Cisco 15327 - Manual (Page 411)

24-3
Ethernet Card Software Feature and Configuration Guide, R7.2
Chapter 24 CE-100T-8 Ethernet Operation
Ethernet Link Integrity Support
To prevent over-subscription, buffer memory is available for each port. When the buffer memory on the
Ethernet port nears capacity, the CE-100T-8 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 Fast Ethernet
interfaces and the attached Ethernet devices. These frames do not continue through the POS ports.
The CE-100T-8 card has symmetric flow control and proposes symmetric flow control when
autonegotiating flow control with attached Ethernet devices. Symmetric flow control allows the
CE-100T-8 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.
Figure 24-2 illustrates pause
frames being sent and received by CE-100T-8 cards and attached switches.
Figure 24-2 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 Ethernet port on the CE-100T-8
card. This particular data rate might occasionally exceed 51.84 Mbps, but the SONET circuit assigned
to the CE-100T-8 port might be only STS-1 (51.84 Mbps). In this example, the CE-100T-8 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.
Ethernet Link Integrity Support
The CE-100T-8 supports end-to-end Ethernet link integrity ( Figure 24-3 ). This capability is integral to
providing an Ethernet private line service and correct operation of Layer 2 and Layer 3 protocols on the
attached Ethernet devices.
End-to-end Ethernet link integrity means that if any part of the end-to-end path fails, the entire path fails.
It disables the Ethernet port on the CE-100T-8 card if the remote Ethernet port is unable to transmit over
the SONET/SDH network or if the remote Ethernet port is disabled.
Failure of the entire path is ensured by turning off the transmit pair at each end of the path. The attached
Ethernet devices recognize the disabled transmit pair as a loss of carrier and consequently an inactive
link or link fail.
Figure 24-3 End-to-End Ethernet Link Integrity Support
115779
STS-N
SONET
Pause Frames
Ethernet
ONS Node ONS Node
Pause Frames
Ethernet
ONS Node
Ethernet port
115780 Tx
Rx
ONS Node
Ethernet port
Tx
Rx
STS-N
SONET