What are the five flag bits in OSPF Hello packet that specify optional capabilities?

This may be a textbook question and probably less important.


If you mean the options field, those are the optional capabilities supported by the router, and it is not only in Hello packets, but also in Database Description packets and LSAs.

The field is detailed in RFC 2328, OSPF Version 2, A.2 The Options field:

A.2 The Options field

The OSPF Options field is present in OSPF Hello packets, Database Description packets and all LSAs. The Options field enables OSPF routers to support (or not support) optional capabilities, and to communicate their capability level to other OSPF routers. Through this mechanism routers of differing capabilities can be mixed within an OSPF routing domain.

When used in Hello packets, the Options field allows a router to reject a neighbor because of a capability mismatch. Alternatively, when capabilities are exchanged in Database Description packets a router can choose not to forward certain LSAs to a neighbor because of its reduced functionality. Lastly, listing capabilities in LSAs allows routers to forward traffic around reduced functionality routers, by excluding them from parts of the routing table calculation.

Five bits of the OSPF Options field have been assigned, although only one (the E-bit) is described completely by this memo. Each bit is described briefly below. Routers should reset (i.e. clear) unrecognized bits in the Options field when sending Hello packets or Database Description packets and when originating LSAs. Conversely, routers encountering unrecognized Option bits in received Hello Packets, Database Description packets or LSAs should ignore the capability and process the packet/LSA normally.

               | * | * | DC | EA | N/P | MC | E | * |

                     The Options field


This bit describes the way AS-external-LSAs are flooded, as described in Sections 3.6, 9.5, 10.8 and 12.1.2 of this memo.


This bit describes whether IP multicast datagrams are forwarded according to the specifications in [Ref18].


This bit describes the handling of Type-7 LSAs, as specified in [Ref19].


This bit describes the router's willingness to receive and forward External-Attributes-LSAs, as specified in [Ref20].


This bit describes the router's handling of demand circuits, as specified in [Ref21].

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