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isisGlobal Config

router isis [instance]

Enters IS-IS router configuration mode for a specific IS-IS instance, allowing configuration of IS-IS routing protocol parameters.

Overview

The 'router isis [instance]' command is used to enter IS-IS router configuration mode on Cisco IOS-XR devices. IS-IS (Intermediate System to Intermediate System) is a link-state routing protocol used within an Autonomous System (AS), similar to OSPF. It is particularly prevalent in service provider networks due to its scalability and support for multiple network layer protocols. The command allows you to configure global parameters for the IS-IS routing process, such as the Network Entity Title (NET), metric style (narrow or wide), LSP (Link State PDU) generation intervals, and SPF (Shortest Path First) timers. In IOS-XR, IS-IS supports multiple instances, each identified by a name. The default instance is unnamed but can be referenced without specifying an instance name. This command is the first step in enabling IS-IS on a router; after entering this mode, you must configure a NET and then enable IS-IS on interfaces using the 'interface' command under the IS-IS configuration. The command is essential for network engineers deploying IS-IS in service provider or large enterprise networks. Understanding how to navigate into this mode and configure basic parameters is fundamental for CCNP and CCIE-level certifications.

Syntax·Global Config
router isis [instance-name]

When to Use This Command

  • Configuring a new IS-IS routing process on a router.
  • Modifying existing IS-IS parameters such as NET, metric style, or LSP timers.
  • Enabling IS-IS on interfaces and setting interface-specific parameters.
  • Troubleshooting IS-IS adjacency issues by reviewing and adjusting configuration.

Parameters

ParameterSyntaxDescription
instance-nameinstance-nameOptional name of the IS-IS instance. If omitted, the default instance is used. The instance name is a string that identifies the IS-IS process. Multiple instances can be configured for different routing domains or administrative purposes.

Command Examples

Entering IS-IS configuration for default instance

router isis
RP/0/RP0/CPU0:router(config-isis)#

The prompt changes to indicate IS-IS configuration mode for the default instance.

Entering IS-IS configuration for a named instance

router isis CORE
RP/0/RP0/CPU0:router(config-isis-CORE)#

The prompt includes the instance name 'CORE', allowing configuration of that specific IS-IS process.

Understanding the Output

The command itself does not produce output; it changes the CLI prompt to indicate that you are in IS-IS router configuration mode. The prompt format is 'RP/0/RP0/CPU0:router(config-isis)#' for the default instance or 'RP/0/RP0/CPU0:router(config-isis-<instance>)#' for a named instance. This mode allows you to configure global IS-IS parameters such as NET, metric-style, LSP generation timers, and more. To exit this mode, use 'exit' or 'end'. The prompt is the primary indicator that you are in the correct context for IS-IS configuration.

Configuration Scenarios

Basic IS-IS Configuration for Default Instance

A single-area IS-IS network with a backbone router.

Topology

Router A --- Router B

Steps

  1. 1.Enter global configuration mode: configure terminal
  2. 2.Enter IS-IS configuration: router isis
  3. 3.Configure NET: net 49.0001.0000.0000.0001.00
  4. 4.Enable IS-IS on an interface: interface GigabitEthernet0/0/0/0
  5. 5.Under interface, enter: router isis
Configuration
!
router isis
 net 49.0001.0000.0000.0001.00
!
interface GigabitEthernet0/0/0/0
 router isis
!

Verify: Use 'show isis neighbors' to verify adjacency.

Watch out: The NET must be unique per router and include the area ID and system ID.

Troubleshooting with This Command

When troubleshooting IS-IS issues, the first step is to verify that the IS-IS process is configured correctly. Use 'show isis' to display global IS-IS information, including the NET and instance status. If the process is not running, check that a NET is configured. Use 'show isis neighbors' to verify adjacencies; if none are formed, check interface configuration and ensure that 'router isis' is enabled under the interface. Also verify that the metric style matches across neighbors (narrow vs wide). Use 'show isis database' to examine the LSDB and check for missing routes. In IOS-XR, logging can be enabled with 'log adjacency changes' to track adjacency events. The 'router isis' command itself does not provide troubleshooting output, but it is the gateway to all IS-IS configuration that affects behavior.

CCNA Exam Tips

1.

Remember that 'router isis' without an instance name enters the default instance, which is common in CCNP scenarios.

2.

In IOS-XR, IS-IS instances are named; the default instance is unnamed but still considered an instance.

3.

Be aware that you must configure a NET (Network Entity Title) for the IS-IS process to become active.

Common Mistakes

Forgetting to specify the instance name when multiple IS-IS processes exist, leading to configuration of the wrong instance.

Assuming 'router isis' is available in global config mode; it is only available after entering 'configure terminal'.

Not configuring a NET, which prevents the IS-IS process from starting.

Platform Notes

In Cisco IOS-XR, IS-IS configuration differs from classic Cisco IOS. IOS-XR uses a hierarchical configuration model and requires explicit commitment of changes. The 'router isis' command is available in global config mode, but unlike IOS, you must use 'commit' to apply changes. Also, IOS-XR supports multiple IS-IS instances, whereas classic IOS typically supports only one. The NET format is the same, but IOS-XR uses a different interface configuration approach: you enable IS-IS on an interface by entering 'router isis' under the interface configuration, not 'ip router isis' as in IOS. Additionally, IOS-XR has a separate 'address-family ipv4 unicast' configuration under IS-IS to control which address families are supported. These differences are important for engineers transitioning from IOS to IOS-XR.

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