dn-bit-check disable

Function

The dn-bit-check disable command disables OSPF from checking the DN bit in LSAs.

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

The dn-bit-check disable command disables OSPFv3 from checking the DN bit in LSAs.

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

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

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

Format

dn-bit-check disable { ase | nssa }

dn-bit-check disable summary [ router-id router-id ]

undo dn-bit-check disable { ase | nssa }

undo dn-bit-check disable summary [ router-id router-id ]

Parameters

Parameter	Description	Value
ase	Indicates that the DN bit in ASE LSAs is not checked.	-
nssa	Indicates that the DN bit in NSSA LSAs is not checked.	-
summary	Indicates that the DN bit in summary LSAs is not checked.	-
router-id router-id	Specifies the ID of a device on which the DN bit in summary LSAs is checked.	The value is in dotted decimal notation.

Views

OSPFv3 view, OSPF view

Default Level

2: Configuration level

Task Name and Operations

Task Name	Operations
ospf	write

Usage Guidelines

Usage Scenario

In the VPN option A scenario, the local PE imports BGP routes to generate LSAs and advertise the generated LSAs 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 need to use the dn-bit-check disable command to disable OSPF from checking 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.

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 ASBR2 from checking the DN bit in LSAs, run the

dn-bit-check disable command.

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

After the dn-bit-check disable command is run, routing loops may occur. If the parameter ase or nssa is specified, the DN bit in ASE LSAs or NSSA LSAs is not checked. You can use the route-tag command to set the same tag value to prevent routing loops. Therefore, run the dn-bit-check disable command only in the scenario specified in Usage Scenario.

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-check disable command only in VPN Option A scenarios.

Precautions

When a PE is connected to an MCE, the MCE does not check the DN bit by default.

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

The dn-bit-set disable command disables OSPF from checking the DN bit in LSAs on the local PE.

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

When a PE is connected to an MCE, the MCE does not check the DN bit in LSAs by default.

Example

# Disable OSPF from checking the DN bit in summary 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-check disable summary router-id 1.1.1.1

# Disable OSPFv3 from checking the DN bit in Type 3 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-check disable summary router-id 1.1.1.1

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