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Understanding How Power Priority Is Determined and Set for Switches Connected to the EX Series Redundant Power System

The Redundant Power System (RPS) is designed to provide backup power to switches that lack built-in redundant power supplies. The RPS provides backup power to switches that either supply power over Ethernet (PoE), which require more power, or switches that do not supply PoE, which require less power. A power supply can either power one PoE device or two non-PoE devices. That means if an RPS is fully loaded with three power supplies, supports PoE switches, and more than three PoE switches have a power failure, some switches will not be powered. You can, however, determine which switches will be powered when an RPS is oversubscribed. When too many connected switches fail, the switches are given power based on their priority. Priority is also reconfigured when any power change takes place. For example, if three switches are already being backed up and another switch has a power failure, the RPS detects this, reconfigures the current top priorities, and allots power accordingly.

Default RPS Priority

While six non-PoE switches can all simultaneously be backed up with three power supplies, only three PoE switches can be backed up (because PoE uses more power). This means that an RPS with four or more PoE switches connected will have to select three of them for backup. You can determine priority by the connector positions you use to connect the switches. By default, an RPS assigns priority to switches based on their switch connector port location, with the leftmost port having the lowest priority and the rightmost port having the highest priority. If the PoE switches shown in Figure 1 all fail, the manufacturing, support, and finance switches will be backed up because they are connected to the rightmost connectors.

Figure 1: Default PoE Switch Priority Is Determined by Connector Port LocationDefault PoE Switch Priority Is Determined by Connector Port Location

Changing the Priority of Switches on an EX Series RPS

There is a way to alter the priority of PoE switches on an RPS without disconnecting the cables. You can optionally reconfigure any of the attached switches from their CLIs to establish a switch’s RPS priority—this CLI configuration overcomes the priority determined by the switch connector port location. Priority ranges from zero (off) to 1 (lowest) through 6 (highest). By default, all switches are configured to 1, the lowest priority. Let’s say that the sales switch is reconfigured from the switch’s CLI for priority 5 (second highest).

Now in Figure 2, with the sales switch configured for RPS 5 from the CLI, the highest priority changes to sales (because 5 is higher than 1), then manufacturing, and then support.

Figure 2: Switch Priority After CLI ConfigurationSwitch Priority After CLI Configuration

When assigning power priority to switches by using the CLI on the switch, keep these points in mind:

  • By default, all switches are assigned priority 1 (lowest) and derive precedence from the location of their connector port on the RPS, with the rightmost port having highest priority.

  • Priority 0 assigned from a switch CLI means that the RPS does not provide any backup power to the switch. Essentially, this turns off RPS support.

  • Priority 6 assigned from a switch CLI is the highest priority and priority 1 is the lowest priority.

  • The CLI command that assigns priority to EX2200 switches is slightly different from the CLI command that assigns priority to EX3300 switches because EX3300 switches can be configured as a Virtual Chassis.

  • If two or more switches are assigned the same priority value from the switches’ CLIs , then the power priority for those switches is determined by the RPS switch connector port location, with the ports to the right receiving priority.

  • If a single power supply is installed, the RPS can provide backup power to one switch out of all the switches connected to the RPS. If you do not need any PoE power backup on any switch, you can increase the number of supported switches to two per power supply. Switches connected to an RPS must be either all PoE or all non-PoE.

  • The RPS discontinues supplying backup power to a lower-priority switch if it detects a backup power need for a higher-priority switch at the same time.