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Open Shortest Path First (OSPF)
Open Shortest Path First (OSPF)
is an Interior Gateway Protocol (IGP) developed to overcome some of RIP's
limitations when it operates in more complex networks.
Where RIP
keeps a table of all known destinations and the number of hops to reach them, OSPF is a link-state
routing method that keeps routing information only for the router's IP domain and its neighboring routers, not the
entire network.
OSPF works best in hierarchical networks, while RIP, which is a simpler protocol to manage, works best in flat
networks. For complete technical details on implementing OSPF, refer to RFCs 1583 and 1850 which define
OSPFv2.
OSPF benefits
faster route convergence
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conserves bandwidth, only sending updates when changes occur
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no hop count limit
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supports hierarchical topologies
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OSPF overview
OSPF splits the network into independent parts called
areas
and connects these areas to a
backbone area
.
Each area is identified with a unique 32-bit
area_id
number imbedded in OSPF packets. The Switch processes
OSPF packets only if one of its interfaces resides in the area advertised by the packet.
Each OSPF router builds a shortest path tree with itself as the root. The router sends updates to its neighboring
routers and verifies that they all have a consistent network map.
Autonomous system
In OSPF, a single IP domain is called an
autonomous system (AS).
The topology within other areas is hidden
from the rest of the autonomous system.
This diagram illustrates the OSPF areas and types of routers described below.
Area 0 - the Backbone
An OSPF network must have an area configured as Area 0, or the backbone area. All areas in an
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