Allied Telesis AT-9724TS - Manual (Page 109)

Figure 6- 100. OSPF Packet Header Format
Field Description
Version No. The OSPF version number.
Type The OSPF packet type.The OSPF packet types are as follows:Type Description Hello Database Description
Link-State Request Link-State Update Link-State Acknowledgment.
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.
Hello Packet
Hello packets are OSPF packet type 1.They are sent periodically on all interfaces, including virtual links, in order to establish and maintain neighbor relationships.
In addition, Hello Packets are multicast on those physical networks having a multicast or broadcast capability, enabling dynamic discovery of neighboring routers.
All routers connected to a common network must agree on certain parameters such as the Network Mask, the Hello Interval, and the Router Dead Interval.
These parameters are included in hello packets, so that differences can inhibit the forming of neighbor relationships.A detailed explanation of the receive
processing for Hello packets, so that differences can inhibit the forming of neighbor relationships.
The format of the Hello packet is shown below:
108 Allied Telesyn AT-9724TS High-Density Layer 3 Stackable Gigabit Ethernet Switch
Version No. Type Packet Length
Octets
0
1 2 3
OSPF Packet Header
Router ID
Area ID
Checksum Authentication Type
Authentication
Authentication
4