Overview of OSPF
Open Shortest Path First (OSPF) applies to
large IPv4 networks. OSPF is a routing protocol based on link status.
Compared with distance vector-based routing protocols, OSPF delivers
a higher convergence rate and supports larger network scale.
Definition
OSPF is an internal network gateway
protocol developed by the Internet Engineering Task Force (IETF) on
the basis of link status. OSPF version 2 (RFC 2328) is for IPv4. OSPF
does not belong to any vendors or organizations and uses the Shortest
Path First (SPF) algorithm to calculate routes. The area concept is
used in OSPF to help reduce the consumption of the router CPU and
memory resources by route selecting protocols and reduce communications
traffic. Large and hierarchical networks can be established using
OSPF.
Objective
As the routers on the network
increase, the Routing Information Protocol (RIP) cannot support large
networks due to routing loops and limited hop numbers. The IETF develops
OSPF to support the large network deployment.
OSPF has the following
advantages:
- Wide application: OSPF is applicable to large-scale networks,
including a network consisting of hundreds of routers.
- Fast convergence: Once the network topology changes, update packets
are transmitted to synchronize the link-state databases (LSDBs) of
all routers in an autonomous system (AS).
- Loop-free: OSPF calculates loop-free routes using the shortest
path tree algorithm and the collected link status information.
- Area partition: An AS is divided into areas to simplify AS management.
Area partition reduces bandwidth consumption because only aggregated
routing information is transmitted between areas.
- Routing types: The following four types of routing are available
in descending order by priority: intra-area, inter-area, Type 1 external,
and Type 2 external.
- Equal-cost routes: OSPF supports multiple equal-cost routes to
the same destination.
- Authentication: Area- and interface-based OSPF packets can be
authenticated to improve packet exchange security.
- Multicast: Multicast packets are transmitted some types of links
to reduce the interference for other devices.