Ospf Packets - Huawei Quidway S3500 Series Operation Manual

Operation Manual - Routing Protocol
Quidway S3500 Series Ethernet Switches
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 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, suppose that the routers are directly connected without other in-between
routing devices in a broadcast network. To enable the individual routers to broadcast
the information of their local statuses to the whole 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.

4.1.3 OSPF Packets

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 an
LSA, which can be used to uniquely identify the LSA. Such 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.
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Chapter 4 OSPF Configuration