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CCNA® Cisco Certified Network Associate: Study Guide, Seventh Edition by Todd Lammle

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Open Shortest Path First (OSPF) Basics

Open Shortest Path First (OSPF) is an open standard routing protocol that’s been implemented by a wide variety of network vendors, including Cisco. If you have multiple routers and not all of them are Cisco (what!), then you can’t use EIGRP, can you? So your remaining CCNA objective options are basically RIP, RIPv2, and OSPF. If it’s a large network, then, really, your only options are OSPF and something called route redistribution—a translation service between routing protocols.

OSPF works by using the Dijkstra algorithm. First, a shortest path tree is constructed, and then the routing table is populated with the resulting best paths. OSPF converges quickly, although perhaps not as quickly as EIGRP, and ...

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