You Truly Get What You Pay with Power Sequencing on PTX
Jul 7, 2016
If you take a first class flight, you can enjoy many privileges: preferential check-in, superb meal service, comfortable seating, the crew’s dedicated attention, you know the drill. These privileges separate first class from economy class. But if the flight is delayed or cancelled, you are equally out of luck, regardless of the ticket. In situations like this, there is no service differentiation!
In networking, there is also a differentiation among various customers and their traffic, called Class of Service (COS). On a port, traffic from different customers can bear a different COS. If there is network traffic congestion on that port, higher COS traffic (normally from higher paying customers) is sent preferentially while lower COS traffic may experience delay or even loss. But what if the whole card is down due to some power supply failure on the router or switch? That’s too bad! All traffic, whether high or low COS, will be stopped and lost. Again, no service differentiation.
On PTX, an innovation is introduced to solve this problem. This innovation is called power sequencing—with it, not only does the traffic on a port have service differentiation, but so do the cards on a chassis during power-on and at power failure, as described below.
Take a look at an example in which the user configures the power-on sequence for FPCs on the chassis. The customer on PFC 3 has paid 50% premium, customer on FPC 7 has paid 30% premium, customer on FPC 2 has bargained hard for a 20% discount, and the user just likes the customer on FPC 4 for no reason. Assume that there are eight FPCs on the chassis, then the user may configure the power-on sequence as: 3, 7, 4, 0, 1, 5, 6, 2. If the user doesn’t configure any power-on sequence, the default sequence would have been 0, 1, 2, 3, 4, 5, 6, 7.
When the chassis boots up, the FPCs are powered on in the order of the configured or default sequence. This is similar to first class passengers getting to board the plane before economy class passengers, isn’t it?
We know that life is always unpredictable. Now let’s say that some power supplies fail. Without this innovation, random FPCs will be shut down and all traffic on these FPCs will be lost, regardless of whether the customer paid the 50% premium or bargained for a 20% discount. This is not fair! Customers would cry.
Here comes the power of power sequencing! When power supplies fail, the FPCs are powered down in the reverse order of the power-on sequence. So in the above example, FPC 2 gets powered down first. If there is still insufficient power, FPC 6 gets powered down, then FPC 5, then FPC 1, etc. The highest paid customer, FPC 3, gets hit last.
As D. Allen, a mystical and Zen Buddhist poet, put, “The most expected thing you can expect is what’s unexpected.” But Juniper has a solution to combat the unexpected, a solution with fairness, a solution better than what even the best airlines offer to their first class customers.