OSPF DD(Database Description) Packet

OSPF DD packets are used to synchronize LSDB between routers in an adjacency relationship. DD packets provide an overview of the LSDB. The DD packet includes a list of LSA headers as an overview of the LSDB. When DD packets are exchanged between routers in an adjacency, they will know which LSAs they do not have with each other.

It is sometimes referred to as a DBD (DataBase Description) packet.
Figure OSPF DD packet overview
Figure OSPF DD packet overview

OSPF DD Packet Format

Type 2 of the OSPF header indicates that it is a DD packet; the format of a DD packet is as follows

Figure OSPF DD Packet format
Figure OSPF DD Packet format

Interface MTU(16bits)

Interface MTU (16 bits) is the MTU size of the interface that outputs DD packets. DD packets may be larger in size and may be split. If the neighbor and MTU size do not match, DD packets may not be exchanged properly. So, If the MTU sizes do not match, the neighbor will get stuck in the ExStart state.

Options(8bits)

Options (8 bits) indicate the various capabilities of the OSPF router. 5 bits following Options are fixed at “00000”.

I-bit

The I in I-bit stands for Initial. Only the first DD packet in a series of DD packet exchanges has I-bit=1.

M-bit

The M in M-bit stands for More, and M-bit=1 indicates that there is more to the DD packet. M-bit = 1 indicates that there is more to the DD packet, and M-bit = 0 at the end of a series of DD packet exchanges.

M/S-bit

MS-bit means Master/Slave; the router that starts exchanging DD packets is the Master. M/S-bit=1 indicates that the router is the Master router. The router with the larger router ID is the Master router.

DD Sequence Number(32bits)

The DD Sequence Number (32 bits) ensures that DD packets are received; the router that serves as the Master determines a unique initial value and increments the sequence number in subsequent DD packet exchanges.

LSA Headers

A list of LSA headers for all LSAs in the LSDB of the router generating the DD packet.

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