How to Overcome Multihomed ISP Link Challenges

By Marc Goodman  |  Posted 2009-07-15

How to Overcome Multihomed ISP Link Challenges

In many organizations today, business-critical applications that had previously been accessed locally are now being accessed over the WAN, creating new concerns over reliable and secure access, as well as appropriate and cost-effective bandwidth allocation. WAN connectivity failures and traffic congestion have become common problems that impact these business-critical applications.

The increasing security vulnerabilities of Web-based applications and the exposure to security attacks are a growing threat. These problems can result in failed business transactions and productivity losses for remote users. But allocating additional budget for additional bandwidth will not solve WAN latency issues and link utilization and ISP network connection failures.

Organizations are also faced with the challenges that are inherent within applications such as voice over IP (VOIP), Outlook Web Access (OWA), Session Initiation Protocol (SIP), Web portals, and others, putting increased stress upon ISP links. They continue to deal with issues such as link availability and application performance over the network links.

Problems such as bandwidth congestion, bandwidth allocation and inefficient link utilization issues are problems that cost businesses in the form of loss of revenue and customer loyalty, not to mention increased operational expenses.

In this article, we'll explore the challenges that organizations face when deploying a multihomed network and how to overcome these challenges with WAN link controllers. They deliver an affordable solution for optimizing the performance of ISP connectivity and ensuring ISP link failover. This approach is becoming more popular for ensuring WAN or Internet uptime to automate the load balancing and failover of multiple, diverse ISP connections.

Multihoming ISP Links

Multihoming ISP links

Today, problems associated with ISP link availability continue to cause organizations to lose millions of dollars each year. However, deploying a solution that is cost effective and operationally efficient can also be a challenge. The following are four alternatives on how to facilitate multihomed networks.

1. Border Gateway Protocol

Typically, larger organizations multihome their sites with two links from two separate ISPs, using Border Gateway Protocol (BGP) to route across the links. While BGP can provide link availability in the case of a failure, it is a slow and complex routing protocol. It is costly to deploy because it requires special Autonomous System (AS) numbers from the ISPs and it requires router upgrades to be installed.

BGP is also not well-suited to provide multihoming and intelligent link load balancing. In the case of a failure, ISP cooperation is often required for link recovery. In general, BGP causes long and unpredictable failover times, which will not meet high availability requirements.

2. WAN link load balancing

Also known as multihoming, WAN link load balancing is a session-based process of directing Internet traffic among multiple and varied network connections. It requires a single WAN link controller located at the main site between the gateway modems/routers and the internal network. It intelligently load balances and provides failover for both inbound and outbound traffic among the network connections. Assuming there are two ISP connections, both network connections can be used at the same time. The benefit here is that you don't pay for bandwidth that is only used as a backup for when an outage occurs.

For example, traffic will go through network connection number one. If the WAN link controller detects that connection number one is overtaxed or failed, it will direct users across the second ISP connection. Intelligent WAN link controllers will continuously spread the traffic across the network connections based on the available resources. For example, with two T1s, it will not wait until the first T1 is overutilized before sending traffic out the second WAN; it will make use of both lines evenly.

Having two 1.5Mbps network connections does not mean that users have 3Mbps of bandwidth available to them. You would have 3Mbps available, but not for a single session. In other words, you would have 3Mbps of available bandwidth, but only 1.5 of throughput could be dedicated to any individual session. A single session will still only have 1.5Mbps of throughput; as with WAN link load balancing, each user can use only one ISP connection at a time.

3. Site-to-site channel bonding

Site-to-site channel bonding is a form of WAN link load balancing with a different approach that can increase the total combined network bandwidth of multiple network connections between two locations. This approach requires a WAN link controller at the main site and also at the remote site. Unlike WAN link load balancing, site-to-site channel bonding conducts continuous health checks (up and down status) of the network connections in use, and uses packet-based load balancing to distribute traffic across all network connections. However, with site-to-site channel bonding, two 1.5Mbps network connections will equal approximately 3Mbps, providing all traffic with the combined throughput from the multiple network connections.

4. Multiple ISPs

Organizations can multihome their sites with two WAN links from the same ISP. While implementing this solution may be a lower cost to deploy, it is not a very efficient solution, as an outage at the ISP will still cause a network failure, or at least create a bottleneck when both links are unavailable or oversubscribed. For greater WAN redundancy, it is best to have two or more different ISPs and load balance and provide failover for traffic among them.

Optimizing Multihomed ISP Links

Optimizing multihomed ISP links

Organizations that need true high availability will find that multihoming ISP links alone will not be sufficient. They will still have to deal with link performance issues. As most organizations have already realized, adding additional bandwidth will not adequately solve the problem. Rather, it creates an endless cycle of buying extra bandwidth while receiving diminishing benefits. To add to the problem, organizations are experiencing extraordinary demand for bandwidth in an effort to deliver new IP services, without being able to affordably and effectively manage the bandwidth.

It is important to find a balance, without having to make a tradeoff between the deployment of applications and efficiently using bandwidth. The challenge comes in being able to easily manage multiple WAN links based on bandwidth costs, capacity and application usage, as well as to incrementally add bandwidth when needed to support appropriate levels of capacity. The following are two common issues to consider:

1. Prioritizing applications

IT departments continually grapple with how to affordably and efficiently prioritize various types of traffic going across their WAN network. They often have a mix of applications, some of which are bandwidth-friendly, while others hog bandwidth and degrade performance. They need a way to prioritize traffic based on the application type, user, location and link, in order to ensure that high-priority applications will always get the bandwidth they need.

2. Oversubscribing bandwidth

IT departments are continually faced with limited bandwidth capacity. Naturally, as bandwidth levels become insufficient, the performance of the applications begins to diminish. This invariably lowers user productivity and can adversely affect business revenue. IT organizations are also dealing with diverse client types (that is, PDAs, cell phones, laptops, etc.) accessing their sites. They require a bandwidth management capability that allows them the flexibility and ease-of-use to set policies to enable the appropriate levels of bandwidth to meet the needs of these diverse client devices.

Marc Goodman is the Director of Marketing at Ecessa. Marc has over 29 years experience in the technology industry, with a history of building industry-leading brands for emerging companies, managing product marketing and marketing communications. Joining Ecessa in 2008, Marc is responsible for leading the company's overall corporate and product marketing.

Prior to Ecessa, Marc ran marketing for KEMP Technologies. From 1998 to 2001, Marc served as senior director of marketing at F5 Networks, where he led the marketing organization, developed an industry-leading brand and managed all marketing functions through a successful IPO. Marc has also served in marketing management positions at UBmatrix, Threshold Networks, SPRY/CompuServe, Cogent Networks, Attachmate and Wall Data. He can be reached at

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