3Com Switch 8800 Configuration Guide
Chapter 19 OSPF Configuration
19-2
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A router uses the SPF algorithm to calculate the shortest path tree with itself as the
root, which shows the routes to the nodes in the autonomous system. The external
routing information is the leave node. A router, which advertises the routes, also
tags them and records the additional information of the autonomous system.
Obviously, the routing tables obtained by different routers are different.
Furthermore, to enable individual routers to broadcast their local state information to
the entire AS, any two routers in the environment should establish adjacency between
them. In this case, however, the changes that any router takes will result in multiple
transmissions, which are not only unnecessary but also waste the precious bandwidth
resources. To solve this problem, “Designated Router” (DR) is defined in the OSPF.
Thus, all the routers only send information to the DR for broadcasting the network link
states in the network. Thereby, the number of router adjacent relations on the
multi-access network is reduced.
OSPF supports interface-based packet authentication to guarantee the security of
route calculation. Also, it transmits and receives packets by IP multicast (224.0.0.5 and
224.0.0.6).
19.1.3 OSPF Packets
OSPF uses five types of packets:
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Hello Packet:
It is the commonest packet, which is periodically sent by a router to its neighbor. It
contains the values of some timers, DR, BDR and the known neighbor.
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Database Description (DD) Packet:
When two routers synchronize their databases, they use the DD packets to describe
their own LSDBs, including the digest of each LSA. The digest refers to the HEAD of
LSA, which uniquely identifies the LSA. This reduces the traffic size transmitted
between the routers, since the HEAD of a LSA only occupies a small portion of the
overall LSA traffic. With the HEAD, the peer router can judge whether it already has had
the LSA.
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Link State Request (LSR) Packet:
After exchanging the DD packets, the two routers know which LSAs of the peer routers
are lacked in the local LSDBs. In this case, they will send LSR packets requesting for
the needed LSAs to the peers. The packets contain the digests of the needed LSAs.
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Link State Update (LSU) Packet:
The packet is used to transmit the needed LSAs to the peer router. It contains a
collection of multiple LSAs (complete contents).
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Link State Acknowledgment (LSAck) Packet
The packet is used for acknowledging the received LSU packets. It contains the
HEAD(s) of LSA(s) requiring acknowledgement.
