OSPF DBD (Database Descriptor) Packets: OSPF Packet Types

Introduction to OSPF DBD

IP routing is the most important aspect of networking. There are several types of protocols which support IP routing using distance vector mechanism or link state mechanism. Ultimate goal of IP routing is to find the best shortest path for packet delivery in the network. Link state routing protocols are more commonly and widely used as they support large enterprises, IPv4, IPv6 networks, CIDR and VLSM addressing models. Some routing protocols use transport protocols such as TCP and UDP for data transmission, however protocols such as Open Shortest path first (OSPF) will not carry data using TCP and UDP.

Today we look more in detail about OSPF DBD (database descriptor) packet type, its purpose, working and features.

Download the OSPF DBD Packet Cheatsheet

Also Refer: OSPF Packet Types: The Ultimate Guide

OSPF Database Descriptor (DBD) Packet: Working & Features 

The link state routing protocols required their link state database should remain synchronized across all routers to function properly. The synchronization starts as soon as adjacency is performed between neighbouring routers. The OSPF uses a database descriptor (DBD) packet to achieve this purpose. The database comprises multiple packets. The poll response procedure is used to describe multiple packet usage. The routers are designated as Master and slaves. Database description is sent by slave router after master sends the database description packet. 

• OSPF router performs summarization of database descriptor (DBD) packets and LSA set belongs to that database.
• When the neighbour views LSA , having a more recent copy of its own database, it requests a new LSA from the neighbour.
• Routers holding LSA do not send packets immediately after hearing hello and parameter check; instead each router will create and send the database descriptor (DBD) packets which contain each LSA header.
• The header includes information for unique identification of LSA and the revision.
• The DD message will use OSPF defined simple error recovery process. Each DD packet has several LSA headers, with an assigned sequence number.
• The receiver acknowledgement happens for received DD packets by sending DD packets with the same sequence number.
• Sender will use a window size of 1 packet before sending the next DD packet and wait for acknowledgement.
• As the relationship between two routers is formed by neighbours it determines which router will be master and which router will be slave during the exchange of database.
• During DD packet exchange the role of master and slave determine routers responsibilities. The LSA header has a local router database.
• DD sequence number is used to sequentially arrange database descriptor (DBD) packet collection. The sequence number increments by 1 until the overall database description is sent. 
• Interface MTU is having MTU value of outgoing interface. For virtual fields this value is set to 0x0000. 

A typical OSPF DBD (database descriptor) Packet

A typical OSPF DBD packet looks like as under:

Continue Reading:

EIGRP Packet Types: The Complete Guide

OSPF Router Types: Detailed Explanation

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ABOUT THE AUTHOR

Rashmi Bhardwaj

I am here to share my knowledge and experience in the field of networking with the goal being – “The more you share, the more you learn.”

I am a biotechnologist by qualification and a Network Enthusiast by interest. I developed interest in networking being in the company of a passionate Network Professional, my husband.

I am a strong believer of the fact that “learning is a constant process of discovering yourself.”
– Rashmi Bhardwaj (Author/Editor)