In which configuration mode is interface [intf] used?

The interface [intf] command is entered in IS-IS Config mode on Cisco IOS-XR devices.

isisIS-IS Config

interface [intf]

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

Definition: interface [intf] is a Cisco IOS-XR is-is config command. 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.

interface [intf] vs address-family ipv4 unicast

Both 'interface [intf]' and 'address-family ipv4 unicast' are submodes entered under the 'router isis' configuration, but they serve distinct purposes: the former applies IS-IS settings to a specific interface, while the latter configures global parameters for the IPv4 unicast address family. Confusion arises because both are accessed from the IS-IS router configuration mode, yet their scopes and impacts differ significantly.

Aspect	interface [intf]	address-family ipv4 unicast
Scope	Interface-specific IS-IS parameters	Global IPv4 unicast address family parameters
Configuration mode	Enters interface configuration sub-mode under IS-IS	Enters address-family configuration sub-mode under IS-IS
Persistence	Settings persist per interface within IS-IS instance	Settings persist for entire IPv4 unicast topology in IS-IS
Precedence	Interface-specific settings override global address-family defaults	Global settings apply unless overridden by interface configuration
Typical use	Enabling IS-IS on an interface or setting interface metric, circuit type	Setting default metric, router-id, or distribute-lists for IPv4 unicast

Use interface [intf] when you need to apply IS-IS parameters to a specific interface, such as enabling IS-IS or setting interface-specific metrics or circuit types.

Use address-family ipv4 unicast when you need to configure global IPv4 unicast routing parameters within the IS-IS process, such as default metric, router ID, or filtering policies.

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.

Practice for the CCNP Service Provider

Test your knowledge with practice questions covering all CCNP Service Provider exam domains.

Practice CCNP Service Provider Questions