isisIS-IS Config

interface [intf]

Enters interface configuration mode under IS-IS to apply interface-specific IS-IS parameters.

Overview

The 'interface [intf]' command under IS-IS router configuration in Cisco IOS-XR is used to enter interface-specific IS-IS configuration mode. This allows network engineers to apply IS-IS parameters on a per-interface basis, such as metric, circuit type, hello timers, authentication, and passive mode. IS-IS (Intermediate System-to-Intermediate System) is a link-state routing protocol that operates within a routing domain. In IOS-XR, the IS-IS process is configured under the 'router isis' hierarchy, and interfaces are enabled for IS-IS by entering this sub-mode. This command is essential for fine-tuning IS-IS behavior on individual links, which is critical for optimizing convergence, controlling adjacencies, and securing routing updates. Platform-specific behavior in IOS-XR includes the requirement to commit changes explicitly and the use of interface path IDs (e.g., GigabitEthernet0/0/0/0). This command fits into troubleshooting workflows when verifying interface-specific configurations, such as checking metric values or circuit types that may affect path selection.

Syntax·IS-IS Config

interface [type] [interface-path-id]

When to Use This Command

Enable IS-IS on a specific interface and configure its metric.
Set the circuit type (Level-1, Level-2, or Level-1-2) on an interface.
Configure authentication on a per-interface basis for IS-IS.
Adjust hello interval and hold-time multipliers for faster convergence.

Parameters

Parameter	Syntax	Required	Description
type	GigabitEthernet \| TenGigE \| FortyGigE \| HundredGigE \| etc.	Required	The interface type, such as GigabitEthernet, TenGigE, etc. Must match the hardware.
interface-path-id	rack/slot/module/port	Required	The physical location of the interface in the format rack/slot/module/port (e.g., 0/0/0/0).

Command Examples

Enter IS-IS interface configuration for GigabitEthernet0/0/0/0

interface GigabitEthernet0/0/0/0

Router(config-isis-if)#

The prompt changes to IS-IS interface configuration mode for the specified interface.

Configure IS-IS metric on an interface

interface GigabitEthernet0/0/0/0
 metric 100

Router(config-isis-if)# metric 100
Router(config-isis-if)#

Sets the IS-IS metric to 100 for this interface.

Understanding the Output

The command itself does not produce output; it changes the CLI prompt to indicate IS-IS interface configuration mode. The prompt changes from 'Router(config-isis)#' to 'Router(config-isis-if)#'. This mode allows configuration of interface-specific IS-IS parameters such as metric, circuit-type, hello-interval, and authentication. A healthy configuration will show the interface in the IS-IS database with the correct parameters. Problems may include mismatched authentication or circuit types causing adjacency failures.

Configuration Scenarios

Basic IS-IS Interface Configuration

Enable IS-IS on a GigabitEthernet interface and set a custom metric.

Topology

Router A --- Gig0/0/0/0 --- Router B

Steps

1.Enter IS-IS router configuration: router isis 1
2.Enter interface configuration: interface GigabitEthernet0/0/0/0
3.Set metric: metric 50
4.Commit the configuration: commit

Configuration

router isis 1
 interface GigabitEthernet0/0/0/0
  metric 50
 !
 commit

Verify: Use 'show isis interface GigabitEthernet0/0/0/0' to verify metric and adjacency.

Watch out: If the interface is not up/up, IS-IS will not form an adjacency; ensure physical connectivity.

Troubleshooting with This Command

When troubleshooting IS-IS adjacency issues, the 'interface [intf]' command is used to verify and adjust interface-specific parameters. Common issues include mismatched circuit types (e.g., one side Level-1, the other Level-2), incorrect metrics causing suboptimal routing, or authentication mismatches. Use 'show isis interface [intf]' to display the current configuration and state. If an adjacency is not forming, check the circuit type with 'circuit-type' command and ensure both sides match. Also verify hello intervals and hold-time multipliers; if they are too aggressive, adjacencies may flap. In IOS-XR, remember to commit changes; otherwise, they are not applied. The 'show isis adjacency' command can confirm whether the neighbor is seen. If the interface is passive, no adjacencies will form; use 'passive' command to disable. For authentication, ensure both sides use the same key and mode (MD5 or HMAC-SHA).

CCNA Exam Tips

Remember that in IOS-XR, the interface command is used under the IS-IS router configuration, not under global interface config.

Know that the metric can be set per interface and defaults to 10 for wide metrics.

Be aware that circuit-type can be Level-1, Level-2, or Level-1-2 (default).

Common Mistakes

Forgetting to enter IS-IS router configuration first; the interface command is only valid under router isis.

Using the wrong interface naming convention; IOS-XR uses physical instance notation like GigabitEthernet0/0/0/0.

Not applying the configuration with 'commit' in IOS-XR; changes are not active until committed.

Platform Notes

In Cisco IOS-XR, the IS-IS interface configuration is done under the 'router isis' hierarchy, unlike classic IOS where it is under interface configuration mode. This is a key difference. IOS-XR also uses a commit model; changes are not active until 'commit' is issued. The interface naming convention in IOS-XR is 'type rack/slot/module/port' (e.g., GigabitEthernet0/0/0/0), whereas classic IOS uses 'type slot/port' (e.g., GigabitEthernet0/0). For equivalent commands on other platforms, Juniper JunOS uses 'set protocols isis interface ge-0/0/0.0' under the [edit] hierarchy. In IOS-XR, the 'interface' command is also used for other routing protocols like OSPF, but the syntax is similar. Version differences: In IOS-XR 6.x and later, wide metrics are default; in earlier versions, narrow metrics may be default. Always check the version.

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