OSPF - DN BIT

Dear User,

When a PE advertises Type 3, Type 5, and Type 7 LSAs to a CE through Area 0, the highest optional bit (called DN bit) of these LSAs must be reset. If the PE advertises these LSAs to a CE through an area that is not Area 0, the DN bit does not need to be reset.

The DN bit is used to prevent loops. A PE does not process the received LSA that contains a DN bit. In a CE dual-homing scenario, this prevents the other PE from learning the same LSA from the CE and so prevents loops.
The PE sets and checks the DN bit in Type 3, Type 5, and Type 7 LSAs.

https://support.huawei.com/enterprise/en/doc/EDOC1000091883/de483cb/what-is-the-function-of-the-dn-bit-in-ospf-lsas

Thanks!

Hello,
Please refer to the link below

https://forum.huawei.com/enterprise/en/what-is-the-purpose-of-setting-the-dn-bit-in-an-ospf-lsa/thread/397729-863#:~:text=The DN bit is used,Type 3 and 5 LSAs.

Thanks

Hello,

The typical scenario is that a CE is dual-homed to PEs, or different CEs in an AS are connected to different PEs. 

As shown in the preceding figure, OSPF runs between PEs and CEs. By default, the DN-bit in the option field of LSAs is used to prevent loops and prevent suboptimal route selection. The DN set takes effect on the PE. 

1. When the PE1 sends LSA3, LSA5, and LSA7 to the CE1, the DN bit in the LSAs is set to 1. 

2. PE2 receives the LSAs with DN bit set to 1 but does not use them for route calculation. 

Thank you guys for the help.