The dn-bit-set disable command disables OSPF from setting the DN bit in an LSA.
The undo dn-bit-set disable command restores the default setting.
By default, OSPF sets the DN bit in an LSA.
| Parameter | Description | Value |
|---|---|---|
| summary | Sets the DN bit in all LSAs. | - |
| ase | Sets the DN bit in an AS-external-LSA. | - |
| nssa | Sets the DN bit in an NSSA LSA. | - |
Usage Scenario
When OSPF is used in an inter-AS VPN optionA scenario, a PE sends an LSA carrying an imported BGP route to the other PE. As defined in RFC 4577, the PE cannot calculate the BGP route by using the LSA due to DN bit suppression. The dn-bit-set disable command or the undo dn-bit-set disable command can be used.
If a PE is connected to an MCE, the MCE needs to calculate routes by using LSAs advertised by the PE. If the MCE must check the DN bit in LSAs, the undo dn-bit-set disable command can be used on the PE.
Precautions
The dn-bit-set disable command is used to disable OSPF from setting the DN bit in an LSA, which causes routing loops. And this command is only supported in the OSPF process on the private network.
If ase or nssa is set in the dn-bit-set disable command, the route-tag command can be used to set the same tag for AS-external LSAs or NSSA LSAs to prevent routing loops.