Rack-Mounting Network Devices
By stretch | Thursday, January 6, 2011 at 2:31 a.m. UTC
Moving the community lab into its new 19-inch rack got me thinking about all of the past installations I've done, and all the mistakes from which I've learned. It seemed like a valid enough subject for a blog article, so here's something of a how-to guide if you're new to rack-mounting equipment.
The term "rack" generally refers to a standard metal frame which measures 19 inches (~48cm) wide between bolt holes or cage nut sockets. Racks have two or four posts and may be enclosed with or without doors, or bare freestanding frames. Racks may be secured to the floor or mounted on casters (wheels). There are also 23-inch racks, found predominantly in telecommunications centers, but 19-inch racks are far more common. Below are several examples of 19-inch racks.
The vertical rails along the interior of a rack have either threaded holes which accept a bolt directly, or square openings for cage nuts (discussed shortly).
A rack unit, abbreviated as RU or simply U, is a single standard unit of vertical rack space. One U is equivalent to 1.75 inches (~44mm) of vertical height. A device which is said to be 2U is 3.5 inches tall; twice the height of a 1U device (1.75in), and half the height of a 4U device (7in).
Note that not all holes (or cage nut sockets) are spaced evenly. It is important to count rack units correctly when preparing to mount gear.
Height varies among racks. A height of 42U or more is commonly considered a "full" rack; a height of 20-24U is considered a "half" rack.
As the name implies, a cage nut consists of two pieces: a square nut enclosed by a small "cage" with wings that hold the cage nut inside the square socket of a rack. There is also a less common type of cage nut which slides over a socket from the side of the rail. Both types position the nut behind the vertical rail, toward the inside of the rack.
Cage nuts and bolts are available with several types of threads. Make sure the bolts and cage nuts you have fit together before beginning to mount something.
Mounting Brackets or "Ears"
Mounting brackets come in various designs, typically unique to the device to which they attach. Brackets typically attach to the sides of a rack-mountable device and provide a structural fin on either side through which bolts attach to the rails or cage nuts. Their 90-degree bends have lent them term "ears."
Pictured below are mounting brackets which will fit several models of Cisco Catalyst switches.
Mounting equipment in a rack can be dangerous, particularly with heavy gear. Below are some guidelines you'll want to follow. Most of these should be common sense.
- Two-post racks not specifically designed to be free-standing should always be secured to the floor.
- Wear hard-toed shoes, either steel or composite toe. (Please note that the idiotic rumor that a steel toe might actually cut off your toes under sufficient load has been proven a myth.)
- Know how much weight you can comfortably lift for a moderate duration of time. Don't attempt to lift more than half of that by yourself; you won't always have both hands free.
- Exercise caution when setting your tools aside. Always keep them below eye level.
- Ensure that both sides of the rack, front and back, are clear before beginning work.
- Never attempt to mount or relocate a powered (or plugged in) device.
- Mount heavier gear toward the bottom of the rack.
Rack Mount Process
As with everything in IT, the success of your rack mount project will depend on diligent preparation. There are several details of which you'll want to be sure before attempting to mount a device.
Rack space. Are there enough consecutive rack units available to accommodate the device(s) to be installed? Don't simply meaure vertical space: check that the proper holes are clear on the rails (some odd equipment might be partially obstructing the space needed). Remember to check that the rack space remains clear as far back as the device is deep.
Power. Check that adequate power outlets of the right nature (AC or DC) and amperage are available. If power will be drawn from a power strip or PDU, ensure that the outlet is administratively on and functioning. Check candidate outlets with a multimeter if you have one.
Air flow. Many racks, particularly in data centers, have been arranged for predominantly front-to-back or side-to-side air flow. Ensure that your device abides by the established layout. Also check that the intake of your device won't be next to the hot air exhaust of another.
Hardware. Ensure that you have all of the required mounting hardware, to include cage nuts, bolts, and mounting brackets, plus spares.
Cabling. Check that all stray cables have been removed or repositioned prior to mounting. The entire space to be occupied by the device should be free of obstruction. Further, ensure that any cables that must be removed will still reach after the device has been mounted.
If you've taken the time to adequately prepare, the installation itself should go smoothly. Begin by inserting cage nuts into the appropriate sockets for the desired units in the rack. If the rack has threaded holes, designate the appropriate holes with a marker or tape.
If the device to be mounted will be situated on horizontal rails, as are many servers, install the rails one at a time. Most rails require a four-post rack for installation.
If the device is to be mounted without rails, it may be beneficial to install a pair of cage nuts and/or bolts in the openings directly below the lowermost socket to be occupied. This will allow you to rest the bulk of the device's weight on the front two rails of the rack while supporting the remainder with a free hand underneath the device.
Keep the device steady in place while securing it to the rack. Secure the bottom two bolts first, while pushing up toward the rear device to ensure the mounting brackets are completely flush with the vertical rails.
Once you have completed installation of all hardware, attach the power and data cabling, in that order, as appropriate.
Collect any stray tools or hardware left around. Replace or resituate any cables or other devices that were temporarily moved for the installation. Finally, update your documentation to indicate any modifications that have been made to the initial installation plan.
January 6, 2011 at 4:06 a.m. UTC
I think it's also worth mentioning to be cognizant of what type/size of screws your rack / cage nuts take.
Older stuff tends to be 10-32. I've seen lots of pre-drilled open post racks tapped for 10-32.
M6 seems to be more popular today. Biggest visual difference is that 10-32 is 'skinny' while M6 is 'fat'.
Finally, if you find yourself in a telco CO, mounting in their 'bays'. a couple of things. First you will probably need 12-24 thread screws. Secondly, you'll need some 23-19 'reducer' plates assuming your gear doesn't have ears for both 23 and 19.
January 6, 2011 at 9:21 a.m. UTC
nice article! About the cage nuts, they are so cheap that I suggest to mount it on every rack unit, even the unused ones, to be more confortable when changing something on the rack. It's always frustrating when you try to insert cage nuts in a single rack unit when the units below and above are occupied!
There are also useful inserting tools for cage nuts.
January 6, 2011 at 12:11 p.m. UTC
Great post - 12 years in the industry and there were a number of things there I didn't know.
I recently had to rack a WAAS appliance and discovered that the rack depth is a something to be careful of.
I completed the entire installation and then determined that the guide rails protruded about 6 inches out of the back of the rack. Never mind any safety risk, the doors wouldn't close - duh!
I found a nearby Rack that appeared larger and would be suitable. I wasn't going to be caught twice - on checking, the rails were about an inch too short and wouldn't reach the back posts.
"Mount" the device on an ACS Appliance. Not elegant, but I wasn't going to fail a change. Of course, I let the DC manager know with the advice that it be revisited. I bet it wasn't and is that way now :-)
Before anyone suggests that the rails expand or shrink, I checked that out. In the first rack, the rear rail connection slid up to the rear post without any problem. But the rail itself was a fixed length. Same answer for the second rack.
January 6, 2011 at 8:50 p.m. UTC
For heavier gear I used to mount a shelf first, then I would hoist the gear up onto it. I would only have to wiggle the ears up maybe an inch at most while I turned those rack screws at my leisure. Beats the hell out of holding a fully loaded chassis up while you fumble with screws.
Keep in mind...most shelves aren't really designed to hold more than 70 - 150 lbs...this is just a temporary aid while you get the gear mounted. You remove the shelf once the gear is secure.
January 6, 2011 at 10:00 p.m. UTC
Two ways to waste space and piss off the poor unfortunate who has to mount another device into an already crowded rack.:
- Mount devices off the 1U boundaries.
- Even better - leave 0.1U or 0.2U spaces throughout the rack. Pretty.
January 6, 2011 at 10:00 p.m. UTC
The amount of rails I have seen that don't fit in 19" racks in unbelievable. Dell ones are usually the worst. It seems that the "standard" rack size is a myth.
January 7, 2011 at 12:30 p.m. UTC
Nice one - I've used the put the screws in under the equipment to hold it while it's racked. Very handy.
I didn't see an obvious tip - when racking big equipment, say a fully populated 6509 - it really helps to take out the cards and power supplies, then rack the chassis, then insert the removed cards and power supplies. Seems kind of obvious, but a lot of people might not think about it.
If you're going really crazy, and work in a big data center, get the Management to spring for an equipment lifter - mention something about your back, OSHA, etc. ;)
January 7, 2011 at 1:32 p.m. UTC
You can move equipment without powering down. Even between data centers and using public transport:
(Sorry it's in German but it's fun to watch).
January 8, 2011 at 6:42 p.m. UTC
Can't tell you how useful this post was. Thanks a lot. It cleared a few things up for me and also sured up some terms I was unsure of.
January 10, 2011 at 4:10 a.m. UTC
Sometimes you may not have quite as many screws as you need to fully attach the devices. In that case, I like to use the mnemonic:
If you can only screw in one hole, make sure it's the bottom.
January 11, 2011 at 2:08 p.m. UTC
One thing to make sure of, when you put the rack ears on a Cisco router or switch, make sure you use the proper screws. Some of them have screws you need to take out to put on the rack ears. if you re-use the original screws, they will stick on so that the switch doesn't it in-between the rack rails when it is flat.
Then you have to tip it so it will fit. Great until someone mounts equipment above and below it and you have to unrack all of it to swap out a dead switch.
January 15, 2011 at 3:48 a.m. UTC
In other news, water is wet, the sky is blue and hot stoves will burn your hands
January 15, 2011 at 3:51 a.m. UTC
@Sam: Understand that while you may be well accustomed to rack-mounting hardware, many others are not, just like you weren't when you first learned. Hence the blog article. No need to be an ass about it.
March 24, 2011 at 5:15 p.m. UTC
Thanks for this very nice article.
It is a good summary of rack mounting best practices.
Do you plan to write one regarding rack cabling ?
December 2, 2011 at 2:30 a.m. UTC
be sure to install cage nuts with the wings horizontal. if the wings are vertical, you will NOT be bale to remove them if equioment is isntall abiove or below the nut
also, chose one of the four common nut sizes, you only ONE size in all your racks. i prefer M6