Cisco Catalyst 4500e Series Installation Manual page 49

Chapter 1 Product Overview
new outlets and costly electrical circuits. PoE also enables businesses to isolate critical devices on a
single power system so that the entire system can be supported by uninterruptable power supply (UPS)
backup.
All new Cisco Catalyst PoE switching modules can support 15.4 W of power per port simultaneously.
Not only do the modules support the IEEE standard, including the optional power classifications, but the
Cisco pre-standard power implementation is also supported to help ensure backward compatibility with
existing Cisco powered devices. The modules are compatible with any chassis and supervisor engine.
Most importantly, the Catalyst 4500 E-series switch has the power supplies and accessories to support
15.4 W per port on every port simultaneously in any fully loaded chassis. (This requires an external power
shelf or a 4200 W dual-input power supply.)
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.
2.
3.
PoE Switching Modules
The Cisco Catalyst 4500 E-series offers switching modules, 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 switching module 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 switching modules. 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.
Although references to "PoE," "inline-power," and "voice" power supplies and switching modules are
synonymous, there are only two versions: Cisco prestandard and IEEE 802.3af compliant. Every Cisco
Catalyst 4500 E-series chassis and PoE power supply supports the IEEE 802.3af standard and the Cisco
prestandard power implementation ensuring backward compatibility with existing Cisco powered
devices. All IEEE 802.3af-compliant switching modules can distinguish an IEEE or Cisco prestandard
powered device from an unpowered network interface card (NIC), ensuring that power is applied only
when an appropriate device is connected.
OL-13972-01
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.
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.
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.
Catalyst 4500 E-Series Switches Installation Guide
System Architecture
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