Cisco 500 series Administration Manual page 158

Port Management
Configuring Green Ethernet
Cisco 500 Series Stackable Managed Switch Administration Guide
• Interactions Between Features
• 802.3az EEE Configuration Workflow
802.3az EEE Overview
802.3az EEE is designed to save power when there is no traffic on the link. In
Green Ethernet, power is reduced when the port is down. With 802.3az EEE,
power is reduced when the port is up, but there is no traffic on it.
802.3az EEE is only supported on devices with GE ports.
When using 802.3az EEE, systems on both sides of the link can disable portions of
their functionality and save power during periods of no traffic.
802.3az EEE supports IEEE 802.3 MAC operation at 100 Mbps and 1000 Mbps:
LLDP is used to select the optimal set of parameters for both devices. If LLDP is
not supported by the link partner, or is disabled, 802.3az EEE still be operational,
but it might not be in the optimal operational mode.
The 802.3az EEE feature is implemented using a port mode called Low Power Idle
(LPI) mode. When there is no traffic and this feature is enabled on the port, the port
is placed in the LPI mode, which reduces power consumption dramatically.
Both sides of a connection (switch port and connecting device) must support
802.3az EEE for it to work. When traffic is absent, both sides send signals
indicating that power is about to be reduced. When signals from both sides are
received, the Keep Alive signal indicates that the ports are in LPI status (and not in
Down status), and power is reduced.
For ports to stay in LPI mode, the Keep Alive signal must be received continuously
from both sides.
Advertise Capabilities Negotiation
802.3az EEE support is advertised during the Auto-Negotiation stage. Auto-
Negotiation provides a linked device with the capability to detect the abilities
(modes of operation) supported by the device at the other end of the link,
determine common abilities, and configure itself for joint operation. Auto-
Negotiation is performed at the time of link-up, on command from management, or
upon detection of a link error. During the link establishment process, both link
partners to exchange their 802.3az EEE capabilities. Auto-Negotiation functions
automatically without user interaction when it is enabled on the device.
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