3com 2500 User Guide

The OSPF protocol uses five types of packets:

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Hello — Router interfaces periodically transmit hello packets to 
identify and maintain communications with their neighbors. 

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Database description — Neighbor routers use database description 
packets to synchronize their link state summary databases. 

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Link state request — To collect network topology data, routers 
transmit link state request packets to their neighbors on the segment. 

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Link state update — On receiving a link state request packet, a 
router floods packets containing its LSA data into the area or 
autonomous system that it serves. The information contained in the 
packets depends on the router’s location and function in the network.

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Link state ack(nowledge) — Routers use these packets to 
acknowledge receipt of link state update packets.

The router acknowledges receiving each LSA in the link state update 
packet.

OSPF routers serve several different, often overlapping, functions: 

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Internal routers — Internal routers connect only to networks that 
belong to the same area. An internal router runs one copy of the OSPF 
algorithm and maintains routing data only for its area.

In Figure 7-1, backbone area 0 and routers 1, 2, 3, and 4 are internal 
routers. In area 1, routers 5 and 6 are internal routers.

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Backbone routers — Backbone routers connect to the backbone 
network and are configured as belonging to the OSPF backbone area. 
Area border routers are always backbone routers, because you must 
configure them as being within the backbone area or connected to it 
by a virtual link.

In Figure 7-1, router 2 and area border routers 1, 2, 3, and 4 are 
backbone routers.