OSPF message options field

Various types of OSPF messages contain what is known as the options field. This field enables OSPF routers to support or not support optional capabilities and to communicate their capability level to other OSPF routers.

With the use of this field, OSPF routers with differing capabilities can be mixed and matched into the same OSPF topology.

OSPF Hello packets, Database Description packets, and all OSPF LSAs contain this options field, and they are used as follows:

  • When used in a Hello packet, an OSPF router is given the ability to reject a neighbor due to a capability mismatch.
  • When used in a Database Description packet, a router can choose not to forward certain LSAs to a neighbor because of limited functionality.
  • When used in an LSA, it allows routers to forward traffic around reduced functionality routers by excluding them from parts of the routing table calculation.

The options field contains 5 bits, which are essentially flags that denote the following:

  • E-bit - This bit describes the way AS-external-LSAs are flooded
  • MC-bit - This bit describes whether IP multicast datagrams are forwarded
  • N/P-bit - This bit describes the handling of Type-7 LSAs
  • EA-bit - This bit describes the router's willingness to receive and forward External-Attributes-LSAs
  • DC-bit - This bit describes the router's handling of demand circuits

More information about these can be found in the related RFC in the links.