Route Redundancy

Designing redundant routes has two purposes: balancing loads and increasing availability. Load Balancing

Most IP routing protocols can balance loads across parallel links that have equal cost. Use the maximum-paths command to change the number of links that the router will balance over for IP; the default is four, and the maximum is six. To support load balancing, keep the bandwidth consistent within a layer of the hierarchical model so that all paths have the same cost. (Cisco Interior Gateway Routing Protocol [IGRP] and Enhanced IGRP [EIGRP] are exceptions because they can load-balance traffic across multiple routes that have different metrics by using a feature called variance.)

A hop-based routing protocol does load balancing over unequal-bandwidth paths as long as the hop count is equal. After the slower link becomes saturated, packet loss at the saturated link prevents full utilization of the higher-capacity links; this scenario is called pinhole congestion. You can avoid pinhole congestion by designing and provisioning equal-bandwidth links within one layer of the hierarchy or by using a routing protocol that takes bandwidth into account.

IP load balancing in a Cisco router depends on which switching mode the router uses. Process switching load-balances on a packet-by-packet basis. Fast, autonomous, silicon, optimum, distributed, and NetFlow switching load-balance on a destination-by-destination basis because the processor caches information used to encapsulate the packets based on the destination for these types of switching modes.

Increasing Availability

In addition to facilitating load balancing, redundant routes increase network availability.

You should keep bandwidth consistent within a given design component to facilitate load balancing. Another reason to keep bandwidth consistent within a layer of a hierarchy is that routing protocols converge much faster on multiple equal-cost paths to a destination network.

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

You can design redundant network links to provide a full mesh or a well-connected partial mesh. In a full-mesh network, every router has a link to every other router, as shown in Figure 2-15. 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. A well-connected partial-mesh network provides every router with links to at least two other routing devices in the network.

Figure 2-15 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 WANs due to the required number of links. In addition, groups of routers that broadcast routing updates or service advertisements have practical limits to scaling. As the number of routing 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 amount limits the number of peer routers that can exchange routing tables or service advertisements. When planning redundancy, follow guidelines for simple, hierarchical design. Figure 2-16 illustrates a classic hierarchical and redundant enterprise design that uses a partial-mesh rather than a full-mesh topology. For LAN designs, links between the access and distribution layer can be Fast Ethernet, with links to the core at Gigabit Ethernet speeds.

Figure 2-15 Full-Mesh Network: Every Router Has a Link to Every Other Router in the Network

Figure 2-16 Partial-Mesh Design with Redundancy

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