Why configure DN bits for OSPF on S series switches

When a PE sends Type 3, Type 5, and Type7 LSAs to a CE through Area 0, the highest optional bit of these LSAs must be reset and is called the DN bit. If the PE sends the LSAs not through Area 0, the highest optional bit of these LSAs can be reset or not.
The DN bit is used to prevent routing loops. A PE does not process the received LSA that contains a DN bit. This prevents the peer PE from learning the same LSA from the CE on a CE dual-homing network, avoiding loops.
PEs reset and check the DN bit of Type 3, Type 5, and Type7 LSAs.

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