Operation Manual – Routing Protocol
H3C S5600 Series Ethernet Switches
Chapter 4 OSPF Configuration
4-2
z
Authentication: OSPF supports interface-based packet authentication to
guarantee the security of route calculation.
z
Multicast transmission: OSPF supports transmitting protocol packets in multicast
mode.
4.1.2 OSPF Route Calculation
Taking no account of area partition, the routing calculation process of the OSPF
protocol is as follows:
z
Each OSPF-supported router maintains a link state database (LSDB), which
describes the topology of the whole AS. According to the network topology
around itself, each router generates a link state advertisement (LSA). Routers on
the network exchange LSAs with each other by transmitting protocol packets.
Thus, each router receives the LSAs of other routers and all these LSAs form the
LSDB of the router.
z
An LSA describes the network topology around a router, whereas an LSDB
describes the network topology of the whole network. Routers can transform the
LSDB to a weighted, directed graph, which reflects the real topology of the whole
network. All routers get exactly the same weighted, directed graph.
z
According to the weighted, directed graph, each router uses the shortest path
first (SPF) algorithm to calculate the shortest path tree with itself as the root. The
tree shows the routes to the nodes in the autonomous system. External routes
are leaf nodes, which are marked with the routers from which they are advertised
to record information outside the AS. The routing tables obtained by different
routers are different.
Furthermore, to enable individual routers to broadcast their local status information
(such as available interface information and reachable neighbor information) to the
whole AS, routers in the AS should establish adjacencies among them. In this case,
the route changes on any router will result in multiple transmissions, which are
unnecessary and waste the precious bandwidth resources. To solve this problem,
designated router (DR) and backup designated router (BDR) are defined in OSPF. For
details about DR and BDR, see section
4.1.6 I.
"
DR/BDR
".
OSPF supports interface-based packet authentication to guarantee the security of
route calculation. In addition, it transmits and receives packets in multicast (224.0.0.5
and 224.0.0.6).
4.1.3 Basic OSPF Concepts
I. Autonomous System
A set of routers using the same routing protocol to exchange routing information
constitute an Autonomous System (AS).