Legacy Cisco (eg C4948's) all mounted like this and weighed a lot, all sagged and looks horrible but is long term stable and no damage to HP or rittal racks. New WAE devices also mount like this and weigh a heap.
I mount 1 or 2 extra pairs of rails between the front and back sets of rails. I then use horizontal support bars (in the top hole of the RU below the sagging device) to support the rear of the device:
That is a Catalyst 4948-10GE about 1/3 of the way down in the picture. The gold-colored bar with holes is the support piece. This rack has 23" rail spacing, so there are 23" to 19" reducers on the equipment and the support bar is also 23", but they are available in 19" as well.
Even new Cisco stuff that has rail like the nexus gear has the worst designed cheapest rails.
Why do network company's do such terrible mounting ??
Cisco has always been a particularly bad offender, from the mounting holes on the 25xx that don't line up, to the 720x which uses a pair of 3mm screws on each side to support a 50 lb router, to the Catalyst 6500 where the ears are so wide you can't mount them side-by-side in adjacent racks. But there are some other terrible vendors as well.
I think part of the problem is that the original rack design was "Telco" racks (one set of rails) where equipment was center-mounted, balanced evenly. Then manufacturers just said to themselves "If we put the same set of mounting holes at the front and back of the unit, customers could mount it that way if they wanted." without thinking of the consequences. It is hard to see why Cisco (for example) hasn't learned yet, though.
