open shortest path first ospf.ppt
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Open Shortest Path First OSPF**Phm Hng Anh-TVT5A
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OSPF Operation Each router discovers its neighbors on each interface. The list of neighbors is kept in a neighbor tableEach router uses a reliable protocol to exchange topology information (LSAs) with its neighbors.LSA : describes a subnet number and mask, the cost, and the other information about the subnet.Each router places the learned topology information in its topology database.Each router runs the SPF algorithm against its own topology database to calculate the best routes to each subnet in the databaseEach router places the best route to each subnet in the IP routing table.**Phm Hng Anh-TVT5A
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Advantages of OSPF OSPF is link-state routing protocolRIP, IGRP and EIGRP are distance-vector (routing by rumor) routing protocols, susceptible to routing loops, split-horizon, and other issues.OSPF has fast convergenceRIP and IGRP hold-down timers can cause slow convergence.Ciscos OSPF metric is based on bandwidth.RIP is based on hop countIGRP/EIGRP bandwidth, delay, reliability, loadOSPF only sends out changes when they occurRIP sends entire routing table every 30 seconds, IGRP every 90 seconds Extra: With OSPF, a router does flood its own LSAswhen it age reaches 30 minutes (later)OSPF also uses the concept of areas to implement hierarchical routing.Open-standard routing protocols.IGRP and EIGRP are Cisco proprietary**Phm Hng Anh-TVT5A
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Routing Hierarchy*Phm Hng Anh-TVT5A*Internal : Routers with all their interfaces within the same area
Backbone : Routers with at least one interface connected to area 0ASBR
ABR : (Area Border Router): Routers with interfaces attached to multiple areas.ASBR :(Autonomous System Boundary Router): Routers that have at least one interface connected to an external internetwork(another AS)
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Packet Format*Phm Hng Anh-TVT5A*
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