3Com 7700 Configuration Manual page 85

LSA describes the network topology around a router, so the LSDB describes the
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network topology of the entire network. Routers can easily transform the LSDB
to a weighted directed graph, which actually reflects the topology of the whole
network. Obviously, all the routers have a graph that is exactly the same.
A router uses the SPF algorithm to calculate the shortest path tree with itself as
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the root, which shows the routes to the nodes in the autonomous system. The
external routing information is leave node. A router, which advertises the
routes, also tags them and records the additional information of the
autonomous system. Therefore, the routing tables obtained by different routers
are different.
OSPF supports interface-based packet authentication to guarantee the security of
route calculation. OSPF also transmits and receives packets by IP multicast.
OSPF Packets
OSPF uses five types of packets:
Hello Packet
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The Hello packet is the most common packet sent by the OSPF protocol. A
router periodically sends it to its neighbor. It contains the values of some
timers, DR, BDR and the known neighbor.
Database Description (DD) Packet
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When two routers synchronize their databases, they use the DD packets to
describe their own Link State Databases (LSDs), including the digest of each
LSA. The digest refers to the HEAD of an LSA, which can be used to uniquely
identify the LSA. Synchronizing databases with DD packets reduces the traffic
size transmitted between the routers, since the HEAD of an LSA only occupies a
small portion of the overall LSA traffic. With the HEAD, the peer router can
judge whether it has already had the LSA.
Link State Request (LSR) Packet
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After exchanging the DD packets, the two routers know which LSAs of the
peer routers are missing from the local LSD's. In this case, they send LSR
packets to the peers, requesting the missing LSAs. The packets contain the
digests of the missing LSAs.
Link State Update (LSU) Packet
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The LSU packet is used to transmit the needed LSAs to the peer router. It
contains a collection of multiple LSAs (complete contents).
Link State Acknowledgment (LSAck) Packet
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The packet is used for acknowledging received LSU packets. It contains the
HEAD(s) of LSA(s) requiring acknowledgement.
Basic Concepts Related to OSPF
Router ID
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To run OSPF, a router must have a router ID. If no ID is configured, the system
automatically selects an IP address from the IP addresses of the current
interface as the router ID.
Designated Router (DR)
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OSPF 77