3Com Switch 7700 Configuration Manual page 114

106 C 5: IP R
HAPTER OUTING
P O
ROTOCOL PERATION
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 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 received 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 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
■
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)
■
In a broadcast network, in which all routers are directly connected, any two
routers must establish adjacency to broadcast their local status information to
the whole AS. In this situation, every change that a router makes results in
multiple transmissions, which is not only unnecessary but also wastes
bandwidth. To solve this problem, OSPF defines a "designated router" (DR). All
routers send information only to the DR for broadcasting the network link
states to the network. This reduces the number of router adjacent relations on
the multi-access network.
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"
Backup Designated Router (BDR)
■
If the DR fails, a new DR must be elected and synchronized with the other
routers on the segment. This process takes a relatively long time, during which
the route calculation is incorrect. To shorten the process, OSPF creates a BDR as
backup for the DR. A new DR and BDR are elected in the meantime. The
adjacencies are also established between the BDR and all the routers on the
segment, and routing information is also exchanged between them. After the
existing DR fails, the BDR becomes a DR immediately.