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Cisco Load Balancing - How to Achieve Cisco Load Balancing on T1 Lines

what is load balancing? per packet load balancing

What is covered in this article?

This article provides a guide to how to configure Cisco routers to load balance internet traffic, or IP traffic, when using two or more T1 lines that are connected in parallel. This is a common challenge faced by network administrators and information technology professionals, but it can be difficult to find a consistent solution that works with the variety of Cisco routers and T1 internet connections available.

What is load balancing?

Load balancing refers to a technique for distributing a workload evenly across two devices, such as hard drives, computer processors, computers, or network links. The goal of load balancing is to achieve the highest amounts of throughput and utilization of resources while minimizing response time and avoiding overload. The use of multiple components with load balancing may result in higher levels of reliability through redundancy. In this case, the load being balanced is internet traffic, and the devices across which it is being balanced are a pair of (or several) parallel T1 information lines.

Per Packet Load Balancing

The ideal goal is to have per packet load balancing. If using this kind of load balancing is both accessible to you and will not result in incompatibility with any of your applications, it is the preferred method. With per packet load balancing, the load balancing that occurs with a router will occur at the third layer, which means it will be related to IP routing. When you choose per packet load balancing, your Cisco router will use a round robin fashion to distribute outbound IP packets. In other words, it will take turns with the interfaces it chooses to route out each outbound IP packet. With per packet load balancing, you will have the most even technique for distributing information across numerous outbound connections.

Despite the advantages of per packet load balancing, there are two challenges that must be addressed for the process to be successful.

The first is that by definition, a router is only capable of load balancing with outbound traffic; it cannot load balance traffic that is inbound. However, it is necessary to have load balancing in both directions. To achieve this, the T1 lines that are connected to the first router, say Router A, must also terminate on Router B. As a result, you must configure load balancing beforehand on not only Router A, but also on Router B. This is not particularly challenging to accomplish if both Router A and Router B are local, or somewhere where you can access or control them through a network. However, if they are not under your direct control, you will need to work with the company or individuals who control the remaining router. An additional consequence of the router restriction is that you cannot use per packet balancing on T1 lines with multiple internet service providers, or ISPs.

The second challenge that must be addressed in Cisco load balancing on T1 lines is that when you use per packet load balancing, it is not uncommon for packets that are part of a particular flow or connection to arrive where they were headed in a sequence that does not match the sequence in which they were originally sent. In other words, packets can and do arrive out of order. For most applications, this will not be noticeable; for a few applications, it may be noticeable now and then, but it will not affect performance. But for a select few applications, it can present significant problems; such applications include streaming media, which have become increasingly popular in recent years. This includes audio streaming, video streaming, and voice over IP or VOIP applications that involve telephone conversations over long distances online. However, Cisco does provide a solution for out of order packets, and recommends the use of Multilink PPP for Cisco load balancing.

Finally, you can use Cisco Express Forwarding, or CEF, to switch packets inside the router. Cisco recommends this method, and it will be required to achieve per packet load balancing. CEF can be enabled via the command “ip cef”, and you can tell if it is enabled via the command “show ip cef”. Following the aforementioned steps will result in load balancing.

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