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Catalyst 4500 Series Switches Installation Guide
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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.
