3Com 5500 SI - Switch - Stackable, Configuration Manual

Page: 236 / 686

236 C
HAPTER
16: IP R
OUTING
P
ROTOCOL
O
PERATION
OSPF Packets
OSPF uses five types of packets:
■
Hello Packet.
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.
When two routers synchronize their databases, they use DD packets to
describe their own LSDBs, 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 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 missing from the local LSDBs. 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.
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.
The packet is used for acknowledging the received LSU packets. It contains the
HEAD(s) of LSA(s) requiring acknowledgement.
Basic Concepts Related
to OSPF
■
Router ID
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. How a router ID is chosen: if the LoopBack interface
address exists, the system chooses the LoopBack address with the greatest IP
address value as the router ID; if no LoopBack interface configured, then the
address of the physical interface with the greatest IP address value will be the
router ID.
■
Designated Router (DR)
In multi-access networks, if any two routers establish adjacencies, the same
LSA will be transmitted repeatedly, wasting bandwidth resources. To solve this
problem, the OSPF protocol regulates that a DR must be elected in a
multi-access network and only the DR (and the BDR) can establish adjacencies
with other routers in this network. Two non-DR routers or non-BDR routers
cannot establish adjacencies and exchange routing information.
When the DR is not manually specified, the DR is elected by all the routers in
the segment. See “Setting the Interface Priority for DR Election” on page 241.