Lunatic Platitudes

If you are deploying VOIP using POE in your enterprise, you need to stop now and dedicate a significant amount of time to planning power. In our case we have Cisco 4506 switches in our wiring closet, all equipped identically as they were purchased all at once as part of our future proofing in anticipation of VOIP. Now VOIP is a reality and I have learned a couple of lessons that I am wanting to both pass along and remember myself. This article contains details specific to the 4506 platform but the concepts are universal.

What is your input power source?

The first thing you need to evaluate is how your switches are powered. My suggestion? Go ahead and order power strips with built in amp gauges. Trip Lite has some really nice units. If you are running 110V circuits they have a sub $200 horizontal rack mount unit part number PD6974 that I recommend. Install these in each of your wiring closets and document the power circuit numbers and what power supply in each switch each circuit is connected to. This will come in handy later on if you have to coordinate changes with your facilities operations. Hopefully you will have a consistent configuration that is not only well documented but that can be easily scaled.

How many phones per switch can I support given my current configuration?

This might seem obvious but it is amazing to me how counterintuitive it seemed to be at first. We always talk about the larger project and how many phones we’re going to deploy, but you need to break it down to a per switch/per configuration aspect. Concentrating on power, I know that I have 1 or 2 switches per floor and that they are all identical in configuration. Cisco provides a very good tool for calculating power requirements called the Cisco Power Calculator. Use this tool to input your exact hardware and input the number of each different type of phone you plan to deploy. We pretty much standardized on the 7945G or 7965G depending on the individual user. This made it very easy to plug in the numbers to calculate what I could do. In my case, I configured a 4506 with a 4013+ sup, 5 blades of 4548G RJ45V and two 4200W power supplies with 100-120V input power. What I found by trial and error is that I could run up to 120 phones on a single power supply with dual 100-120V power inputs.

How does the catalyst budget power and what is my redundancy model?

Don’t gloss over this part. There are two power redundancy modes, 4 power inputs, 2 power supplies and a lot of options to utilize all those resources. Cisco tells you in documentation that you should run in power mode “redundant” and not “combined” because you can overload the switch and lose redundancy should a power circuit fail. In reality, I have found that not only can input power fail, individual power supplies can fail in such a way as to experience reduced output levels. What I said at the end of the last section was that I found that I could deploy 120 Cisco 7945/65G phones on a single 4506 chassis as long as I always had at least 2 110V input power connections. Now, I can connect those two power inputs into a single power supply and run all day long within my power budget, but I have no hardware redundancy. In this way my power mode would be “redundant” or “combined” because it doesn’t matter with only a single power supply. I have another option in that I could spread those two 110 volt circuits across a pair of power supplies, but now if I lose a power supply, I lose half my power and I can no longer run 120 phones. Plus, with two power supplies each wiht a single 110V power input source, I have to run “combined” mode to take advantage of both inputs together.

The 4506 will manage power based on a budget taken from the lowest common denominator. In other words, let’s say you have three 110V inputs connected to two power supplies. Meaning one power supply has a single input. If your power mode is redundant, the switch will budget power based on a single 110V circuit even though three are connected. If you run that same hardware configuration with “power redundancy-mode combined” you will suck from all three indiscriminately and you will be able to support more than 120 phones. Cisco implies this is a dangerous situation and they are correct in saying that. The fact is, this is not redundant once the 121st phone is connected. When you reach the 121st phone on a 4506 switch with 110V inputs and 4200W power supplies there is NO REDUNDANCY POSSIBLE in power. The simple fact is that even with 4 110V inputs and combined mode, you will not be able to sustain a failure in a power supply. What to do? PLAN!

The most important thing you can do in a project as big as a VOIP deployment is to plan every detail up front. Document how many phones your current infrastructure can support in a given area of your company. It’s that simple. If you are in a similar situation to me, and need to support more than 120 phones in a space that has a single 4506, you have to document the risks to management and options to mitigate that risk. In my case, I documented that power could be upgraded from 110V to 208V dryer circuits, or a second or third switch could be added to the floor in order to spread the POE ports between a second chassis, bringing my total supported phones up to 240.

My Recommendations

I realize this is yet another rambling post, but I’ve gone through a good bit of frustration with POE and VOIP lately on my 4506’s. My ultimate recommendation is two fold. First, document everything you have exactly and maintain that document. When negotiating expensive power renovations you must be completely well informed. Second, specific configurations must be made with the actual user experience in mind.

Let me explain. If you have a mandate that only 120 phones can exist on a 4506 because of 4200W power supplies and 110V inputs (dual) you are implying that you can sustain TWO input failures or a SINGLE power supply failure, but nothing else. If you follow Cisco’s recommendation to run “power redundancy-mode redundant” you cannot sustain ANY other failure condition. I have seen TWICE now where a power supply will be running and providing power to the switch in a degraded mode and go unnoticed. As soon as an engineer goes to move a power cable or perform some other maintenance, they impact users and I have to explain. My recommendation is that you MONITOR your power situation and phone counts and run “power redundancy-mode combined” and ensure that even if you have to do maintenance on a degraded platform that your users don’t get impacted by you. I am sure you are important but if your users have to suffer everytime you have to actually work I guarantee folks are going to start questioning why you exist.

Conclusion

VOIP is a big project.

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