Packet loss with Fast Reroute
Do we still lose packet with fast reroute? One of my students asked me this question. And I would like to share the answer with everyone.
Before we discuss whether or not we lose packet with fast reroute mechanisms, let’s remember what fast reroute is.
It is pertinent to know that fast reroute is not the same as fast convergence. With fast convergence, we can touch protocol timers, squeeze them, and maybe tweak FIB prefix instalment method in order to install golden prefixes before the link prefixes. And in the end, we have less than a second convergence time.
This may be enough for your network applications. But note that with fast convergence, a backup path is not pre-installed in the FIB.
Fast reroute can provide 50ms convergence time. Yes, it can. And this is the most important difference from the end result point of view.
But from the technology implementation point of view, the difference is that with fast reroute, a backup path is pre-installed in the FIB (Forwarding Information Base).
So, if your fast reroute mechanism works based on a tunnel (Some FRR mechanisms don’t require a tunnel), a backup tunnel is pre-installed.
And that is why fast reroute is known as a proactive, fast convergence is a reactive protection method.
Let’s take a look at the question again.
Do we still lose packet with fast reroute?
The answer is YES. As I said, fast reroute can provide 50ms convergence time depending on mainly the fast failure detection time. This 50ms is the time between the failure and success of using a backup path.
All the traffic in this 50 ms is lost. The application timeout can tolerate this. But I think it is important to know that there will be packet loss with fast reroute.