Enhance your Career in Networking With IPinBits!!!​

OSPF Packets: DBD

Let’s open the DBD packet and use what’s inside, why DBD is required and how DBD are exchanged

DBD: DATABASE Description packet, which has message type 2 inside OSPF Hellos. The below capture is for DBD, and the contents inside DBD.

Why DBD packets are used?

  • OSPF is a link state routing protocol, its needs to maintain visibility of the entire network or entire area.
  • DBD packets contain the summary of link state database, DBD are used to compare the databases. They do a quick scan of what’s present in the database and what is not.
  • During the DBD packet exchange, a Master/ Slave relationship is established between the Neighbors. The router with the highest Router ID becomes the Master and initiates DBD packet exchange.
  • The Interface MTU should match on Neighbors otherwise FULL adjacency is not reached it gets stuck in the Exstart state.

Refer the following article for more on MTU Mismatch in OSPF.

Let’s check the DBD content:

Interface MTU– Contains the MTU value of the outgoing interface. For virtual-links, this field is set to 0x0000. (2-bytes)
Options– Same as Options field in a Hello packet (1-byte)
I- Initial Bit. Indicates this is the first in the series of DBD packets (1-bit)
M- More bit. Indicates whether the DBD packet is the last in the series of packets. Last packet has a value of 0, while all previous packets have a value of 1. (1-bit)
MS- Master/ Slave bit. Master=1, Slave=0 (1-bit)
DD Sequence Number– Used to sequence the collection of DBD packets. The initial value should be unique. The sequence number then increments by 1 until the complete database description has been sent. (4-bytes)
LSA Header– This field contains the LSA headers describing the local router’s database. (variable length).

Related blog posts