-IS is disabled and no adjacencies will be formed on any interface:Router# configure terminalEnter configuration commands, one per line. End with CNTL/Z.Router(config)# router isis area1Router(config-router)# protocol shutdownRouter(config-router)# end
The following router output now shows that both adjacencies are gone.Router# show clns neighborsSystem Id Interface SNPA State Holdtime Type Protocol
When the no protocol shutdown command is entered, the adjacencies will again be formed on both interfaces:Router(config)# router isis area1Router(config-router)# no protocol shutdownRouter(config-router)# endRouter# show clns neighborsSystem Id Interface SNPA State Holdtime Type Protocolsouth Et3/1 0002.7dd6.1c21 Up 24 L1L2 IS-ISnorth Et3/2 0004.6d25.c056 Up 24 L1L2 IS-IS
Where to Go Next
•To customize IS-IS for your network design, see the "Customizing IS-IS for Your Network Design" module.
•To customize IS-IS for achieving fast convergence and scalability, see the following modules:
•To enhance IS-IS network security, see the "Enhancing Security in an IS-IS Network" module.
No new or modified standards are supported, and support for existing standards has not been modified.
MIB MIBs Link
To locate and download MIBs for selected platforms, Cisco IOS releases, and feature sets, use Cisco MIB Locator found at the following URL:
Feature Information for Configuring a Basic IS-IS Network
Table 1 lists the features in this module and provides links to specific configuration information. Only features that were introduced or modified in Cisco IOS Release 12.2(1), 12.0(3)S, or a later release appear in the table.
For information on a feature in this technology that is not documented here, see the "Integrated IS-IS Features Roadmap" module.
Not all commands may be available in your Cisco IOS software release. For release information about a specific command, see the command reference documentation.
Use Cisco Feature Navigator to find information about platform support and software image support. Cisco Feature Navigator enables you to determine which Cisco IOS and Catalyst OS software images support a specific software release, feature set, or platform. To access Cisco Feature Navigator, go to http://www.cisco.com/go/cfn. An account on Cisco.com is not required.
Note Table 1 lists only the Cisco IOS software release that introduced support for a given feature in a given Cisco IOS software release train. Unless noted otherwise, subsequent releases of that Cisco IOS software release train also support that feature.
Table 1 Feature Information for Configuring a Basic IS-IS Network
Cisco and the Cisco Logo are trademarks of Cisco Systems, Inc. and/or its affiliates in the U.S. and other countries. A listing of Cisco's trademarks can be found at www.cisco.com/go/trademarks. Third party trademarks mentioned are the property of their respective owners. The use of the word partner does not imply a partnership relationship between Cisco and any other company. (1005R)
Any Internet Protocol (IP) addresses and phone numbers used in this document are not intended to be actual addresses and phone numbers. Any examples, command display output, network topology diagrams, and other figures included in the document are shown for illustrative purposes only. Any use of actual IP addresses or phone numbers in illustrative content is unintentional and coincidental.
© 2011 Cisco Systems, Inc. All rights reserved.