In networking, we rely on routing protocols to compute best path. That is to ask, from the perspective of a given router in a routing domain, what is the best way to reach a destination? Best path is typically computed using simplistic metrics like hop count, cost, bandwidth, and delay. Given several paths to get to the destination, the best path is chosen based on the most desirable metric computed along those several paths.
Traditional “best path” thinking is effective, insofar as it goes. It scales to a large number of devices and destinations. It is resilient. It is mature. However, it has its limitations.
- Routing protocols compute best path for a given destination, not a given application.
- Best path computations are not influenced by real-time events such as lossy links or link congestion.
- In general, lower bandwidth links are not used to carry traffic (i.e. are not computed as the best path and are therefore unused), EIGRP’s variance feature being a notable exception.
Now, routing protocols can be influenced by outside forces, such as IP SLA. Complex routing policies can be built with BGP. Engineers can create custom routing policies or build traffic engineered paths that flout best path calculations. I recognize that. But, generally speaking, routing protocols were not built to route application flows. They were built to route individual packets.
Software defined WAN brings a much more sophisticated metric to the computation of best path.
- Best path is computed on a per-application basis, not per-destination. The best path for a bulk traffic application might be different from the best path for a voice application, even if heading to the same destination.
- Real-time link capabilities can be factored into the decision.
- Any link, no matter the available bandwidth compared to other links, can be used in an active/active manner.
The way best path computation changes is one of several things that has grabbed my attention about SD-WAN technology. I will be talking about SD-WAN in a webinar with Silver Peak CEO David Hughes June 23, 2015 @ 8PT/11ET. I’ll kick off the webinar with ~20 minutes of my take on SD-WAN as a technology, and then I’ll be interviewing David about Silver Peak’s specific SD-WAN offering, Unity Edge Connect for the remainder of the event. I will do my best to make the event flow in a Packet Pushers style.