OSPF Packet Types

OSPF is an open-standard, classless routing protocol that converges quickly and uses cost as a metric. It encapsulates the five different types of OSPF messages inside IP packets, using IP protocol 89.

OSPF (Open Shortest Path First) is a link-state routing protocol. It means, that every router builds a database of connected links and their properties. Then, that knowledge is exchanged between all the routers in a given area - so that all the routers know about all the links and their properties.


IETF OSPF working group was created in 1987. RFC 1131 was the first protocol specification and was published in 1989. The more important OSPF specifications include:

• RFC 1131, "OSPF specification" - initial revision

• RFC 1247, "OSPF Version 2" - corrects most OSPFv1 drawbacks. Not backward compatible.

• RFC 1587, "The OSPF NSSA Option" - includes not-so-stubby area specification.

• RFC 1850, "OSPF Version 2 Management Information Base" - allows for remote SNMP management.

• RFC 2328, "OSPF Version 2" - final revision.

 

1. Hello: Identifies neighbors and serves as a keepalive. Establish communication with directly attached neighbors.

2. LSA – Links State Advertisement : These messages are used by OSPF to share info about network and propagate it through all OSPF domain.

There are in general 11 types of LSA but most common and important one are LSA 1,2,3,4,5,7

Type 1 – Router LSA
Type 2 – Network LSA
Type 3 – Summary LSA
Type 4 – ASBR Summary LSA
Type 5 - External LSA
Type 6 – Group Membership - Multicast LSA
Type 7 – NSSA External Link LSA
Type 8 – External Attributes LSA for BGP
Type 9 – Opaque LSA (link-local scope) - used for Graceful Restart

Type10 – Opaque LSA (area-local scope) - used for Traffic Engineering
Type 11 - Opaque LSA (AS scope)



Detail description of each LSA 




3. DBD - Database Description: Sometimes it calls short list of router IDs. Sends a list of LSAs and compare to others routers. If someone has new or fresh information than I do please provide me that info. This information is used to compare information about the network.

DBD uses three different bits (options): 

I – Initial Bit. Decide who is starting DBD packet exchange.
M – More Bit. Indicates if this is the last or no DBD packet. “I need MORE info, please continue sending packets”/
MS- Master / Slave Bit. Decide who is Master or Slave during EXTART communication. 

4.  LSR - Link State Request : These messages are used to “ask” other router to send some info about prefixes (LSA). When router receives DBD from other router and see that there is some newer LSA or some LSA about prefix that it doesn’t know then router sends LSR = please give this and that info – LSAs.

5. LSU - Link State Update : This is package of LSAs. When router receives requests LSR it can send single LSA or couple LSAs => LSU.


6. LSAck - Link State Acknowledgment : OSPF requires acknowledgment for the receipt of each LSA. Multiple LSAs can be acknowledged in a single LSAck packet.



OSPF traffic is multicast to either of two addresses: 
224.0.0.5 for all OSPF routers
224.0.0.6 for all OSPF DRs.