attached-bit (IS-IS topology view)

Function

The attached-bit avoid-learning command prevents an IS-IS Level-1 router from generating a default route after it receives LSPs carrying Attached (ATT) bit 1.

The undo attached-bit avoid-learning command restores the default configuration.

The attached-bit advertise command configures a rule for setting the Attached (ATT) bit in LSPs.

The undo attached-bit advertise command restores the default configuration.

By default, an IS-IS Level-1 router generates a default route only after it receives LSPs carrying ATT bit 1.

By default, the Level-1-2 router sets the ATT bit in LSPs using the default rule.

Format

attached-bit { { advertise { always | never } } | avoid-learning }

undo attached-bit advertise

undo attached-bit avoid-learning

Parameters

Parameter	Description	Value
always	Indicates that the ATT bit is set to 1.	-
never	Indicates that the ATT bit remains at 0.	-

Parameter

Description

Value

always

Indicates that the ATT bit is set to 1.

never

Indicates that the ATT bit remains at 0.

Views

IS-IS topology view

Default Level

2: Configuration level

Task Name and Operations

Task Name	Operations
isis	write

Usage Guidelines

Usage Scenario

An ATT bit is a field in LSPs. An ATT bit identifies whether a Level-1 area is attached to other areas. A Level-1-2 router sets the ATT bit to 1 to notify the routers in the Level-1 area of its attachment to a Level-2 backbone area. After a Level-1 router receives the LSPs carrying the ATT bit 1 from the Level-1-2 router, the Level-1 router generates a route with the same destination address as the default route of the Level-1-2 router. Traffic can be forwarded along this route.

To prevent the Level-1 router from advertising the default route when the ATT bit is set to 1, run the attached-bit avoid-learning command.

To prevent the Level-1 router from advertising default routes to the routing table, use either of the following methods:

Run the attached-bit advertise never command on the Level-1-2 router to disable the router from sending LSPs with the ATT bit 1.
Run the attached-bit avoid-learning command on the Level-1 router that is connected to the Level-1-2 router.
The difference between the preceding commands lies in that the attached-bit avoid-learning command applies to a specified router.

Prerequisites

An IS-IS process has been created and the IS-IS view has been displayed using the isis command.

Precautions

The ATT bit is defined in both Level-1 and Level-2 LSPs, but it can be set to 1 only in Level-1 LSPs. Additionally, only the Level-1-2 router can set the ATT bit. Therefore, the attached-bit avoid-learning command takes effect only on the Level-1-2 router.

Example

# Enable a Level-1-2 router to set the ATT bit to 1 in the LSPs to be sent in IS-IS process 1.

<HUAWEI> system-view
[~HUAWEI] isis 1
[*HUAWEI-isis-1] cost-style wide
[*HUAWEI-isis-1] topology red topology-id 10
[*HUAWEI-isis-1-topology-red] attached-bit advertise always

# Disable IS-IS-enabled router from advertising default routes to a routing table when the ATT bit is set to 1.

<HUAWEI> system-view
[~HUAWEI] isis 1
[*HUAWEI-isis-1] cost-style wide
[*HUAWEI-isis-1] topology red topology-id 10
[*HUAWEI-isis-1-topology-red] attached-bit avoid-learning