isisIS-IS Interface Config

circuit-type level-2-only

Configures the IS-IS interface to operate as a Level-2-only circuit, disabling Level-1 adjacency formation on that interface.

Overview

The 'circuit-type level-2-only' command in Cisco IOS-XR is used to configure an IS-IS interface to operate exclusively as a Level-2 circuit. IS-IS (Intermediate System to Intermediate System) is a link-state routing protocol that supports a two-level hierarchy: Level-1 (intra-area) and Level-2 (inter-area). By default, interfaces on IOS-XR routers are configured as Level-1-2, meaning they can form both Level-1 and Level-2 adjacencies. However, in many network designs, it is beneficial to restrict an interface to a single level to reduce routing overhead, simplify configuration, and improve scalability.

This command is typically used on core routers or backbone links where only Level-2 adjacencies are needed. For example, in a large enterprise or service provider network, core routers often connect to other core routers via high-speed links. Setting these interfaces to Level-2-only ensures that they do not participate in Level-1 routing, which is usually confined to access or distribution layers. This reduces the number of hello packets and routing updates exchanged, conserving bandwidth and CPU resources.

When to use this command: It is appropriate when an interface connects two routers that are both in the same Level-2 area or when the interface is part of the backbone. It is also used in migration scenarios where a network is transitioning from a flat Level-1-2 design to a hierarchical Level-2-only backbone. In troubleshooting workflows, verifying the circuit type helps identify why an adjacency is not forming as expected. For instance, if a Level-1-only router is connected to a Level-2-only interface, no adjacency will form. Understanding this command is essential for designing and troubleshooting IS-IS networks on IOS-XR platforms.

Syntax·IS-IS Interface Config

circuit-type level-2-only

When to Use This Command

In a multi-level IS-IS network, configure core routers with only Level-2 adjacencies to reduce routing overhead.
On a WAN link connecting two Level-2 backbone routers, ensure only Level-2 adjacencies are formed.
When an interface is used solely for inter-area traffic, avoid unnecessary Level-1 hello packets.
In a migration scenario from Level-1/Level-2 to Level-2-only, apply this command to simplify the design.

Parameters

Parameter	Syntax	Required	Description
level-2-only	level-2-only	Required	Specifies that the interface should only form Level-2 adjacencies. This is the only option for this command; there is no 'level-1-only' variant in IOS-XR (use 'circuit-type level-1' instead).

Command Examples

Configure Level-2-only on GigabitEthernet0/0/0/0

router isis 1
 interface GigabitEthernet0/0/0/0
  circuit-type level-2-only
  commit

The command is applied under the IS-IS interface configuration mode. No output is shown upon successful commit; use 'show isis interface' to verify.

Verify Level-2-only configuration

show isis interface GigabitEthernet0/0/0/0

GigabitEthernet0/0/0/0
  Circuit Type: Level-2
  ...

The output shows 'Circuit Type: Level-2', confirming the interface is configured as Level-2-only.

Understanding the Output

The 'show isis interface' command displays the circuit type for each IS-IS interface. The 'Circuit Type' field indicates whether the interface is Level-1, Level-2, or Level-1-2. A healthy Level-2-only interface shows 'Level-2'. If the field shows 'Level-1-2' or 'Level-1', the configuration is not applied or has been overridden. This output is crucial for verifying that the intended circuit type is active, especially after configuration changes or during troubleshooting of adjacency issues.

Configuration Scenarios

Configuring a Core Router Backbone Interface

A core router in an ISP network connects to another core router via a 10G link. The network uses a two-level IS-IS hierarchy with Level-1 in access areas and Level-2 in the backbone.

Topology

Core1 --- 10G --- Core2
Both routers are Level-2 backbone routers.

Steps

1.Enter IS-IS router configuration mode: 'router isis 1'
2.Enter interface configuration mode: 'interface GigabitEthernet0/0/0/0'
3.Apply the circuit type: 'circuit-type level-2-only'
4.Commit the configuration: 'commit'

Configuration

router isis 1
 interface GigabitEthernet0/0/0/0
  circuit-type level-2-only
  commit

Verify: Use 'show isis interface GigabitEthernet0/0/0/0' to confirm 'Circuit Type: Level-2'.

Watch out: If the remote router's interface is configured as Level-1-only, no adjacency will form. Ensure both ends are compatible.

Troubleshooting with This Command

When troubleshooting IS-IS adjacency issues on IOS-XR, the 'circuit-type' configuration is a common point of failure. Use 'show isis interface' to verify the circuit type on each interface. If an adjacency is not forming, check that the circuit types on both ends are compatible: Level-2-only interfaces only form adjacencies with other Level-2 or Level-1-2 interfaces. A Level-1-only interface will not form an adjacency with a Level-2-only interface. Additionally, ensure that the IS-IS process is configured with the correct levels (e.g., 'is-type level-2-only' under the router process) if the entire router should only participate in Level-2. The 'show isis adjacency' command can also help; if an adjacency is missing, the 'show isis interface' output will indicate the circuit type. If the circuit type is incorrect, reconfigure it and commit. Also, verify that the interface is not passive (under 'passive-interface') as that would prevent hello packets. In summary, always start troubleshooting by checking the circuit type and ensuring consistency between neighbors.

CCNA Exam Tips

Remember that 'circuit-type level-2-only' is configured under the interface, not the router IS-IS process.

In CCNP exams, know that Level-2-only interfaces do not form Level-1 adjacencies, reducing hello overhead.

Be aware that the default circuit type in IOS-XR is Level-1-2; changing it to Level-2-only affects adjacency formation.

Common Mistakes

Applying the command under the router IS-IS process instead of the interface configuration mode.

Forgetting to commit the configuration after applying the command, leaving it unapplied.

Assuming Level-2-only disables all Level-1 functionality; it only affects the specific interface.

Platform Notes

In Cisco IOS-XR, the 'circuit-type' command is available under IS-IS interface configuration mode. Unlike classic Cisco IOS, where the command is 'isis circuit-type level-2-only' under interface configuration, IOS-XR uses a hierarchical configuration style with 'router isis' and then 'interface'. The default circuit type in IOS-XR is Level-1-2. There is no 'level-1-only' option; instead, use 'circuit-type level-1' to set Level-1-only. This command is supported from IOS-XR release 4.0 onwards. On other platforms like Juniper Junos, the equivalent is 'level 2' under 'isis' interface configuration. Understanding these differences is crucial for multi-vendor environments.

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