Minimizing Downtime

In addition to facilitating load balancing, redundant routes minimize network downtime.

As already discussed, you should keep bandwidth consistent within a given layer of a hierarchy to facilitate load balancing. Another reason to keep bandwidth consistent within a layer of a hierarchy is that routing protocols converge much faster if multiple equal-cost paths to a destination network exist.

By using redundant, meshed network designs, you can minimize the effect of link failures. Depending on the convergence time of the routing protocols being used, a single link failure will not have a catastrophic effect.

A network can be designed as a full mesh or a partial mesh. In a full mesh network, every router has a link to every other router, as shown in Figure 4-6. A full mesh network provides complete redundancy and also provides good performance because there is just a single-hop delay between any two sites. The number of links in a full mesh is n(n-1)/2, where n is the number of routers. Each router is connected to every other router. (Divide the result by 2 to avoid counting Router X to Router Y and Router Y to Router X as two different links.)

Figure 4-6 Full Mesh Network: Every Router Has a Link to Every Other Router in the Network

A full mesh network can be expensive to implement in wide-area networks due to the required number of links. In addition, practical limits to scaling exist for groups of routers that broadcast routing updates or service advertisements. As the number of router peers increases, the amount of bandwidth and CPU resources devoted to processing broadcasts increases.

A suggested guideline is to keep broadcast traffic at less than 20 percent of the bandwidth of each link; this will limit the number of peer routers that can exchange routing tables or service advertisements. When planning redundancy, follow guidelines for simple, hierarchical design. Figure 4-7 illustrates a classic hierarchical and redundant enterprise design that uses a partial mesh rather than a full mesh architecture. For LAN designs, links between the access and distribution layer can be Fast Ethernet, with links to the core at Gigabit Ethernet speeds.

Figure 4-6 Full Mesh Network: Every Router Has a Link to Every Other Router in the Network

(6*5)/2=15 Circuits

Figure 4-7 Partial Mesh Design with Redundancy

Figure 4-7 Partial Mesh Design with Redundancy

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