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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 ach
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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
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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 o
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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 followin
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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
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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 3750- X 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
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Use Case for the Cisco StackPower Feature Figure 6. Power Stack of Four Switches The following use cases will help you understand the Cisco StackPower technology and its main features. 1 Switches A, B, C, and D have the following configuration and power draw requirements : ● Switch A requires 946W to provide full PoE on all of its 48 ports. It has only one 1100W power supply; extra power capacity of 154W is available. ● Switch B requires 206W, since it is a non-PoE switch. It has o
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Although the power stack in the example has extra capacity that gets in the power pool, it is not sufficient to power the switch D and PoE devices attached to it. To get the system in balance and to have a positive power budget, we must add more power to the stack; a 350W power supply will solve the problem but it must be installed in slot A of switch D (see the “Best Practices” section). With the addition of a 350W power supply we can make the power budget positive and have additional 241
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“show env power all” SW PID Serial# Status Sys Pwr PoE Pwr Watts 1A NG3K-PWR-1100WAC LIT1337057T OK Good Good 1100/0 1B No Input Power Bad N/A 235/0 2A NG3K-PWR-1100WAC LIT13370597 OK Good Good 1100/0 2B NG3K-PWR-715WAC LIT133705CC OK Good Good 715/0 Intelligent Power Management Placing all of the available power in a single pool has advantages: a load (the switches in the power stack and their respective power-able devices) is applied to the pooled power, making an efficient use of
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A maximum of 8400W is possible in a pool of power in a stack of four switches loaded with two 1100W power supplies each. Obviously, it is excessive; even if the stack consists of four Catalysts 3750-X-48P switches requiring full PoE+. Let’s do the math: 4 switches x 48 ports x 30W = 5760W: a lot less than the available 8400W If only one power supply is installed in each switch, we get 4200W in the power pool. Now we fall short, but the reality is that not everyone needs full PoE+ on ever
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Figure 8. Power Share Mode In the following scenario, the Cisco StackPower technology aggregates three power supplies on four switches (1100W + 1100W + 1100W + 0W = 3300W). The load requirement is 500W per switch (500W + 500W + 500W + 500W = 2000). Figure 9. Power sharing among 4 Switches Power budget is 3300W Allocated power is 2000W Available unallocated power is 1300W Should power requirements increase in any of the switches of the power stack, the Cisco StackPower feature
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Power Share “Strict” Mode The power sharing mode has an inherent capability to oversubscribe itself in case of power supply failures. That is, when an imbalance occurs between available power budgets and allocated power, more power is allocated than is available. By definition, the system should never have a negative power budget, but it is possible that a power supply failure brings the power budget below the allocated power. Such a condition will trigger a series of alarming messages to
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configured, but it would not offline any power supply from the system. This action needs to be performed by an administrator of the power stack. The following scenario is similar to the power-sharing mode in figure 9. The Cisco StackPower technology aggregates three power supplies (1100W + 1100W + 1100W + 0W = 3300W). The load requirement is 500W per switch (500W + 500W + 500W + 500W = 2000). Figure 11. Redundant power Power budget is 3300W Reserved power 1100W (equal to the size of
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Immediate load shed occurs when a failure could cause the power stack to fail very quickly, for example a massive power failure where multiple power supplies go offline for any reason and the available power cannot sustain the system. Graceful load-shedding can occur when a power supply fails and forces the power stack to re-evaluate its power allocations. Switches and powered devices may shut down in order of their default or configured priority, starting with devices with priority 27, u
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The table shows the default priorities used by StackPower that can be overwritten by an administrator. As an example, a power stack can assign priority 2 to a switch-A, priority 10 to high priority ports, and priority 20 to low priority ports. Each switch in the stack will have a different priority set for the switch, high priority ports, and low priority ports respectively. The priority levels are not repeated on any switch or port. That is switch-A with has priority level 10 for its high
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Power supply slots are named left to right, that is, power supply slot A in on the left and power supply slot B is on the right (closest to the edge of the switch). This practice allows us to distribute power sources across all switches in the stack. Otherwise, a single failure in the power stack would cause larger power demands than what StackPower is designed to handle. Any switch can only share up to 2000W of power to a neighbor, whether the power is to be consumed by the neighbor or to
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The recommendation is to perform the calculation of required power in the power stack and add the amount of reserved power (size of the largest power supply in the power stack) and compare the requirement with the available power in the power stack. 6. Off-lining power supplies are supported but it is not automatic. Cisco IOS has the necessary commands to manage the power supplies and administrator intervention is required due to the multiple use cases that can be derived. Cisco IOS has m
