Friday, April 10, 2009

OSPF Definition and advantage and disadvantages

OSPF Definition:
Open Shortest Path First (OSPF), is a routing protocol used to determine the correct route for packets within IP networks. It was designed by the Internet Engineering Task Force to serve as an Interior Gateway Protocol replacing RIP.
Advantages of OSPF:
Changes in an OSPF network are propagated quickly.
OSPF is heirarchical, using area 0 as the top as the heirarchy.
OSPF is a Link State Algorithm.
OSPF supports Variable Length Subnet Masks (VLSM).
OSPF uses multicasting within areas.
After initialization, OSPF only sends updates on routing table sections which have changed, it does not send the entire routing table.
Using areas, OSPF networks can be logically segmented to decrease the size of routing tables. Table size can be further reduced by using route summarization.
OSPF is an open standard, not related to any particular vendor. Disadvantages of OSPF:
OSPF is very processor intensive.
OSPF maintains multiple copies of routing information, increasing the amount of memory needed.
Using areas, OSPF can be logically segmented (this can be a good thing and a bad thing).
OSPF is not as easy to learn as some other protocols.
In the case where an entire network is running OSPF, and one link within it is "bouncing" every few seconds, OSPF updates would dominate the network by informing every other router every time the link changed state (I've done this). OSPF routers check the status of other routers on the network by sending a small hello packet at regular intervals. If a router does not respond to the hello packet, it is assumed dead, and routing updates are sent to every other router by using a multicast address.
In the case where there are no network changes, OSPF will use very little bandwidth (only sending hello packets). As soon as there is an outage, however, OSPF will flood the network as the change is sent to every router (and then every router notifies every other router about the change). This system of near silence when possible and flooding when necessary ensures that routing information gets propagated throughout the network as quickly as possible.

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