Understanding the network.pdf - Back to Home

• Better reliability—OSPF routers construct their own routing
table describing network reachability from their perspective
form within the network. All network routes are stored in the
topological or link state database, where they can be retrieved
quickly in the event that a routing change must be made. RIP,
as you might recall, discards all routes except the best one,
making it dependent on a routing update in order to construct a
new route in the event that the old route is no longer viable.
• Fast convergence—After the network has converged and all the
routers have constructed their own routing tables and network
maps, updates are sent out only when changes in the network
topology occur. When changes occur, they are flooded across all
network links so each router can adjust for the change in the
network topology immediately.
• Unlimited network size—No maximum hop distance is enforced
with OSPF. Network size is constrained only by available router
processing power and Layer 2 medium limitations.
• VLSM support—OSPF provides full support for VLSM.
• Type of Service routing—OSPF supports Layer 4 Quality of
Service routing. ToS/QoS was part of the original TCP/IP suite,
but was never compatible with RIP. This lack of support resulted
in lack of implementation. ToS/QoS is experiencing something
of a revival now that network-based applications are becoming
more and more performance sensitive.
• Low bandwidth usage—OSPF uses multicast instead of local
network broadcasts to send out OSPF messages. This limits the
impact of OSPF messages to only OSPF-speaking hosts.
• Dynamic load balancing and route selection—OSPF uses a cost
metric to determine route desirability. Cost is based on the link
speed. Links of equal cost are load balanced, and in cases where
there are links of unequal cost between similar destinations, the
lowest cost link is preferred. If the lowest cost link is down, the
next lowest cost link will be used.
The price that must be paid for these advantages is the requirement
for more processing power and memory than is needed for distance
vector protocols. OSPF is an Open Systems protocol and an open
standard (that is, in the public domain). It was introduced by the
Internet Engineering Task Force (IETF) in 1989 in RFC 1131 (OSPF
v1), and made a standard in RFC 2328 (OSPF v2). OSPF was intended
to be an updated Open Systems alternative to RIP and to provide an
Open Systems alternative to Cisco's proprietary IGRP and EIGRP
protocols.