Information Technology Reference
In-Depth Information
The “a-” prefixes on the “full” and “100” indicate that this port has not been hard-coded (configured) for
a specific duplex mode or speed. Therefore, it is willing to autonegotiate its duplex mode and speed if
the device it is connected to (its link partner) is also willing to autonegotiate its duplex mode and speed.
Also note that the status shows “connected” on both ports, which means that a link pulse has been
detected from the other port. The status can show “connected” even if duplex has been incorrectly
negotiated or misconfigured.
Step 4
To demonstrate what happens when one link partner is autonegotiating and the other link partner is not,
the speed on port 1/1 in Switch A will be set to 10 Mb by using the
set port speed 1/1 10
command.
Switch-A> (enable)
set port speed 1/1 10
Port(s) 1/1 speed set to 10Mbps.
Switch-A> (enable)
Hard-coding the speed on a port disables all autonegotiation functionality on the
port for speed and duplex.
Note
When a port has been configured for a speed, its duplex mode will automatically be configured for the
mode that it had previously negotiated—in this case, full duplex. Therefore, entering the
set port speed
1/1 10
command caused the duplex mode on port 1/1 to be configured as if the command
set port duplex
1/1 full
had also been entered. This is explained in the next step.
Now you must understand the meaning of the “a-” prefix in the Duplex and Speed status fields.
The absence of the “a-” prefix in the status fields of the output from the
show port 1/1
command on
Switch A shows that the duplex mode is now configured for full-duplex operation, and the speed is now
configured for 10 Mb.
Step 5
Switch-A> (enable)
show port 1/1
Port Name Status Vlan Level Duplex Speed Type
----- ------------------ ---------- ---------- ------ ------ ----- ------------
1/1 connected 1 normal full 10 10/100BaseTX
The
show port 1/1
command on Switch B indicates that the port is now operating at half-duplex and 10
Mb.
Step 6
Switch-B> (enable)
show port 1/1
Port Name Status Vlan Level Duplex Speed Type
----- ------------------ ---------- ---------- ------ ------ ----- ------------
1/1 connected 1 normal a-half a-10 10/100BaseTX
This step shows that it is possible for a link partner to detect the speed at which the other link partner is
operating, even though the other link partner is not configured for autonegotiation. Sensing the type of
electrical signal that is arriving to see if it is 10 Mb or 100 Mb does this. This is how Switch B determined
that port 1/1 should be operating at 10 Mb.
It is not possible to detect the correct duplex mode in the same way that the correct speed can be detected.
In this case, where Switch B's 1/1 port is configured for autonegotiation and Switch A's is not, Switch
B's 1/1 port was forced to select the default duplex mode. On Catalyst Ethernet ports, the default mode
is autonegotiate and, if autonegotiation fails, then is half-duplex.
This example also shows that a link can be successfully connected when there is a mismatch in the
duplex modes. Port 1/1 on Switch A is configured for full-duplex operation, while port 1/1 on Switch B
has defaulted to half-duplex operation. To avoid this, always configure both link partners.
The “a-” prefix on the Duplex and Speed status fields does not always mean that the current behavior
was negotiated. Sometimes it means only that the port has not been configured for a speed or duplex
mode.
