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10-7
Cisco ONS 15600 Reference Manual, R7.2
Chapter 10 Ethernet Operation
10.6 Autonegotiation
flow control, it generates and obeys pause frames. When it uses asymmetric flow control, it only
generates pause frames. Through management control, the Ethernet interface allows an operator to turn
flow control ON or OFF (the default is ON).
When an Ethernet interface is set to be capable of generating PAUSE frames, it starts generating them
at a rate of X PAUSE frames per second when the ingress buffer passes above a certain level called the
High Water Mark. A PAUSE_TIME is specified in each PAUSE frame. The values of the PAUSE_TIME
parameter and the rate X parameter are determined by the system and are not user settable.
It is expected that the peer Ethernet interface stops sending new frames after receiving a PAUSE frame
for the amount of time as specified in the PAUSE_TIME. When an Ethernet interface is actively
generating PAUSE frames, it stops generating them when the ingress buffer passes below a certain level,
called the Low Water Mark. It is expected that the peer Ethernet interface starts sending new frames after
the PAUSE frames stop.
When an Ethernet interface is set to be capable of receiving PAUSE frames, it temporarily suspends
transmission of any new Ethernet frames upon reception of a PAUSE frame from its peer. Note that the
transmission of any Ethernet frames already begun are completed.
When an Ethernet interface has temporarily PAUSED transmission of Ethernet frames, it resumes
transmission of the frames when no new PAUSE frame is received within the PAUSE_TIME specified
in the last received PAUSE frame.
The High Water Mark and Low Water Mark values are both operator provisionable. Legal values are as
follows:
•
Low Water Mark: from 10 kB to 74 kB
•
High Water Mark: from 11 kB to 75 kB
The Low Water Mark is always lower than the High Water Mark.
The ASAP line card utilizes tail drop when frames need to be dropped. This means that the last frames
received are the first to be dropped. This is implemented with a Last In First Out (LIFO) buffer.
10.6 Autonegotiation
The Ethernet interfaces on the ASAP card are capable of autonegotiating for full duplex (only) operation
as per IEEE 802.3z. The default provisioning is to autonegotiate the duplex mode and persists in
memory. Autonegotiation can be provisioned to be ON or OFF.
When flow control is turned on (and so is autonegotiation), the Ethernet interface autonegotiates and
advertises all modes that it supports. The mode used by the Ethernet interface is the one agreed to by the
Ethernet peers. If an agreement on the supported flow control mode cannot be reached, the transmitter
is turned off. Autonegotiation is depicted in Figure 10-4.