3Com 8807 Configuration Manual page 304

304 C 31: OSPF C
HAPTER
OSPF Packets
LSA Type
ONFIGURATION
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).
OSPF uses five types of packets:
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.
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.
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.
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).
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.
Five basic LSA types
As mentioned previously, OSPF calculates and maintains routing information from
LSAs. RFC2328 defines five LSA types as follows: