Allied Telesis AT-8550 - Manual (Page 250)

Using the Command-Line Interface
B-6
VLANs You cannot add a port to a VLAN:
If you attempt to add a port to a VLAN and get an error message
similar to
localhost:7 # config vlan marketing add port
1:1,1:2
ERROR: Protocol conflict on port 1:5
you already have a VLAN using untagged traffic on a port. Only one
VLAN using untagged traffic can be configured on a single physical
port.
VLAN configuration can be verified by using the following command:
show vlan
The solution for this error is to remove ports 1 and 2 from the VLAN
currently using untagged traffic on those ports. If this were the
“default” VLAN, the command would be
localhost:23 # config vlan default del port
1:1,1:2
which should now allow you to re-enter the previous command
without error as follows:
localhost:26 # config vlan red add port
1:1,1:2
VLAN names:
There are restrictions on VLAN names. They cannot contain
whitespaces and cannot start with a numeric value unless you use
quotation marks around the name. If a name contains whitespaces,
starts with a numeric, or contains non-alphabetical characters, you
must use quotation marks whenever referring to the VLAN name.