OSPF Messages

OSPF Messages/Packets

Before you start

Objectives : You will be able to understand how OSPF work, and different function and feature of OSPF messages.

Prerequisites: You have to know about OSPF protocol.

Tags : OSPF , Routing Protocol, Hello, DBD, LSR, LSU, LSAck.

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1. Hello Message

Hello Protocol or message is used to discover neighbor. By default it is send whenever OSPF process is enabled on any router. In this message there are various component which is send to neighbor router, and then the other router match these component or parameter and then become neighbor or each other.

Contents of HELLO message :

• OSPF version
• Network mask
• HELLO interval
• Type 1
• CHECKSUM
• Area ID
• Router ID
• Dead interval
• Stub flag
• Priority
• Authentication type
• Authentication
• Designated router
• Backup designated router
• Neighbor Router ID
• Neighbor router priority

Requirement for OSPF neighborship:

• Area ID must match
• Timers must match
• Authentication must match
• Subnetmask must match
• Stub flag must match
• MTU size must match
• Router ID must not match
• Network type should match

2. DBD (Database Description)

DBD (Database Description) is used to exchange all type of LSA's for one OSPF neighbor to another OSPF neighbor. With the help of DBD each router exchange initial topology so that each router comes to know about topology used in the network.
It is also known as Type 2 message.

3. Link-State Request (LSR)

If a new link comes up or a old link goes down, then OSPF routers, request to a new Database in the form of LSAs. The request from one router to another router is made by LSR message.
It is also known as Type 3 Message.

4. LSU (Link-State Update)

As we discuss in Type 3 message, if a link comes up or goes down, then OSPF routers immediately send this information to all the neighbor of that router. This information is flood in the form of LSU message or packet.
It is also known as Type 4 Message.

5. LSAck (Link-State Acknowledgment)

Now, we comes to know that OSPF protocol exchange the data in the form of various LSAs. So, whenever a LSA is send from one Router to another Router, then the sender Router receive the acknowledgment of that particular LSA.
It is also known as Type 5 Message.