When it comes to fast convergence, first thing that we need to understand what is convergence? Convergence is the time between failure and the recovery.
Link, circuits, routers, switches all eventually fails. As a network designers, our job is to understand the topology and whenever there is qrequirement, add backup link or node. Of course, not every network, or not every place in the network requires redundancy though. But let's assume, we want redundancy, thus we add backup link or node and we want to recover from the failure as quickly as possible, by hoping before Application timeout.
But what is the time for us to say , this network is converging fast. Unfortunately, there is no numerical value for it. So, you cannot say, 30 seconds , or 10 seconds , or 1 second is fast convergence. Your application convergence requirement might be much below 1 second. Thus, I generally call ' Fast Convergence' is the convergence time faster than default convergence value.
Let's say, OSPF on Broadcast media is converging in 50 seconds, so any attempt to make OSPF convergence faster than 50 seconds default convergence value is OSPF Fast Convergence on Broadcast media.There are in general 4 steps for making the convergence faster, so 4 steps for Fast Convergence.
Four necessary steps in fast convergence1. Failure detection
Layer 1 Failure detection mechanisms:
- Carrier delay
- Debounce Timer
- Sonet/SDH APS timers
- Layer 3 Failure detection mechanisms:
- Protocol timers (Hello/Dead)
Propagation of failure throughout the network. Here LSP throttling timers come into play. You can tune LSA throttling for faster information propagation. It can be used to slow down the information processing as well. Also LSP pacing timers can be tuned for sending update much faster.