Connectivity to the Internet through more than one upstream ISP (Internet Service Provider) is referred to as multi-homing (or dual-homing in case of two ISPs). Multi-homing is generally required to increase the reliability of the Internet connection by reducing the reliance on a single provider and eliminating single-point-of –failure in the IP network. Dual- or multi-homing can also be used to load-balance the Internet traffic and improve performance.
While there are some techniques that can be used to archive dual-homing for special applications, the use of BGP routing to connect to multiple providers is the only effective technique to achieve general dual-homing for IPv4 networks. This report will focus exclusively on the use of BGP to connect to multiple providers.
BGP provides the ability for the network traffic going or coming from the Internet to be forwarded to any of the available ISPs. Unlike internal routing, BGP does not select routes based on shortest path to the destination but on the number of ASs (Autonomous Systems) the represent the networks from source to destination. BGP may be also configure to implement other routing policies to, for example, prefer some routes over others.
To improve the reliability of the Internet connection, an organization may choose to connect to two or more ISPs and split the Internet traffic equally among them. In the case where one provider’s link fails, outgoing traffic will automatically be routed to the remaining link(s). Other networks will be notified, through BGP updates, of the failed link and incoming traffic will be routed through another ISP link as well. In this architecture, there must be enough capacity in the remaining active links to be able to carry all the traffic from the failed link with causing congestion, which results in dropped packets and degradation of service. This means than in a dual-homing scenario, each link must carry the entire organization’s Internet traffic volume.
The organization may find an advantage in connecting to two ISP of unequal bandwidth. BGP may be configured to use one ISP as the main route where all outgoing and incoming traffic is directed. The backup ISP of small bandwidth will be activated only in the case of the main ISP’s failure and only selected traffic is routed through this link while the main ISP is being repaired. The advantage of this approach is to reduce the expenses needed to establish a second full capacity link.
Dual- or Multi-homing can be also used to improve the performance of the Internet connectivity by the carful choice of the ISPs and the proper configuration of BGP. For an organization that serve customers in diverse geographic locations, or it has branches both locally and abroad, BGP peering with multiple ISPs can ensure that traffic to each geographic location will go through the best route. This configuration will reduce the latency experiences by the users in each geographic region.
To enable multi-homing using BGP, an organization must have its own public IP address block and a public Autonomous System (AS) number before connections to two or more separate ISPs are established. Generally, ISPs do not accept or announce IPv4 address blocks smaller than /24 (255 addresses) through BGP. The organization must receive its public ASN from the regional Registry of Internet Numbers (ARIN in North America). The IPv4 can be obtained directly from the regional authority or from one of the ISPs. In the latter case other ISPs must agree to announce the IPv4 block in BGP.
A key problem to avoid in multi-homing is creating two apparently independent links from completely different ISPs using a common infrastructure such as link or a router in the organization’s network. This will actually form a single point of failure and considerably reduce the reliability benefits from multi-homing. Another problem to watch for is connecting to two ISPs, which in turn connect to a third, common ISP. The failure of the distant ISP may result in simultaneous outage or degradation of service on both links.