dn-bit-check

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 RFC 4577, 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.

Configuration Impact

When 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.

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 the PEs.

In this scenario, you can run the dn-bit-set disable command to set the DN bit on or remove the setting of the DN bit from the local PE.