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Internet Redundancy: Multi-Homing with BGP or Static Routing

Does your company need to Multi-Home? Internet Multi-Homing is essentially two or more redundant connections to the Internet. Let’s take a look at some of the ways this can be accomplished.

Factors to Consider When Designing Redundant Connections

First and foremost it is important to understand the physical design of your redundancy. There are several variables to consider when designing redundant connections. These variables can range from simple and cost-effective to complex and cost prohibitive. In general, you could use a single or dual router with diverse local loops (paths) to your ISPs.

When using a single router, you may want to consider a router that has redundancy built in. This would include the option for redundant power supplies and route processors. Depending on your requirements, you may need multiple routers with this level of redundancy built in.

Path Diversity
Another important factor to consider is the path diversity. Before leasing a diverse local loop, determine the full path the connection will take from the ISP to your building. Also, be aware of the building entrance. It is important the local loops enter the building on diverse access entrances. These local loops may be completely diverse from the ISPs to your curb, but as they enter the building they could be brought in through the same entrance.

If your organization is geographically diverse, you may consider bringing redundant Internet connections into separate facilities. This would protect your organization if there are issues with a network in a specific geographical area. While those issues are being resolved, your organization can seamlessly reroute through the redundant facilities connection.

Multi-Home Routing Options: Static Routing vs. BGP

Each of the options above will require some sort of mechanism or protocol in place to determine the best path to the Internet. Let’s take a look at some of the options necessary to make use of your diverse Internet connections.

Static Routing
The simplest way to multi-home would be with static routes. This requires that you bring two diverse Internet connections from your ISPs into the same router. There would be a primary static route and a secondary static route. The primary route is used if the main connection is up, and the secondary route would be configured to come up only if the main connection is down. This is sometimes called a “floating static route.”

This option is simple but less scalable than BGP. It works well for organizations that need redundant Internet at a single location. It is also suggested only when connecting to a single ISP on redundant local loops. It is possible to use this method when connecting to multiple ISPs, but it requires an additional firewall “NAT” function responsible for changing the public IP Address when switching between ISPs. Check with your firewall manufacturer for support with redundant ISP configurations.

Three Benefits of Border Gateway Protocol
The Border Gateway Protocol (BGP) is the routing protocol used for large networks that are made up of multiple independent routing domains or autonomous systems (AS). The most common network that relies on BGP is the Internet. Each ISP has a unique presence on the Internet identified by an Autonomous System number. In general, BGP determines the best path based on the fewest number of autonomous systems needed to transit before reaching a final destination.

When you require redundancy to the Internet, BGP presents many options that static routing is unable to achieve. These include load-balancing and diversity between hardware, providers, and geographical areas. It also utilizes a TCP/IP connection to communicate with its peers. This allows it to determine the full end-to-end reachability of a peer. Static routing can only monitor an interface locally, and if the connection goes down indirectly upstream, it will not sense the loss of connectivity and deactivate the route.

  1. Hardware Redundancy
    BGP allows you to bring diverse Internet connections into multiple routers. Each router would peer with upstream provider(s) over what is called an E-BGP (external) link. Internally, your routers would peer with each other over an I-BGP (internal) link. This allows them to all be in sync, and to alert each other when a BGP path is down.
  2. Provider Diversity
    BGP allows you to diversify with multiple providers. When moving toward this solution, there are some requirements that must be met. First, you must obtain a unique Autonomous System number from the American Registry for Internet Numbers (ARIN). Second, you have to obtain a block of public IP Addresses from ARIN, or be reassigned a minimum of one /24 (Class C) IPv4 address range from your upstream provider. This is because the BGP Internet routing table will not accept anything smaller than a /24 IPv4 address range. Once these requirements are met, you are able to configure BGP to allow for redundancy between multiple providers. For more information on obtaining an AS number or IPv4 address range, go to http://www.arin.net.
  3. Load Balancing
    If your organization is geographically diverse, BGP will allow you to take in Internet connections to several locations, from several providers all over the world, while maintaining a single “presence” on the internet. It also allows you to load balance between multiple connections and providers. In this scenario you may have certain routes using one connection, while other routes are using an alternative connection. It is common to engineer traffic out a provider that is “closer” to destinations in certain geographical areas, or have less “AS hops” necessary to transit. This allows you to reduce latency and increase application performance.

In conclusion, static routing is the simpler way to multi-home for organizations working from a single location. However, BGP is much more scalable and flexible for large, multi-campus companies and organizations.