dn-bit-set

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 RFC 4577, setting of the DN bit is restricted. The peer PE may fail to calculate a route. In this situation, the dn-bit-set disable command can be used to set the DN bit on or remove the setting of the DN bit from the local PE.
• 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, the dn-bit-set disable command can be used to set the DN bit on or remove the setting of the DN bit from the local PE.

To prevent routing loops, the OSPF multi-instance process uses a bit as a flag. The bit is called DN bit.

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 converted from type 7 LSAs even if the DN bit is set in type 7 LSAs.

Precautions

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

The dn-bit-check disable command can be used to control whether OSPF running on the peer PE checks the DN bit when calculating routes.