Protocol Packets; Router Types - 3Com CoreBuilder 2500 User Manual

Protocol Packets

Router Types

The OSPF protocol uses five types of packets:
Hello — Router interfaces periodically transmit hello packets to
identify and maintain communications with their neighbors.
In nonmulticast networks, routers find neighbors by sending unicast hello
packets to other statically configured routers.
Database description — Neighbor routers use database description
packets to synchronize their link state summary databases.
Link state request — To collect network topology data, routers
transmit link state request packets to their neighbors on the segment.
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.
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:
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.
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.
Elements of OSPF Routing 7-7