Cisco Systems STACKT150CM User Manual
White Paper
Cisco StackPower: Efficient Use of Power
Introduction
The Cisco StackPower™ technology is an innovative feature that aggregates all of the available power in a stack of
switches and manages it as one common power pool for the entire stack. It is one of the main features introduced
in the Cisco® Catalyst® 3750-X Series Switches (Figure 1).
Figure 1. Cisco StackPower
This white paper explains how Cisco StackPower manages power more efficiently, allowing customers to achieve
more granular control of their power use and to realize savings that reduce the total cost of ownership (TCO) of
their Cisco Catalyst switches.
This Whitepaper covers the following topics:
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Benefits of Cisco StackPower
●
Technology Overview
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Use Case for Cisco StackPower
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Intelligent Power Management
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Cisco StackPower Topology
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Modes of Operation
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Intelligent Load Shedding
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Best Practices
Benefits of Cisco StackPower
The benefits of the Cisco StackPower technology are immediately tangible, and so are the savings.
Consider a stack of switches with each switch requiring a slightly higher power budget for some extra Power over
Ethernet (PoE) devices randomly scattered in the stack. Purchasing extra second power supply for each switch
that needs some additional power would be inefficient and expensive. With the Cisco StackPower solution, a
common pool of power is made available and additional power can be automatically redirected to the appropriate
switch based on the available power budget in the common power pool.
© 2012 Cisco and/or its affiliates. All rights reserved. This document is Cisco Public Information. Page 1 of 17
Cisco StackPower technology immediately produces savings by reducing the number of power supplies required
per switch and the number of outlets required in the wiring closet. Additional savings accrue from minimizing
energy wastage due to inefficiency of power supply operation at lower loads and reduction in cooling requirements
in the wiring closet. Cisco StackPower also eliminates the need for external power shelves, thus freeing up
additional space and power outlets in the wiring closet.
Cisco StackPower also allows the deployment of larger power pools by using an eXpandable Power System (XPS
2200). This system allows for a star topology which shares power with up to 9 switches. See Cisco StackPower
Topology section.
Cisco StackPower technology provides the following additional benefits:
●
Abstracts the location of a power supply from its physical location in a stack of switches, allowing for better
utilization of available power capacity
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Maximizes the efficiency of power supplies - aggregated loads allow power supplies to operate at optimum
efficiency, considerably reducing power supply wastage over time
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Provides or complements the power required for PoE+ to any port in a stack
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Enables a scalable PoE+ infrastructure
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Offers a pay-as-you-grow architecture, similar to the Cisco StackWise® technology
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Offers improved reliability, availability, and efficiency via the eXpandable Power System (XPS) to up to nine
switches
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Enables a “zero-footprint” redundant power system (RPS)
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Offers greater redundancy, 1+N vs. 1:N redundancy with RPS
●
Allows off-lining of power supplies when extra capacity is available in the system
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Helps achieve a higher mean time between failures (MTBF) due to improved efficiency
●
Lowers TCO by reducing the number of power supplies needed, the number of devices in the rack, the
amount of heat in the wiring closet, and the number of AC outlets required in the wiring closet
Technology Overview
The key aspect to the new Cisco StackPower technology is the way power is supplied and distributed to a switch
in the stack. A switch requires power to be provided at different voltage levels, such as 5V DC and 48V DC, and a
traditional power supply provides those voltages. These requirements make the power supply more complex, and
this complexity affects efficiency.
Cisco StackPower technology implies a new approach to power supply design and power distribution in a switch,
but its effects are most significant in a stack of switches.
The Cisco Catalyst 3750-X and 3560-X Series Switches are designed for power supplies that provide a single
power voltage. This approach simplifies the power supply design and allows aggregation of power, from power
supplies in a single switch and across switches in a power stack. Cisco StackPower technology creates a pool of
power that shares a common load consisting of all the switches in the power stack. This capability to manage
power as share resource is unique to a stack of switches that can operate as a single unit. Therefore StackPower
technology is available only on the Catalyst 3750-X switches.
© 2012 Cisco and/or its affiliates. All rights reserved. This document is Cisco Public Information. Page 2 of 17
Note that all power available in the power stack is combined into one single large pool of power, and the stack
becomes a large single load to the power pool (Figure 2).
Figure 2. Cisco StackPower: One Power Pool, One Load
A surplus of power in a power stack enables features such as Zero-Footprint RPS and 1+N redundancy instead of
the classic 1:N redundancy with dedicated external RPS.
Redundancy with Cisco StackPower is better because the redundant power is already inline (1+N), as opposed to
being switched from one source to another, as in a classic RPS (1:N). The 1+N) redundancies are less susceptible
to problems because the power is already available inline.
StackPower operation
Switches deployed in a power stack discover each other and exchange messages to figure out how much power is
available in the stack (power budget), to set priorities (or use default priority values), and to start booting Cisco
IOS® Software on all switches, depending upon the power budget available in the stack.
The boot-up sequence of events is as follows:
1. Switches are interconnected in a ring topology and power is applied.
2. All switches power up their Stack Power infrastructure (50W each, 42W in initial IOS release 12.2(53)SE2).
3. All switches participate in the power stack, exchange discovery packets, information messages regarding
power resources and priorities.
4. The total budget in the power stack is discovered.
5. The power stack reserves 30W (42W in initial IOS release 12.2(53)SE2) in case a new member is
dynamically added to the stack. This amount is reserved once per stack, not per switch (unless there is only
one switch in the stack).
6. Budgets are distributed based on total power budget, power draw requirements, and switch priorities.
7. Switches that receive an allocation of power proceed to boot the Cisco IOS Software.
8. Switches that did not receive a power allocation will remain in the power stack without booting until more
power is added to the power budget.
© 2012 Cisco and/or its affiliates. All rights reserved. This document is Cisco Public Information. Page 3 of 17
Cisco Catalyst 3750-X and 3560-X Series Switches do not consume more power than the rest of the Catalyst 3750
family of switches, but the power budget required is higher. This higher budget allows each switch to budget
enough power for the switch itself, to power a high power network module (if present), and to power its
downstream neighbor’s StackPower logic (MCU), which is the minimum number of components in a switch to form
a power stack without booting Cisco IOS Software.
The following table shows the power budget requirement for each 3560-X and 3750-X model
Table 1. Power Budget requirements for Cisco 3560-X and 3750-X Catalyst switches
Product ID Power Budget Requirement
WS-C3560X-48 WS-C3560X-48P WS-C3750X-48 WS-C3750X-48P 223W
WS-C3560X-24 WS-C3560X-24P WS-C3750X-24 WS-C3750X-24P 190W
WS-C3750X-12S 192W
WS-C3750X-24S 230W
Note that with initial IOS release 12.2(53)SE2 all models had a power budget requirement of 206W.
For example, a standalone Cisco Catalyst 3750-X-48P switch (Figure 3) budgets 223W, this budget does not
necessarily reflect the actual power consumption (see the data sheet for details).
Figure 3. Cisco Catalyst 3750-X-48P
Adding a new switch to a power stack
Cisco StackPower technology adds resiliency to the stack by reserving an amount of power enough to bring up the
MCU of any Catalyst3750-X Series switch. This helps ensure that if and when a new switch without any power
supplies is added to the power stack and does not have any power, it can join the power stack using the power
that has been reserved for that purpose. This guarantees the inclusion of a new switch in the power stack; if the
power stack has enough power in the power pool, Cisco StackPower will allocate power to the new switch to boot
Cisco IOS Software.
Zero-Footprint RPS
The ability to provide redundancy without the need for an actual RPS is called Zero-Footprint RPS. The power
stack discovers the members of the stack, aggregates power from all available sources in the stack, and subtracts
from the power pool an amount of power equal to the largest power supply in the stack. It does not subtract the
power supply itself nor it turn off any power supply.
In the simplest example, a power stack is formed by using a special cable to connect switches to each other to
form a closed ring, similar to the topology for a Cisco StackWise stack (Figure 4). Up to four switches can be part
of a power stack in a ring topology, and up to nine switches can share power in a star topology by using an
eXpandable Power System (XPS) (Figure 5). Current flows through the cables that form the power stack and feed
switches in need of power or complements power requirements of other switches in the stack. It is a safe system
with plenty of circuit breakers spread around the printed circuit board to cut off current to different components in
the system or if needed, to the system itself.
© 2012 Cisco and/or its affiliates. All rights reserved. This document is Cisco Public Information. Page 4 of 17
Figure 4. Ring Topology
Figure 5. Star Topology, Using an XPS
The Cisco StackPower cables are thick but flexible, and they carry power as well as a data signal to provide a
communications channel among the switches in the power stack. Table 1 shows Cisco StackPower and XPS
Cables.
Table 2. Cisco StackPower and XPS Cables
Product ID Description
CAB-SPWR-30CM 30cm StackPower cable
CAB-SPWR-150CM 150cm StackPower cable
CAB-XPS-58CM 58cm Short XPS cable
CAB-XPS-150CM 150cm Short XPS cable
StackPower and XPS cables are keyed and have colored bands on the ends to help you understand what cable
plugs into what switch or XPS.
A StackPower cable has colors bands on the cable ends, green on one end and yellow on the other end. This
cable can be used between Catalyst 3750-X switches and an XPS. Note that the connector with the blue band can
only connect into a Catalyst 3750-X switch while the connector with the yellow band can connect to a Catalyst
3750-X or an XPS.
© 2012 Cisco and/or its affiliates. All rights reserved. This document is Cisco Public Information. Page 5 of 17
An XPS cable has a color band on the cable ends as well, red on one end and yellow on the other end. This cable
can only be used to connect Catalyst 3750-X or 3560-X switch to an XPS. Note that the connector with the red
band can only connect to an XPS while the connector with the green and blue band can connect to Catalyst 3750X or 3560-X switch.
The Cisco Catalyst 3750-X Series Switches come with four power supply options and any of them can be used on
any switch in a stack and in any combination. Table 2 lists these options.
Table 3. Power Supply Options for the Cisco Catalyst 3750-X Series Switches
Product ID Description
C3KX-PWR-350WAC 350W AC power supply
C3KX-PWR-715WAC 715W AC power supply
C3KX-PWR-1100WAC 1100W AC power supply
C3KX-PWR-440WDC 440W DC power supply
The following are some important concepts for Cisco StackPower technology:
●
The Cisco StackPower feature is responsible for the negotiation and distribution of power from a common
power pool among the switches participating in the power stack.
●
The intelligent load shed is a mechanism used by Cisco StackPower to decide what devices must power
down when the available power drops below the allocated power levels. A priority scheme is used to set
different levels which become useful when:
◦
Load shedding in case the power budget falls below the allocated power levels.
Initial allocation of power to boot up into Cisco IOS Software
◦
Priorities for powered devices and switches can be changed to values other than the pre-configured
default values.
●
Physical placement of power supplies in a switch is independent of the power required for that switch.
●
A switch may not be allowed to boot up with Cisco IOS Software if all available power in the common pool
has been already allocated. But this switch continues to participate in the power stack while waiting for
more power to become available to allow it to boot.
●
The power stack reports all of the Cisco StackPower information to the stack master; therefore, configuring
a power stack that spans multiple data stacks is not recommended (see the “Best Practices” section).
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