Information Technology Reference
In-Depth Information
The previous output from Switch B shows the Duplex field as “a-half” and the Speed field as “a-10,”
which indicates that the port is operating at 10 Mb in half-duplex mode. In this example, however, the
link partner on this port (port 1/1 on Switch A) is configured for full-duplex mode and 10 Mb. Therefore,
it was not possible for port 1/1 on Switch B to have autonegotiated its current behavior. This proves that
the “a-” prefix indicates only a willingness to perform autonegotiation, not that autonegotiation actually
took place.
Step 7
The following message about a duplex mode mismatch is displayed on Switch A after the speed on port
1/1 was changed to 10 Mb. The mismatch was caused by Switch B's 1/1 port defaulting to half-duplex
mode because it sensed that its link partner was no longer performing autonegotiation.
%CDP-4-DUPLEXMISMATCH:Full/half duplex mismatch detected o1
It is important to note that this message is created by the Cisco Discovery Protocol (CDP), not the 802.3
autonegotiation protocol. CDP can report problems that it discovers, but it typically doesn't
automatically fix them.
A duplex mismatch may or may not result in an error message. Another indication of a duplex mismatch
is rapidly increasing FCS and alignment errors on the half-duplex side, and “runts” on the full-duplex
port (as seen in a
sh port {mod_num/port_num}
).
Step 8
In addition to the duplex mismatch error message, you may also see the following spanning-tree
messages when you change the speed on a link. A discussion of the Spanning-Tree Protocol is beyond
the scope of this document; see the section found later in this chapter “Troubleshooting Spanning-Tree
Protocol and Related Design Considerations,” for more information.
%PAGP-5-PORTFROMSTP:Port 1/1 left bridge port 1/1
%PAGP-5-PORTTOSTP:Port 1/1 joined bridge port 1/1
Step 9
To demonstrate what happens when the duplex mode has been configured, the mode on port 1/1 in Switch
A will be set to half-duplex mode using the
set port duplex 1/1 half
command.
Switch-A> (enable)
set port duplex 1/1 half
Port(s) 1/1 set to half-duplex.
Switch-A> (enable)
The
show port 1/1
command shows the change in the duplex mode on this port.
Switch-A> (enable)
sh port 1/1
Port Name Status Vlan Level Duplex Speed Type
----- ------------------ ---------- ---------- ------ ------ ----- ------------
1/1 connected 1 normal half 10 10/100BaseTX
At this point, ports 1/1 on both switches are operating at half-duplex mode. Port 1/1 on Switch B,
however, is still configured to autonegotiate, as shown in the following output of the
show port 1/1
command.
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
The next step shows how to configure the duplex mode on port 1/1 in Switch B to half-duplex mode.
This is in keeping with the recommended policy of always configuring both link partners in the same
way.
Step 10
To implement the policy of always configuring both link partners for the same behavior, this step now
sets the duplex mode to half-duplex and the speed to 10 on port 1/1 in Switch B.
