{思科CCIE-RS}OSPF DN Bit用于MPLS VPN LSA3防环（扩展） BY乱雪纷飞

The previously unused bit in the OSPF LSA Options Field is referred to as the DN Bit. This bit is set on Type 3, 5, and 7 LSA when the MP-BGP routes are redistributed into OSPF. When the other PE Router receives the LSA from a CE router Type 3, 5, or 7 LSA with the DN Bit set, the information from that LSA is not used in the OSPF route calculation.

Based on the network topology, PE2 sets the DN Bit for the redistributed LSA and this LSA is never considered for route calculation in OSPF Process 2 on PE3. So PE3 never redistributes this route back into MP-BGP.