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OSPFv3 Messages

OSPFv3 Messages

OSPFv3 runs on top of IPv6 and so IPv6 Next-Header value for OSPF packets is 89. The IPv6 ALL SPF Routers multicast address is FF02::5, and ALL DR Routers multicast address is FF02::6.

OSPFv3 Standard Header format:

There are 5 different OSPF packet types. All OSPF packet types begin with a standard 16 byte header.

Version: 3
Type: Type of OSPF packet
1- Hello Message
2- Database Description message
3- Link State Request
4- Link State Update
5- Link State Acknowledgement
Packet Length: Length of OSPF packet in bytes. It includes the standard 16 bytes as well.
Router ID: The 32-bit Router ID of the packet source
Area ID: A 32-bit Area ID indicating the area that this packet belongs to. Every packet belongs to a single area.
Checksum: Standard 16-bit checksum
InstanceID:¬†Enables multiple instances of OSPF to be run over a single link. It has local significance only. Received packets whose Instance ID is not equal to the receiving interface’s Instance ID, are discarded.

OSPFv3 Hello Message:

Hello messages are OSPF packets with Type 1. Hello messages are sent periodically on all interfaces in order to establish and maintain neighbor relationships. All routers connected to a common link agree on certain parameters – Hello and Router Dead Intervals, Area ID, Router ID and Instance ID. These parameters should be agreed upon to form neighborship.

 

InterfaceID: 32-bit number uniquely identifying the interface
Rtr Pri:¬†This router’s Router Priority. It is used in DR and BDR election. If set to 0, the router is ineligible for DR or BDR election.
Options: The options supported by the router.
HelloInterval: Periodic interval at which Hello Messages will be sent
RouterDeadInterval: Number of seconds after which a router will be deemed dead if no packets are received from the neighbor.

Designated Router ID: The Router ID of the DR for the network. If no DR is present, it is set to 0.0.0.0
Backup Designated Router ID: The Router ID of the BDR for the network. If no BDR is present, it is set to 0.0.0.0
Neighbor ID: The Router IDs of each neighbors from whom valid Hello packets have been seen recently on the network.

OSPFv3 Database Description Message:

DBD messages are OSPF packets with Type 2. DBD messages are exchanged when an adjacency is initialised. It contains the contents of the link-state database (LSDB). Multiple packets may be used to send the complete database. One of the router acts as a Master and other as a slave. The Master sends the DBD packets to the slave which it acknowledges.

Options: Options supported by this router
Interface MTU: The size of the largest IPv6 packet that can be sent through the outgoing interface, without fragmentation. It should be set to 0 for virtual-links.
I-The Init bit: Indicates this is the first DBD packet
M- The More bit: Indicates more DBD packets to follow
MS- The Master/Slave bit: When set to 1 indicates this router is the Master router. Otherwise, this router is the Slave.
DD Sequence Number: Used to sequence the collection of DBD packets. The first packet with I-bit set is unique.

The rest of the packet consists of the list of LSDB’s pieces.

OSPFv3 Link-State Request messages:

Link-state Request messages are OSPF packets with Type 3. A router sends Link-State Request packets if it finds that parts of its LSDB is out-of date.

Each LSA requested is identified by LS Type, Link-State ID and Advertising Router.

OSPFv3 Link-State Update message:

Link-State Update messages are OSPF packet with Type 4. These packets implement the flooding of LSAs. The Link-State Update message are acknowledged using Link-State Acknowledgement messages. These messages are sent to ALL SPF Routers multicast address. However, if re-sending is required, then they are unicast.

#LSA: The number of LSAs included in this update.

The body of the Link-State Update packet consists of the list of LSAs.

OSPFv3 Link-State Acknowledgement Message:

Link-State Acknowledgement messages are OSPF packets with Type 5. These messages are sent to acknowledge the Link-State Update messages received by the router. They could be sent to ALL SPF Routers multicast address or as a unicast.

Multiple LSAs can be acknowledged using a single message.

 

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