Solve the bandwidth dilemma

Multilink and multihoming architectures provide choices for enhancing performance.

The evolution of technology and economics has illustrated a fundamental paradigm shift in the way the enterprise must manage ISP bandwidth utilization. New applications require higher throughput, yet IT administrators often are left with an ever-increasing need for bandwidth, accelerating user demand and more stringent restrictions on network operating costs.

There are several alternatives available to aid the IT administrator in improving this common IT predicament.

The most frequent solution has been to over-specify link speed, which allows bandwidth overhead above the average utilization level. As this model evolved, ISPs responded with “burstable” billing plans, which provide a dedicated access level, plus additional bandwidth on demand billed at a higher burst rate.

An alternative is multilink architecture, where a number of links to a single ISP are provisioned, allowing throughput at the approximate sum of the individual links. This can add additional bandwidth without the loop costs involved in migrating to the next circuit “tier.”

Another common multilink methodology utilizes fair-weighted round-robin addressing, which allows a per-packet load share across a number of links. This method has a lower load on the router’s processor, and yields excellent performance when packet sizes remain relatively constant and the telco provider has newer facilities in the area.

Provisioning links through multilink architecture to several ISPs will yield not only additional throughput, but increased resiliency, as well. This is known as “multihoming.” The core principle behind multihoming is redundancy vs. adding bandwidth in a cost-effective model.

There is no single “best practice” in architecting such a network. IT administrators and network architects should begin with an understanding of several component factors:

• What is the size and profile of the user base? This will include: host users, Web servers or other resources resident on the LAN, remote application services used by local hosts, and VoIP or other streaming protocols.
• What is the utilization profile? Are there multiple applications in constant use, or are there variances in traffic patterns based on time-of-day?
• What quality of service is required?
• Will the existing access router support the required load?

When considering the multihoming option, remember that a single access router constitutes a single point of failure. Adding redundant platforms will not only add to initial deployment cost, but also factor into ongoing operational expenses and the general complexity of the solution.

Not all ISPs support all protocols; some prefer to maintain vendor-agnostic policies. If your final architecture relies on one of these proprietary technologies, be sure the ISP is aware of this and understands its implications.

Other factors specific to the company’s business include: Is your inbound and outbound traffic requirement asymmetric? Are there periodic spikes in traffic at known intervals? Is your bandwidth demand subject to fluctuation from some outside event? What is your growth plan over 12-18 months?

Next, evaluate the ISP. Quality ISPs have quality engineering staff available–do not hesitate to involve them.

Finally, consider a hybrid architecture. For example, a series of multiple links to multiple providers could be provisioned–such as a 2x2xT-1, where two T-1s each are provided by two separate ISPs. This allows the best of all possible worlds–increased bandwidth with loop cost savings, resiliency via multihoming and flexible options when migrating circuits.

Several new technologies have recently become available that enable least-cost routing over multiple links. By assigning multiple metrics to each link (such as bandwidth cost, burst cost, hop count), these systems are capable of dynamically allocating traffic across multiple links. With these technologies, the system looks at the state of link utilization at any given interval, and directs traffic across a particular link based on the metrics assigned.

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Teejay Riedl is director of solutions technology for the broadband services business unit at Verio, Springfield, Va.