ISP Multihoming Explained
Internet access for organisations today is no longer about connectivity for email and
web browsing. A stable Internet connection is a vital component in the chain of IT systems
required to conduct business. Typically, in the past, the focus around Internet connectivity
has been on cost, with vendors providing solutions allowing organisations to spread their traffic
across consumer and enterprise products.
This approach is all good and well, and can provide significant cost savings, especially
when employee traffic is directed over low-cost consumer products such as ADSL, however,
when you are conducting B2B business through front end servers hosted in your DMZ,
resilience becomes a major concern. In this scenario, a dead Internet link can mean loss
of revenue and even, potentially more serious, brand damage.
In this paper we discuss a number of methods that can be used to improve the resilience
of an Internet link. While this sounds like it should be a simple case of connecting to multiple
Internet Service Providers, the devil as they say, is in the detail.
Mission critical Internet
Business networks have been mission critical for some time now and the focus on resilience
and business continuity has always been top of any CIO's mind, however, the general areas of
interest for this focus were restricted to internal networks and systems.
With more and more business being conducted either directly via the web or via B2B over
Internet links to systems hosted in DMZ's, it is simply no longer permissible for an Internet
link to be down. Loss of access to the Internet can have a direct impact on revenue generation,
especially today as the business operating models begin shifting towards off site cloud
computing and software as a service.
A solution to the problem
Multihoming is essentially a method whereby a company can connect to more than one ISP
at the same time. The concept was born out of the need to protect Internet access in the event
of either an ISP link failure or an ISP internal failure. In the earlier days of Internet access,
most traffic was outbound with the exception of email. An Internet link failure left internal
users with no browsing capability and with email backing up on inbound ISP mail gateways.
Once the link was restored so was browsing and email delivery. The direct impact to the
business was relatively small and mostly not revenue effecting. Early solutions to this problem
were to connect multiple links to the same ISP, but while this offered some level of link resilience,
it could provide no safeguards against an internal ISP failure.
Today, however, most organisations deploy a myriad of on-site Internet accessible services
such as VPN's, voice services, webmail and secure internal system access while also making
use of business critical off site services such as software as a service (SaaS) and other cloud
Furthermore, while corporate front-end websites are traditionally hosted offsite with web
hosting firms, the real-time information on the corporate websites and B2B sites is provided
by back-end systems based in the corporate data centre or DMZ. Without a good quality
Internet connection, these vital links would be severed.
Varied requirements and complexity
That said, the requirement for multihoming are varied and could range from the simple
need for geographic link diversity (single ISP) to full link and ISP resilience where separate
links are run from separate data centres to different ISP's. While the complexity varies for
each option, the latter forms the most complex deployment option, but affords the highest availability,
with the former providing some degree of protection, but does requiring a higher grade of ISP.
A major component of the complexity comes in around IP addressing. The way the Internet
IP addressing system works is that each ISP applies for a range of addresses from the central
Internet registrar in their region. They would then allocate a range of IP addresses, called
an address space, to their customers from this pool. It goes without saying that no two ISP's
can issue the same address space to a customer.