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xStack DES-3800 Series Layer 3 Stackable Fast Ethernet Managed Switch
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Adjacencies on Point-to-Point Interfaces
OSPF Routers that are linked using point-to-point interfaces (such as serial links) will always form adjacencies. The concepts of
DR and BDR are unnecessary.
OSPF Packet Formats
All OSPF packet types begin with a standard 24-byte header and there are five packet types. The header is described first, and
each packet type is described in a subsequent section.
All OSPF packets (except for Hello packets) forward link-state advertisements. Link-State Update packets, for example, flood
advertisements throughout the OSPF routing domain.
• OSPF packet header
• Hello packet
• Database Description packet
• Link-State Request packet
• Link-State Update packet
• Link-State Acknowledgment packet
OSPF Packet Header
Every OSPF packet is preceded by a common 24-byte header. This header contains the information necessary for a receiving
router to determine if the packet should be accepted for further processing.
The format of the OSPP packet header is shown below:
OSPF Packet Header
Type
Packet Length
Router ID
Area ID
Checksum
Authentication Type
Authentication
Authentication
Version No.
Figure 8- 19. OSPF Packet Header Format
Field Description
Version No.
The OSPF version number
Type
The OSPF packet type. The OSPF packet types are as follows: 1 means
Hello packet, 2 means Database Description packet, 3 means Link State
Request packet, 4 means Link State Update packet, 5 means Link State
Acknowledgement packet.
Packet Length
The length of the packet in bytes. This length includes the 24-byte header.
Router ID
The Router ID of the packet’s source.
Area ID
A 32-bit number identifying the area that this packet belongs to. All OSPF
packets are associated with a single area. Packets traversing a virtual link
are assigned the backbone Area ID of 0.0.0.0
Checksum
A standard IP checksum that includes all of the packet’s contents except for
the 64-bit authentication field.
Authentication Type
The type of authentication to be used for the packet.
Authentication
A 64-bit field used by the authentication scheme.