Specific Scenarios: Designing Redundancy, Symmetry, and Load Balancing

By now you recognize the general ways in which the design goals of redundancy, symmetry, and load balancing intersect with and potentially conflict with one another. How is it possible to balance traffic among multiple links and still achieve a single entrance and exit point as symmetry mandates? This becomes even harder when multiple links are spread out over multiple routers in the autonomous system. The routing attributes described in Chapter 5, "Tuning BGP Capabilities," are the tools for implementing the desired redundancy, symmetry, and load balancing. It is the responsibility of the operator to choose and configure the correct attributes and filtering to achieve the desired outcome.

This section presents specific scenarios and attempts to configure them in such a way as to optimize redundancy, symmetry, and load balancing. The scenarios are not representative of every possible network configuration, and the design solutions shown here are not the only ones possible. But the lessons they illustrate can be applied to other scenarios and will help you understand and implement better and more efficient designs.

The first scenario is a simple case followed by increasingly complex scenarios. Note that there is a fine line between a customer and provider in many cases because a provider can be the customer of another provider. The principal distinction is this: customers obtain Internet connectivity by connecting to providers, but do not themselves offer connectivity to other customers. Providers offer Internet connectivity services and can themselves be customers of other providers.

The scenarios to be considered in the following subsections are further divided depending on whether the customer is receiving minimal or no routes, partial routes, full routes, or some combination of these from the providers. In the case where the customer is accepting minimal or no routes (called default only), you can assume that the customer can still learn the 0/0 route or a couple of aggregate routes that enables him to statically set a default. Partial routing usually consists of the provider's local routes and the provider's other customers' routes. Full routing means all Internet routes in existence—about 42,000 routes in 1996. A combination of these scenarios can occur where a customer can receive a default route and partial routes from the same provider, or partial routes from one provider and full routes from another and so on.

Scenario 1: Single-Homing

Single-homed customers have sites that connect to the Internet via a single connection to a service provider. Figure 6-8 illustrates such a situation. These customers can usually be adequately served by pointing defaults toward the provider. The provider can also install static routing to reach the customer. This method is the least expensive and the most effective. The customer router does not need to learn any of the Internet routes. This substantially reduces memory usage and processing overhead. In this case, there is no issue of route symmetry because traffic has a single entrance and exit point.

Figure 6-8  Simple single-homed site situation.

Single-homed sites generally rely on a single connection to the Internet. Backup is not an issue. If the connection is lost, the customer can tolerate the outage until it is fixed. Obviously, such an arrangement would not satisfy mission-critical data communication requirements. A single-homed site with no backup access would not be appropriate for applications needing high levels of reliability.

Scenario 2: Multihoming to a Single Provider

A customer with multiple connections to the Internet via the same provider is considered to be multihomed to a single provider. For multihoming to a single provider, assume that BGP is used as a routing protocol. Although it is not necessary in all cases, it is recommended.

Default Only, One Primary, and One Backup Link

In this scenario, the customer configures default routing toward the provider and is not accepting partial or full routes. The customer can run default to both connections. In figure 6-9, the customer wants to use one link as the primary traffic conduit and the other as a backup in case the primary link goes down. (If there were more than two connections to the provider, the customer could set up multiple defaults with varying preference levels.)

Figure 6-9  Basic mulithoming/single provider scenario.

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Troubleshooting:  
Ch. 11, pp. 373-376. Default Only, One Primary, and One Backup Link

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Customer's Outbound Traffic

In the scenario of figure 6-9, where a single router is used to connect to the provider in multiple locations, multiple static defaults with different distance values can be used, as already discussed in figure 6-6. The default with the lower distance will be the primary. The 0/0 default route or few aggregate routes can also be learned dynamically from the provider to enable the customer to set the default. Local preference can be used to prefer one default over the other.

Assume in figure 6-9 that the default to NY is more preferred than the default to SF. In normal operations, the customer will use the NY link as the primary link and the SF link as a backup.

For outbound traffic, load balancing is not an option because all traffic is sent over the primary line, and the secondary is kept as backup.

Absence of load balancing is offset by the fact that the customer's router requires less memory and processing power.

Customer's Inbound Traffic

The customer can advertise its networks to the provider via BGP. The provider will have two paths to reach the customer. Which path it chooses affects the customer's inbound traffic. Usually, the provider's default behavior (assuming that all attributes are the same) is for traffic to flow back to the customer's AS depending on which of the provider's exit points it is closest to. If traffic toward the customer is closer to the NY link, then it will enter the customer's AS via NY. If it is closer to SF, then it will enter via SF.

All the previous factors are outside the customer's control. Customers who want to override these influences and control incoming traffic via one path or the other can do so by advertising their routes with different metrics. The provider will direct its traffic toward the customer based on the metric value. In figure 6-9, the customer is advertising its routes with a metric of 50 toward NY and a metric of 100 toward SF. As such, traffic toward the customer will take the NY route.