Backup Paths

To maintain interconnectivity even when one or more links are down, redundant network designs include a backup path for packets to travel when there are problems on the primary path. A backup path consists of routers and switches and individual backup links between routers and switches, which duplicate devices and links on the primary path.

When estimating network performance for a redundant network design, you should take into consideration two aspects of the backup path:

• How much capacity does the backup path support?

• How quickly will the network begin to use the backup path?

You can use a network-modeling tool to predict network performance when the backup path is in use. Sometimes the performance is worse than the primary path, but still acceptable.

It is quite common for a backup path to have less capacity than a primary path. Individual backup links within the backup path often use different technologies. For example, a leased line can be in parallel with a backup dialup line or ISDN circuit. Designing a backup path that has the same capacity as the primary path can be expensive and is only appropriate if the customer's business requirements dictate a backup path with the same performance characteristics as the primary path.

If switching to the backup path requires manual reconfiguration of any components, then users will notice disruption. For mission-critical applications, disruption is probably not acceptable. An automatic failover is necessary for mission-critical applications. By using redundant, partial-mesh network designs, you can speed automatic recovery time when a link fails.

One other important consideration with backup paths is that they must be tested. Sometimes network designers develop backup solutions that are never tested until a catastrophe happens. When the catastrophe occurs, the backup links do not work. In some network designs, the backup links are used for load sharing as well as redundancy. This has the advantage that the backup path is a tested solution that is regularly used and monitored as a part of day-to-day operations. Load sharing is discussed in more detail in the next section.

Continue reading here: Load Sharing

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