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Catalyst2950 and Catalyst2955 Switch Software Configuration Guide
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Chapter17 Configuring VLANs
Configuring VMPS
•If the VLAN in the database does not match the current VLAN on the po rt and active hosts exist on
the port, the VMPS sends an access-denied or a port-shutdown response, depending on the secure
mode of the VMPS.
If the switch receives an access-denied response from the VMPS, it continues to block traffic from the
MAC address to or from the port. The switch continues to monitor the packets directed to the port and
sends a query to the VMPS when it identifies a new address. If the switch receives a port-shutdown
response from the VMPS, it disables the port. The port must be ma nuall y r e-en abl ed by using the CLI,
CMS, or SNMP.
You can also use an explicit entry in the configuration table to deny access to specific MAC addresses
for security reasons. If you enter the none keyword for the VLAN name, the VMPS sends an
access-denied or port-shutdown response, depending on the VMPS secure mode setting.
Dynamic Port VLAN Membership
A dynamic (nontrunking) port on the switch can belong to only one VLAN, with a VLAN ID from 1 to
1005. When the link comes up, the switch does not forward traffic to or from this port until the VMPS
provides the VLAN assignment. The VMPS receives the source MAC address from the first packet of a
new host connected to the dynamic port and attempts to match the MAC addre ss to a VLAN in the VMPS
database.
If there is a match, the VMPS sends the VLAN number for that port. If the client switch was not
previously configured, it uses the domain name from the first VTP packet it receives on its trunk port
from the VMPS. If the client switch was previously configured, it includes its d omain name i n the query
packet to the VMPS to obtain its VLAN number. The VMPS v eri f ies t ha t the do main n ame i n the pa cke t
matches its own domain name before accepting the request and responds to the client with the as signed
VLAN number for the client. If there is no match, the VMPS either denies the request or shuts down the
port (depending on the VMPS secure mode setting).
Multiple hosts (MAC addresses) can be active on a dynamic port if they are all in the same VLAN;
however, the VMPS shuts down a dynamic port if more than 20 hosts are active on the port.
If the link goes down on a dynamic port, the port returns to an isolated state and does not belong to a
VLAN. Any hosts that come online through the port are checked again thro ugh the VQP with the VM PS
before the port is assigned to a VLAN.
VMPS Database Configuration File
The VMPS contains a database configuration file that you create. This ASCII text file is stored on a
switch-accessible TFTP server that functions as a server for VMPS. The file contains VMPS
information, such as the domain name, the fallback VLA N na me , a nd the MAC-address- to-V LAN
mapping. The switch cannot act as the VMPS, but you can use a C ataly st 5 000 or Ca ta lyst 60 00 ser ies
switch as the VMPS.
You can configure a fallback VLAN name. If you connect a device with a MAC address that is not in the
database, the VMPS sends the fallback VLAN name to the client. If you do not configure a fallback
VLAN and the MAC address does not exist in the database, the VMPS send s an access-denied response.
If the VMPS is in secure mode, it sends a port-shutdown response.
Whenever port names are used in the VMPS database configuration file, the server must use the switch
convention for naming ports. For example, Fa0/4 is fixed Fast Ethernet port number 4. If the switch is a
cluster member, the command switch adds the name of the swi tc h bef or e the type . For exam ple ,
es3%Fa0/4 refers to fixed Fast Ethernet port 4 on member switch 3. When port names are required, these
naming conventions must be followed in the VMPS database configuration file when it is configured to
support a cluster.