Load Balancing

Load balancing deals with the capability to divide data traffic over multiple connections. A common misconception about balancing is that it means an equal distribution of the load. Equal distribution of traffic is elusive enough even in situations in which traffic flows in a network that is under a single administration. Given the multiple players that traffic has to touch, equal distribution of traffic is difficult to achieve in most scenarios. Load balancing tries to achieve a traffic distribution pattern that will best utilize the multiple links that provide redundancy. Achieving this requires a good understanding of what traffic you are trying to balance, incoming or outgoing.

It is important not to think of traffic as a single entity. Traffic should be thought of as two separate entities, inbound and outbound. With respect to an autonomous system, inbound traffic is received from other ASs, whereas outbound traffic is sent to other ASs.

Suppose that you are connected to two ISPs and traffic is overloading your link to ISP1. Your first question should be: Which traffic, inbound or outbound? Are you receiving all your traffic from ISP1, or are you sending all your traffic toward ISP1?

The patterns of inbound and outbound traffic go hand-in-hand with the way you advertise your routes and the way you learn routes from other ASs. Inbound traffic is affected by how the AS advertises its networks to the outside world, whereas outbound traffic is affected by the routing updates coming in from outside ASs. Make sure that you fully understand this behavior, because it will be the basis of all future discussions. From now on, whenever we talk about taking steps to affect inbound traffic, we are really talking about applying attributes to outbound routing announcements because how our routes are learned by others affects how traffic is routed inbound. Similarly, whenever we talk about taking steps to affect outbound traffic, we are talking about applying attributes to inbound routing announcements because how our network learns routes affects how outbound traffic is routed. Figure 7-7 illustrates how inbound and outbound traffic behaves.

Figure 7-7. Inbound and Outbound Decisions


As you can see, the path for outbound traffic to reach NetA depends on where NetA is learned. Because NetA is received from both SF and NY, your outbound traffic toward NetA can go via SF or NY.

On the other hand, the path for inbound traffic to reach your local networks, NetB and NetC, depends on how you advertise these networks. If you advertise NetC over the NY link only, incoming traffic toward NetC will take the NY link. Similarly, if you advertise NetB over the SF link only, traffic toward NetB will take the SF link. Although this scenario appears optimal for traffic entering the AS, there is no provision for redundancy for the two advertised networks.

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