dn-bit-set disable

Function

The dn-bit-set disable command disables OSPF from setting the DN bit in LSAs.

The undo dn-bit-set disable command enables OSPF to set the DN bit in LSAs.

The dn-bit-set disable command disables OSPFv3 from setting the DN bit in LSAs.

The undo dn-bit-set disable command enables OSPFv3 to set the DN bit in LSAs.

By default, OSPF is enabled to set the DN bit in LSAs.

By default, OSPFv3 is enabled to set the DN bit in LSAs.

Format

dn-bit-set disable { summary | ase | nssa }

undo dn-bit-set disable { summary | ase | nssa }

Parameters

Parameter	Description	Value
summary	Sets the DN bit in Type 3 LSAs.	-
ase	Sets the DN bit in AS-external-LSAs.	-
nssa	Sets the DN bit in NSSA LSAs.	-

Parameter

Description

Value

summary

Sets the DN bit in Type 3 LSAs.

ase

Sets the DN bit in AS-external-LSAs.

nssa

Sets the DN bit in NSSA LSAs.

Views

OSPFv3 view, OSPF view

Default Level

2: Configuration level

Task Name and Operations

Task Name	Operations
ospf	write

Usage Guidelines

Usage Scenario

The dn-bit-set disable command can be used in the following scenarios:

In the VPN option A scenario, the local PE imports BGP routes to generate LASs and advertise the generated LASs to the peer PE. According to standard protocols, setting of the DN bit is restricted. The peer PE may fail to calculate a route. In this situation, you can run the dn-bit-set disable command to disable OSPF from setting the DN bit in LSAs or run the undo dn-bit-set disable command to enable OSPF to set the DN bit in LSAs.
When a PE is connected to an MCE, the MCE needs to calculate routes advertised by the PE. By default, the MCE does not check the DN bit. In this situation, you can run the undo dn-bit-set disable command on the local PE to enable OSPF to set the DN bit in LSAs.
To prevent routing loops, the OSPF multi-instance process uses a bit as a flag. The bit is called DN bit.
You are advised to run the dn-bit-set disable command only in the following scenario:
In a VPN Option A scenario, the local ASBR (ASBR1) imports BGP routes to generate LSAs, advertises the LSAs to the peer ASBR (ASBR2), and exchange OSPFv3 routes with ASBR2. According to the standard protocol, ASBR2 cannot calculate the BGP routes imported by ASBR1 due to the DN bit check mechanism, causing traffic loss. To disable ASBR1 from setting the DN bit, run the dn-bit-set disable command.
When a PE is connected to an MCE, the MCE needs to calculate routes advertised by the PE. By default, the MCE does not check the DN bit. You can run the dn-bit-set disable command on the local PE to disable OSPFv3 from setting the DN bit in LSAs.
OSPFv3 multi-instance processes use a bit flag called the DN bit to prevent routing loops.

Prerequisites

A VPN instance has been created using the ip vpn-instance vpn-instance-name command and the VPN instance view is displayed.
The BGP-VPN instance IPv6 address family view has been enabled using the ipv6-family vpn-instance-name command in the VPN instance view.
The ospfv3[ process-id ] [ vpn-instance vpn-instance-name ] command has been run to enable an OSPFv3 multi-instance process and enter its view.

Configuration Impact

When the dn-bit-set disable command is used to disable OSPF from setting the DN bit in LSAs, routing loops may occur. If the parameter ase or nssa is specified, the DN bit in ASE LSAs or NSSA LSAs is not set. You can use the route-tag command to set the same tag value to prevent routing loops. Therefore, it is recommended that the dn-bit-set disable command be used in only the scenarios specified in Usage Scenario.

If the dn-bit-set disable ase command is configured, the DN bit is not set in type 5 LSAs that are translated from type 7 LSAs even if the DN bit is set in type 7 LSAs.

After the dn-bit-check disable command is run to disable OSPFv3 from checking the DN bit in LSAs, routing loops may occur. If ase or nssa is configured, you can set the same VPN route tag for imported VPN routes using the route-tag command in the OSPFv3 view to prevent routing loops. You are advised to run the dn-bit-set disable command only in VPN Option A scenarios.

Precautions

The dn-bit-set disable command can be configured for only private OSPF processes. The configuration of this command takes effect only on PEs.

The dn-bit-check disable command can be used to prevent OSPF running on the peer PE from checking the DN bit when calculating routes.

The dn-bit-set disable command can be run only in OSPFv3 VPN processes and takes effect only on PEs.

Example

# Disable OSPF from setting the DN bit in ASE LSAs.

<HUAWEI> system-view
[~HUAWEI] ip vpn-instance huawei
[*HUAWEI-vpn-instance-huawei] ipv4-family
[*HUAWEI-vpn-instance-huawei-af-ipv4] quit
[*HUAWEI-vpn-instance-huawei] quit
[*HUAWEI] ospf 100 vpn-instance huawei
[*HUAWEI-ospf-100] dn-bit-set disable ase

# Disable OSPFv3 from setting the DN bit in AS-external-LSAs.

<HUAWEI> system-view
[~HUAWEI] ip vpn-instance huawei
[*HUAWEI-vpn-instance-huawei] ipv6-family
[*HUAWEI-vpn-instance-huawei-af-ipv6] quit
[*HUAWEI-vpn-instance-huawei] quit
[*HUAWEI] ospfv3 100 vpn-instance huawei
[*HUAWEI-ospfv3-100] dn-bit-set disable ase

**Table 1** Description of the **dn-bit-set disable** command output
Item	Description
Set ASE	Enables or disables the DN bit of all ASE LSAs.
Set NSSA	Enables or disables the DN bit of all NSSA LSAs.
Set Summary LSA	Enables or disables the DN bit of all summary LSAs.
Router ID	ID of a router. The DN bit in summary LSAs generated by this router is not checked.