OSPF v3 Packet Formats

OSPF v3 Packet Formats

The "Main OSPF Packet Header" is the same for all 5 types of packets (with exception of the Type field) whereas the following sub-headers will vary from type to type and are shown below the Main OSPF Packet Header.

There are 5 OSPF Packet formats as follows:

Type  Description
1         Hello
2         Database Description
3         Link State Request
4         Link State Update
5         Link State Acknowledgement

The five different formats for each "Type" of OSPF v3 packet are listed below:

Type 1: The Hello Packet

Type 2: The Database Description Packet

Type 3: The OSPF Link State Request Packet

Type 4: The OSPF Link State Update

Type 5: The OSPF Link State Acknowledgement Packet

The OSPFv3 (24 Bit) Options Field

This "Options Field" is used in OSPF Hello packets, Database Description packets, and certain LSAs (router-LSAs, network-LSAs, inter-area-router-LSAs, and link-LSAs).

(Note: Previous OSPF versions {v1 & v2} DO NOT support all of the options/fields listed here.)

Explanation of the bits in the Options field:

There are currently only 7-bits assigned.

• V6-bit: "V6" stands for IP[v6] routing calculations are to be used.
• E-bit: "E" stands for [E]xternal as in AS-External-LSA flooding as specified in OSPFv2.
• x-bit: This is currently deprecated. It was previously used by MOSPF.
• N-bit: "N" stands for [N]SSA (Not So Stubby Area) and used for routers which are attached to NSSA networks.
• R-bit: "R" stands for [R]outer and specifies whether the router is Active or not.
• DC-bit: "DC" stands for [D]emand [C]ircuts 
• *-bits: These two bits are reserved for migration of OSPFv2 protocol extensions.

The remaining 16-bits have yet to be assigned.

OSPF in broadcast and non-broadcast networks

In broadcast multiple-access networks, neighbor adjacency is formed dynamically using multicast hello packets to 224.0.0.5. A DR and BDR are elected normally, and function normally.
For non-broadcast multiple-access networks (NBMA) defined the two official modes of OSPF:

• nonbroadcast
• point-to-multipoint

Cisco has defined the following three additional modes for OSPF in NBMA topologies:

• point-to-multipoint nonbroadcast
• broadcast
• point-to-point