25-3
Ethernet Card Software Feature and Configuration Guide, R7.2
January 2009
Chapter 25 CE-1000-4 Ethernet Operation
Autonegotiation and Frame Buffering
Note Many Ethernet attributes are also available through the network element (NE) defaults feature. For more
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 re duce 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 pau se 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 e nable 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 prop oses 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 perio d of time. The sending
station waits the requested amount of time before sending m ore data. Figure 25-2 illustrates pause
frames being sent and received by CE-1000-4 cards and attached switches.
Figure 25-2 Flow Control
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 c ircuit
assigned to the CE-1000-4 port might be only STS-1 (51.84 Mbps). In thi s 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 con trol
pauses. The CE-1000-4 does not have a mode where it would sen d flow control pauses but not be able
to receive flow control pauses.
115779
STS-N
SONET
Pause Frames
Ethernet ONS Node ONS Node
Pause Frames
Ethernet