Chapter 4. Implementing OSPF in a Single Area
| Terms you'll need to understand:
Techniques you'll need to master:
The Internet Engineering Task Force (IETF) first developed the Open Shortest Path First (OSPF) routing protocol in 1988. The most recent version of this link-state protocol is OSPF version 2 and is defined by RFC 2328. The RFC that describes the modifications to OSPF that support IPv6 is RFC 2740. OSPF employs the Shortest Path First algorithm to fill its routing table with information and share it with other routers in the network. OSPF distributes routing information primarily between internal routers in a single autonomous system (AS). Therefore, OSPF is categorized as an Interior Gateway Protocol (IGP). An AS is simply a collection of universally administered OSPF networks that share a common routing strategy. These systems are often broken down into areas. An area is a logical grouping of network segments that consists of a single AS. An area is also a subset of an AS and is often deployed to decrease the number of routing updates as well as improve convergence time. All the routers in an OSPF area share common administration and link-state databases. An area provides an added level of routing protection and minimized network traffic. This chapter focuses on implementing the OSPF routing protocol in a single area. The first OSPF area that you configure will be Area 0. The area ID 0 is used to represent this area. A backbone router (BBR) is a router that has at least one interface linked to Area 0. Therefore, Area 0 is commonly referred to as the backbone area. A backbone is simply the portion of a network that functions as the primary path for source traffic to flow to other networks. This area will serve as the core or backbone through which all other areas in the autonomous system will transit. The most important criteria for the backbone area are stability and redundancy. Stability is achieved through maintaining a reasonable backbone size. Every router in the backbone has to recompute its routes after every change in the link-state topology. You can minimize the probability of a change as well as reduce the CPU cycles required to recompute routes by keeping the backbone as small as is feasible . The average number of routers in each area (including the backbone) of newer , large enterprise environments is about 150. Redundancy is critical in the backbone; therefore, backbones should be designed so that no single link failure can cause link-state database problems. All routers in the backbone should be connected directly to other backbone routers. You should avoid placing hosts such as workstations, file servers, or other shared resources in the backbone area to enable smoother internetwork scalability and provide greater stability. Stability can also be maintained by ensuring that your routers have enough CPU and memory to handle the potential load. Each OSPF router has a database that depicts the AS topology. Each participating router has an identical copy of this link-state database. OSPF routers use a flooding process to distribute their local state throughout the AS. Whenever a change to the topology occurs, OSPF's link-state algorithm quickly recalculates the routes, using as little traffic as possible. OSPF also offers support for equal-cost multipath routing and authenticated packet exchanges. OSPF is a widely used routing protocol that is known for its rapid convergence speed, support for variable-length subnet masking (VLSM), scalability, efficient use of bandwidth, and improved path determination techniques. Here are the general objectives that will be covered in this chapter:
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EAN: 2147483647
Pages: 170