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OSPF Options Field


OSPF Options Field




  • DN = Down bit : used in MPLS Layer 3 VPNs. If a route learnt from a customer network via OSPF is advertised across a BGP / MPLS VPN using Multiprotocol BGP, and is advertised back to a customer network via OSPF, there is a possibility of a loop to occur in which the OSPF route is redistributed back to the VPN service provider network via BGP. The DN bit prevents this type of routing loop.
  • O = The “O” bit is set when the originating router supports Type 9,10 and 11 opaque LSAs. Not used in normal OSPF implementations
  • DC = The “DC” bit is set when the originating router supports OSPF over Demand Circuits. Not used in normal OSPF implementations
  • L = Indicates whether the OSPF packet contains a LLS (Link-Local signalling) data block. This bit is set only in Hello and DBD packets.
  • N = The N bit is used only in Hello packets. The N bit is set when the originating router supports Type-7 NSSA-External-LSAs. Neighboring routers with mismatched N bit value will not form neighbor relationship. This restriction ensures that all OSPF routers within an area support NSSA capabilities. When the N bit is set to 1, the E bit must be 0.
  • P = The P bit is used only in Type-7 NSSA-External-LSA headers. Due to this reason, the N and P bits can share the same position in the Options Field. The P (Propogate) bit is set to inform the NSSA ABR to translate Type-7 LSAs into Type-5 LSAs.
  • MC = The MC bit is set when the originating router supports Multicast extensions to OSPF (MOSPF). Again not used in normal OSPF implementations.




The above fields can be found in the packet capture, under OSPF, as below:





OSPF Options Field

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