The basics of the commonly used mainstream dynamic routing protocol OSPF and OSPF message types. Learn about them in one minute

1. Basic Concepts of OSPF

OSPF is based on IP protocol, protocol number 89

(1) Open Shortest Path First (OSPF) is a link-state-based interior gateway protocol developed by the IETF.

(2) OSPF is a link-state based protocol with the advantages of fast convergence, loop-free routing, and scalability. It has become an excellent internal gateway protocol and has been quickly accepted and widely used.

(3) OSPFv2 is based on IPv4 and its scalability is based on LSA extensions. If it is to be used in other networks, it must be redeveloped, such as OSPFv3 in IPv6.

(4) Based on the IP protocol, reliability cannot be guaranteed, so it is necessary to implement its own confirmation mechanism and authentication mechanism.

(5) Based on the IP protocol, as IP becomes more popular, OSPF has become a mainstream IGP protocol, generally used in enterprise networks, and engineers in enterprise networks are more familiar with it.

2. OSPF Message Types

Each OSPF message consists of an OSPF header plus an OSPF payload. OSPF supports multicast message transmission, where 224.0.0.5 is the IP multicast address reserved for OSPF devices, and 224.0.0.6 is the IP multicast address reserved for OSPF DR/BDR devices.

• Version: Version 2
• Message Type: message type
• Packet Length: OSPF packet length
• Source OSPF Router: The router-id of the router that sent this message
• Area ID: The area where the outbound interface that sends this message is located
• Checksum: checksum, used for verification
• Auth Type: authentication type, including Null (0), Simple (1) and MD5 (2) authentication types
• Auth Data: authentication data, used to store authentication keys, Key ID, etc.

3. Hello Message

Used to discover, establish and maintain neighbor relationships:

• Network Mask: The network mask of the interface that sends the message
• Hello Interval [sec]: Hello periodic sending time
• Options: Option bits, used to support different functions
• DN：Down
• E: External Routing: If set, it means that 5 types of LSA are supported.
• N: NSSA Routing: If set, it indicates that 7 types of LSA are supported.
• Router Priority: Router priority, used for DR/BDR election, the default value is 1
• Router Dead Interval [sec]: Hello dead time = 4*hello time
• Designated Router: DR router, identified by Router-id
• Backup Designated Router: BDR router, identified by Router-id
• Active Neighbor: Active neighbor list, identified by Router-id

4. DD message

Describes the summary information of LSDB

• Interface MTU: 0
• Options: Option settings
• DB Description: DD message description set
• (I) Init: If set, it indicates that it is the first DD message.
• (M) More: If set, it indicates that there will be subsequent DD messages with LSA summary information.
• (MS) Master: If set, it indicates that the one who sent the DD message is the Master.
• DD Sequence: DD message sequence number, used to identify DD messages. Only the Master generates the sequence number, which increases by 1 each time it is sent.

5. LSR Message

The LSA used for requesting a response contains the three elements of the requested LSA.

6. LSU message

LSA used to request updates from neighbors, containing the actual LSA.

7. LSACK Message

Used to confirm the received LSA and only contains the summary information of the LSA.