Multi-Homed Networks
What is multi-homing?
Having multiple connections to the Internet via multiple providers to provide a reliable and high throughput service. Multi-homed networks are increasingly popular because they provide networks with better reliability and performance to the end-user. Better reliability results from the fact that the network is protected in case one of the Internet links or access routers fails. If a connection with one ISP is lost or degraded, traffic is automatically redirected to links that are properly functioning. When more bandwidth is required, it is easy and cost-effective to add more links. There are up to five ways to multi-home, but the two most common are:
- Using multiple links with a single IP address. This requires the use of multiple routers and a protocol called the Border Gateway Protocol (BGP). With this type of multi-homing, the end-site announces this address space to its upstream links. When one of the links fails, the protocol notices this on both sides and traffic is no longer sent over the failing link. For reasons due to the complexity of the technique, It can take as long as 30 minutes to redirect traffic to a functioning link. Usually this method is used to multi-home a single site and not for single hosts.
- Using multiple links, multiple IP addresses. This method uses a specialized Link Load Balancer appliance (such as PowerLink) between the firewall and the link routers. No special configuration is required in the ISP's routers. Using a link load balancer (also referred to as a "WAN link controller" or "Internet load balancer") allows you to use all the links at the same time to increase the total available bandwidth (bandwidth aggregation) and detects link saturation and failures in real-time to redirect traffic. Incoming balancing is usually performed with real-time DNS (Domain Name System) resolution.
How do you multi-home without BGP?
ISPs and large enterprises have mult-ihomed for years using BGP to connect to multiple Internet backbones. But BGP has many restrictions. For one, it requires that ISPs cooperate with each other and set up "peering" agreements between routers, but because of the performance impact to their networks, many are not willing to do so. BGP also require expensive routers, designated address blocks and an Address Space Number( ASN), which are sometimes not available to small businesses. And BGP requires that gateway hosts exchange dynamic routing tables, which must be constantly synchronized and which can lead delays of up to 30 minutes in changing the direction traffic is sent.
WAN link controllers such as PowerLink multi-home use Network Address Translation (NAT) to unify traffic coming from, and going to different destination IP addresses on the Internet. The PowerLink can be configured with at least one routable IP address for each router/WAN link that is connected to the network.
The biggest benefit of PowerLink multi-homing resides in the ability to achieve outgoing and incoming load balancing and failover without defining BGP routing tables or utilizing any of the underlying complicated routing techniques. The ability to offer this functionality without the expensive and complicated networks/equipment necessary to achieve BGP is what makes the PowerLink such an exceptional value, especially for small and medium sized organizations.
For more information, see our white paper on multi-homing without BGP.
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