Part 1 of the posting shows the challenges of dual ISP design, the traditional approach of outbound load sharing based on entire internet routing table will largely depend on the particular ISPs. And when routes received from ISPs have different characteristics such as AS Path and metric, the result is often undesirable. To achieve better outcome by design, in part 2 we will start with a simple alternative.
Replacing the entire Internet routing table with just the default route is an extremely simple method that offers a number of advantages.
Load balancing
Instead of getting the entire Internet routing table, only default route is received and installed in the routing table. As a result, IGP can load balance to two equal cost default routes. For outbound traffic, the simple design achieve near 50/50 load balancing, as well as resiliency.
Simplicity, Stability and Resource Efficiency
The design is extremely simple to implement and support. Resource usage on devices can be greatly reduced, from holding tens and thousands of Internet routes, to just default route. Route flapping and any disruptive convergence due to instability in any parts of the Internet is virtually eliminated.
The simplicity advantage is well suited for a large number of enterprise data centers.
Disadvantage
The design essentially “splits” the Internet in half, by two equal cost default routes to dual ISPs. Therefore, the exit point may not be optimal, especially for networks directly attached to an ISP, which may require the longer path to get to.
For vast majority of applications, the selection of exit ISP is not noticeable. However, lower latency access of large amount of media content may be highly desirable when a direct path is available. An optimized solution is presented in part 3.
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