Autonegotiation And Frame Buffering; Flow Control - Cisco ONS 15454 SDH Configuration Manual

Chapter 25 CE-1000-4 Ethernet Operation
Many Ethernet attributes are also available through the network element (NE) defaults feature. For more
Note
information on NE defaults, refer to the "Network Element Defaults" appendix in the Cisco ONS 15454
Reference Manual or the Cisco ONS 15454 SDH Reference Manual.

Autonegotiation and Frame Buffering

On the CE-1000-4 card, Ethernet link autonegotiation is on by default. You can also enable and disable
autonegotiation under the card-level Provisioning tab of CTC.
The CE-1000-4 supports field-programmable gate array (FPGA) buffering to reduce data traffic
congestion. FPGA buffering supports SONET/SDH oversubscription. When the buffer nears capacity,
the CE-1000-4 card uses IEEE 802.3x flow control to transmit a pause frame to the attached Ethernet
device. Flow control and autonegotiation frames are local to the Gigabit Ethernet interfaces and the
attached Ethernet devices. These frames do not continue through the POS ports.

Flow Control

The CE-1000-4 supports IEEE 802.3x flow control and allows you to enable symmetric flow control,
enable asymmetric flow control, or to disable flow control. The configuration is done in CTC at the port
level.
By default the CE-1000-4 card uses symmetric flow control and only proposes symmetric flow control
when autonegotiating flow control with attached Ethernet devices. Symmetric flow control allows the
CE-1000-4 cards to respond to pause frames sent from external devices and to send pause frames to
external devices.
The pause frame instructs the source to stop sending packets for a specific period of time. The sending
station waits the requested amount of time before sending more data.
frames being sent and received by CE-1000-4 cards and attached switches.
Figure 25-2
This flow-control mechanism matches the sending and receiving device throughput to that of the
bandwidth of the STS circuit. For example, a router might transmit to the Gigabit Ethernet port on the
CE-1000-4 card. This particular data rate might occasionally exceed 51.84 Mbps, but the SONET circuit
assigned to the CE-1000-4 port might be only STS-1 (51.84 Mbps). In this example, the CE-1000-4 card
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-1) is efficient because frame loss can be controlled to a large extent.
Asymmetric enables the CE-1000-4 to receive flow control pauses, but not generate flow control pauses.
This mode supports a link partner that cannot receive flow control pauses but can send flow control
pauses. The CE-1000-4 does not have a mode where it would send flow control pauses but not be able
to receive flow control pauses.
Flow Control
ONS Node
Ethernet
Pause Frames
Cisco ONS 15454 and Cisco ONS 15454 SDH Ethernet Card Software Feature and Configuration Guide, R8.0
ONS Node
STS-N
SONET
Pause Frames
Autonegotiation and Frame Buffering
Figure 25-2 illustrates pause
Ethernet
25-3