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CCNP Routing Studies: Basic Open Shortest Path First

Chapter Description

Understanding basic Open Shortest Path First (OSPF) routing principles not only applies to the CCNP certification but to all Cisco-based certifications. A concrete understanding of how OSPF routing works is fundamental for any small or large network. This chapter focuses on a number of objectives falling under the CCNP routing principles.

Configuring OSPF in a Single Area

When configuring any OSPF router, you must establish which area assignment to enable the interface for. OSPF has some basic rules when it comes to area assignment. OSPF must be configured with areas. The backbone area 0, or, must be configured if you use more than one area assignment. You can configure OSPF in one area; you can choose any area, although good OSPF design dictates that you configure area 0.

To enable OSPF on a Cisco router and advertise interfaces, the following tasks are required:

Step 1

Use the command router ospf process ID to start OSPF.

Step 2

Use the network command to enable the interfaces.

Step 3

Identify area assignments.

Step 4

(Optional) Assign the router ID.

Example 3-1 displays OSPF with a process ID of 1 and places all interfaces configured with an IP address in area 0. The network command network area 0 dictates that you do not care ( what the IP address is, but if an IP address is enabled on any interface, place it in area 0.

Example 3-1 Configuring OSPF in a Single Area

router ospf 1

network area 0

The following is a list of reasons OSPF is considered a better routing protocol than RIP:

  • OSPF has no hop count limitations. (RIP has 15 hops only.)

  • OSPF understands variable-length subnet masks (VLSMs) and allows for summarization.

  • OSPF uses multicasts (not broadcasts) to send updates.

  • OSPF converges much faster than RIP, because OSPF propagates changes immediately.

  • OSPF allows for load balancing with up to six equal-cost paths.

  • OSPF has authentication available. (RIPv2 does also, but RIPv1 does not.)

  • OSPF allows for tagging of external routes injected by other autonomous systems.

  • OSPF configuration, monitoring, and troubleshooting have a far greater IOS tool base than RIP.


OSPF does have some disadvantages, including the level of difficulty and understanding required to configure, monitor, and troubleshoot it. The other two factors are the memory and Central Processing Unit (CPU) requirements that can affect even high-end router performance. You can configure more than one OSPF process, but you must be mindful that the SPF calculations associated with multiple OSPF processes can consume a considerable amount of CPU and memory.

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