Ospf Route Calculation; Basic Ospf Concepts - 3Com 5500-EI PWR Install Manual

OSPF Route Calculation

Taking no account of area partition, the routing calculation process of the OSPF protocol is as follows:
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.
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.
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
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).

Basic OSPF Concepts

Autonomous System
A set of routers using the same routing protocol to exchange routing information constitute an
Autonomous System (AS).
Router ID
To run OSPF, a router must have a router ID. A router ID can be configured manually. If no router ID is
configured, the system will automatically select an IP address from the IP addresses of the interfaces as
the router ID. A router ID is selected in the following way:
If loopback interface addresses are configured, the system chooses the latest configured loopback
interface IP address as the router ID.
If no loopback interface is configured, the first configured IP address among the IP addresses of
other interfaces will be the router ID.
OSPF Packets
OSPF uses five types of packets:
0 "DR/BDR introduction".
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