Sim-Ex? Tutorial for CCNA: OSPF Routing Fundamentals

Definition: OSPF is a routing protocol used to determine the best route for delivering the packets within an IP networks. It was published by the IETF to serve as an Interior Gateway Protocol replacing RIP. The OSPF specification is published as Request For Comments (RFC) 1247.

Note that OSPF is a link-state routing protocol, whereas RIP and IGRP are distance-vector routing protocols. Routers running the distance-vector algorithm send all or a portion of their routing tables in routing-update messages to their neighbors.?

OSPF sends link-state advertisements (LSAs) to all other routers within the same area. Information on attached interfaces, metrics used, and other variables is included in OSPF LSAs. OSPF routers use the SPF (Shortest Path First) algorithm to calculate the shortest path to each node. SPF algorithm is also known as Dijkstra algorithm.

Advantages of OSPF

• OSPF is an open standard, not related to any particular vendor.
• OSPF is hierarchical routing protocol, using area 0 (Autonomous System) at the top of the hierarchy.
• OSPF uses Link State Algorithm, and an OSPF network diameter can be much larger than that of RIP.
• OSPF supports Variable Length Subnet Masks (VLSM), resulting in efficient use of networking resources.
• 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, which in turn conserves network bandwidth.
• Using areas, OSPF networks can be logically segmented to improve administration, and decrease the size of routing tables.?

Disadvantages of OSPF:

• OSPF is very processor intensive due to implementation of SPF algorithm. OSPF maintains multiple copies of routing information, increasing the amount of memory needed.
• OSPF is a more complex protocol to implement compared to RIP.

OSPF Networking Hierarchy:

As mentioned earlier, OSPF is a hierarchical routing protocol. It enables better administration and smaller routing tables due to segmentation of entire network into smaller areas. OSPF consists of a backbone (Area 0) network that links all other smaller areas within the hierarchy. The following are the important components of an OSPF network:

• Areas
• Area Border Routers
• Backbone Areas
• AS Boundary Routers
• Stub Areas
• Not-So-Stubby Areas
• Totally Stubby Area
• Transit Areas

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