Cisco 15327 - Manual (Page 384)

23-4
Ethernet Card Software Feature and Configuration Guide, R7.2
Chapter 23 E-Series and G-Series Ethernet Operation
Gigabit EtherChannel/IEEE 802.3ad Link Aggregation
When both autonegotiation and flow control are enabled, the G-Series card proposes symmetrical flow
control to the attached Ethernet device. Flow control may be used or not depending on the result of the
autonegotiation.
If autonegotiation is enabled but flow control is disabled, then the G-Series proposes no flow control
during the autonegotiation. This negotiation succeeds only if the attached device agrees to no flow
control.
If autonegotiation is disabled, then the attached device’s provisioning is ignored. The G-Series card’s
flow control is enabled or disabled based solely on the G-Series card’s provisioning.
This flow-control mechanism matches the sending and receiving device throughput to that of the
bandwidth of the STS/VC circuit. For example, a router might transmit to the Gigabit Ethernet port on
the G-Series card. This particular data rate might occasionally exceed 622 Mbps, but the SONET circuit
assigned to the G-Series port might be only STS-12c (622 Mbps). In this example, the ONS 15454 sends
out a pause frame and requests that the router delay its transmission for a certain period of time. With
flow control and a substantial per-port buffering capability, a private line service provisioned at less than
full line rate capacity (STS-24c) is efficient because frame loss can be controlled to a large extent. The
same concept applies to the ONS 15454 SDH or ONS 15327.
The G-Series cards have flow control threshold provisioning, which allows a user to select one of three
watermark (buffer size) settings: default, low latency, or custom. Default is the best setting for general
use and was the only setting available prior to Software R4.1. Low latency is good for sub-rate
applications, such as voice-over-IP (VoIP) over an STS-1. For attached devices with insufficient
buffering, best effort traffic, or long access line lengths, set the G-Series to a higher latency.
The custom setting allows you to specify the buffer size of Flow Ctrl Lo and Flow Ctrl Hi thresholds.
The range is 1 to 511 units, where 1 unit is equal to 192 bytes. Make sure that the value of Flow Ctrl Lo
is lesser than Flow Ctrl Hi with a difference of at least 160 units between the two values to ensure
packets are not dropped. The flow control high setting is the watermark for sending the Pause On frame
to the attached Ethernet device; this frame signals the device to temporarily stop transmitting. The flow
control low setting is the watermark for sending the Pause Off frame, which signals the device to resume
transmitting. With a G-Series card, you can only enable flow control on a port if autonegotiation is
enabled on the device attached to that port.
Note External Ethernet devices with autonegotiation configured to interoperate with G-Series cards running
releases prior to Software R4.0 do not need to change autonegotiation settings when interoperating with
G-Series cards running Software R4.0 and later.
Gigabit EtherChannel/IEEE 802.3ad Link Aggregation
The G-Series supports all forms of link aggregation technologies including GEC, which is a Cisco
proprietary standard, and the IEEE 802.3ad standard. The end-to-end link integrity feature of the
G-Series allows a circuit to emulate an Ethernet link. This allows all flavors of Layer 2 and Layer 3
rerouting to work correctly with the G-Series.
Figure 23-2 illustrates G-Series GEC support.