Redundancy (and Power Backup) in Juniper MX Series Routers

A whole lot of people are using the word “redundant” incorrectly. Unique individual? Redundant. Advance planning? Redundant. Two powers in a Juniper MX480 chassis? NOT redundant. For full redundancy in the larger MX units, you’re going to have to have all the power slots populated. Here’s how it works.

For the MX480, there are four power supply slots, PEM0 through PEM3, each with its own specific role. PEM0 is life support, providing juice for the fan, the SCB/RE slots, and the first two DPC/MPC slots, (zero and one). Juniper calls that Zone 0. The power slot next to that is PEM1, and it provides power to slots two through five. Juniper calls that Zone 1. If you want redundancy for life support in Zone 0, that’s going to happen only on PEM2. Redundancy for the line cards on Zone 1 happens on PEM3.

Got that? You can’t add a second power and expect redundancy on the 480. You can’t even add a third, unless you’ve only got the SCBs and the first two cards populated. Of course if that’s all you’re going to fill, you’ve probably gotten yourself too much chassis (see the explanation of the MX240 below).

If you’re deploying an MX960, the basic idea is the same, but the specifics are a little different. PEM0 powers Zone 0, which includes the lower fan tray, SCBs one and two, and DPC/MPC slots six through eleven. PEM1 in Zone 1 powers the upper fan tray, SCB0 and DPC/MPC slots zero to five. Paying attention? That means you can’t even run life support on one power in the MX960. Both fans won’t operate in that scenario. Redundancy on the 960 utilizes the same zones as the MX480: PEM2 backs up PEM0, and PEM3 acts as a failover for PEM1. That much hardware needs four power supplies.

Two more things to note, here. Firstly, even if you have high-powered cards and invest in high capacity power supplies, the issue of redundancy is the same. Buying a 4100 Watt power isn’t going to save you from a catastrophic meltdown (redundant! Are there any other kinds of meltdowns?), because if that thing fails in Zone 1, the brains of the outfit are flat lining. You still need another power in each zone to prevent failure.

The second thing is that the MX240 works differently. It’s a little fella, and uses less energy than its big brothers, so PEM0 runs life support, and PEM1 acts as a failover, with PEM2 and PEM3 just giving you more ways to protect your interests. Best way to see if you need extra PEMs is to calculate your power requirements by adding up the needs of the base unit and any additional components . Over the wattage of the power supply you chose? Then that second one will take up the slack, but it won’t provide redundancy. You’ll need a third one to make sure your hardware is provided for in the event of a single power supply failure. Those bigger units might need to be filled to capacity (which is redundant) to become redundant, but with the 240 you can save yourself a little money with some careful planning.

The concepts are easy, but the details can be a little tough. More questions? We’re here to help. You can’t get redundancy with rough estimates (even though that phrase is redundant). You have to plan ahead (redundant!) to get the right end result (redundant!) and ensure you’ve got a system that meets your needs.