Backbone Design Golden Rules

Use the following guidelines when designing an OSPF backbone (area 0):

• Understand that area 0 is a transit area, not a destination for traffic.

• Ensure that the stability of the backbone area is maintained and monitored.

• Ensure that redundancy is built into the design whenever possible.

• Ensure that OSPF backbones are contiguous.

• Keep this area simple. Fewer routers are better.

• Keep the bandwidth symmetrical so that OSPF can maintain load balancing.

• Ensure that all other areas connect directly to area 0.

• Restrict all end-user (host) resources from area 0.

The backbone must be at the center of all other areas, that is, all areas must be connected to the backbone. This is because OSPF expects all areas to inject routing information into the backbone, and in turn, the backbone disseminates that routing information into other areas. Figure 4-25 illustrates the flow of routing information in an OSPF network.

Figure 4-25 Flow of Information in an OSPF Network, in Which the Backbone Is the Key

Inter-area routes

Inter-area routes (Summary routes)

Figure 4-25 Flow of Information in an OSPF Network, in Which the Backbone Is the Key

Inter-area routes

Inter-area routes (Summary routes)

External routes

In Figure 4-25, all areas are directly connected to the backbone. Stability and redundancy are the most important criteria for the backbone. Keeping the size of the backbone reasonable results in stability. This is desirable because every router in the backbone needs to recompute its routes after every link-state change. Keeping the backbone small reduces the likelihood of a change and reduces the amount of CPU cycles required to recompute routes.

Redundancy is important in the backbone to prevent partition when a link fails. Good backbones are designed so that no single link failure can cause a partition (that is, the backbone becomes isolated). OSPF backbones must be contiguous. All routers in the backbone should be directly connected to other backbone routers. Avoid placing hosts (such as workstations, file servers, or other shared resources) in the backbone area. Keeping hosts out of the backbone area simplifies internetwork expansion and creates a more stable environment because a host's normal operation (morning or evening, power up or down) causes unnecessary LSA traffic.

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