Cisco Systems 4500 manual PoE Over-subscription

Chapter 1 Product Overview

System Architecture

PoE Over-subscription

With the advent of powered devices requiring as much as 15.4 W and the different combinations of power supplies and chassis port densities, it becomes quite possible to over-subscribe the PoE capacity of the power supplies. This temporary over-subscription typically occurs when a power supply configured in combined mode fails or when the user has not kept track of the powered devices and plugs in one too many. The best practice is to design a PoE system in which all devices receive the power needed at all times. When a power supply is over-subscribed—more power is being drawn from it than it can supply—the power supply shuts down. There are several ways to predictably manage a temporary PoE over-subscription:

1.Configure unused ports to never receive PoE. This prevents a user from inadvertently plugging a powered device into a port and causing problems for other powered devices.

2.Configure ports to be in static mode. This is for ports that have highest priority, such as phones for executives or wireless access points. If ports need to be disabled because of a power shortage, auto ports are disabled before static ports.

3.Configure the maximum wattage on ports to be less than the default, based on the maximum power consumption of the powered device. This disallows devices demanding unexpected amounts of power and also stretches the finite resources of the power supplies. For example, the default port wattage is 15.4 W. By configuring a maximum of 7 W, twice as many PoE powered devices can be supported with the same power supply.

PoE Line Cards

The Cisco Catalyst 4500 series offers line cards, power supplies, and accessories required to deploy and operate a standards-based PoE internetwork. PoE provides –48 V DC power over standard Category 5 unshielded twisted-pair (UTP) cable up to 100 meters when an IEEE 802.3af-compliant or Cisco pre-standard powered device is attached to the PoE line card port. Instead of requiring wall power, attached devices such as IP phones, wireless base stations, video cameras, and other IEEE-compliant appliances can use power provided by the PoE line cards. This capability gives network administrators centralized control over power and eliminates the need to install outlets in ceilings and other out-of-the-way places where a powered device may be installed.

	Catalyst 4500 Series Switches Installation Guide
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