OSPF uses link-state technology , as
opposed to distance vector technology used by protocols such as RIP .
Link-state routers maintain a common picture of the network and exchange
link information upon initial discovery or network changes. Link-state
routers do not broadcast their routing tables periodically like distance
vector routing protocols do. While RIP is appropriate for small
networks, OSPF was written to address the needs of large, scalable
internetworks. OSPF addresses the following issues:
- Speed of convergence - In
large networks, RIP convergence can take several minutes, since the
entire routing table of each router is copied and shared with
directly connected neighboring routers. In addition, a distance vector
routing algorithm may experience hold down and/or route-aging
periods. With OSPF, convergence is faster because only the routing
changes (not the entire routing table) are flooded rapidly to other
routers in the OSPF network.
- Support for Variable-Length Subnet
Masking (VLSM) - RIPv1 is a classful protocol and does not
support VLSM. In contrast, OSPF, a classless protocol, supports VLSM.
(Note: RIPv2 supports VLSM.)
- Network size - In a RIP
environment, a network that is more than 15 hops away is considered
unreachable. Such limitations restrict the size of a RIP network to
small topologies. On the other hand, OSPF has virtually no
reachability limitations and is appropriate for intermediate to
large size networks.
- Use of bandwidth - RIP
broadcasts full routing tables to all neighbors every 30 seconds.
This is especially problematic over slow WAN links because these
updates consume bandwidth. Alternately, OSPF
multicasts minimally sized link-state updates and sends the updates
only when there is a network change.
- Path Selection - RIP selects
a path by measuring the hop count, or distance, to other routers. It
does not take into consideration the available bandwidth on the link
or delays in the network. In contrast, OSPF selects optimal routes
using cost as a factor ("cost" is a metric based on
bandwidth).
- Grouping of members - RIP
uses a flat topology and all routers are part of the same network.
Thus, communication between routers at each end of the network must
travel through the entire network. Unfortunately, changes in even
one router will affect every device in the RIP network. OSPF, on the
other hand, uses the concept of "areas" and can
effectively segment a network into smaller clusters of routers. By
narrowing the scope of communication within areas, OSPF limits
traffic regionally and can prevent changes in one area from
affecting performance in other areas. This use of areas allows a
network to scale efficiently.
Although OSPF was written for large
networks, implementing it requires proper design and planning, which is
especially important if your network has more than 50 routers. At this
size, it is important to configure your network to let OSPF reduce
traffic and combine routing information whenever possible.
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