Understanding Dynamic Vlan Membership - Cisco Catalyst 4500 Series Software Configuration Manual

Chapter 17 Configuring VLANs, VTP, and VMPS
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Understanding Dynamic VLAN Membership

When a port is configured as "dynamic," it receives VLAN information based on the MAC-address that
is on the port. The VLAN is not statically assigned to the port; it is dynamically acquired from the VMPS
based on the MAC-address on the port.
A dynamic port can belong to one VLAN only. When the link becomes active, the switch does not
forward traffic to or from this port until the port is assigned to a VLAN. The source MAC address from
the first packet of a new host on the dynamic port is sent to the VMPS as part of the VQP request, which
attempts to match the MAC address to a VLAN in the VMPS database. If there is a match, the VMPS
sends the VLAN number for that port. If there is no match, the VMPS either denies the request or shuts
down the port (depending on the VMPS security mode setting). See the
page 17-20
Multiple hosts (MAC addresses) can be active on a dynamic port if all are in the same VLAN. If the link
goes down on a dynamic port, the port returns to the unassigned state and does not belong to a VLAN.
Any hosts that come online through the port are checked again with the VMPS before the port is assigned
to a VLAN.
For this operation to work, the client device must be able to reach the VMPS. A VMPS client sends VQP
requests as UDP packets, trying a certain number of times before giving up. For details on how to set the
retry interval, refer to section "Configuring the Retry Interval" on page 26.
The VMPS client also periodically reconfirms the VLAN membership. For details on how to set the
reconfirm frequency, refer to section "Administering and Monitoring the VMPS" on page 27.
A maximum of 50 hosts are supported on a given port at any given time. Once this maximum is exceeded,
the port is shut down, irrespective of the operating mode of the VMPS server.
The VMPS shuts down a dynamic port if more than 50 hosts are active on that port.
Note
Default VMPS Client Configuration
Table 17-4
Table 17-4
Feature
VMPS domain server
VMPS reconfirm interval
VMPS server retry count
Dynamic ports
Default VMPS Client Configuration, page 17-23
Configuring a Switch as a VMPS Client, page 17-24
Administering and Monitoring the VMPS, page 17-27
Troubleshooting Dynamic Port VLAN Membership, page 17-28
for a complete description of possible VMPS responses.
shows the default VMPS and dynamic port configuration on client switches.
Default VMPS Client and Dynamic Port Configuration
Catalyst 4500 Series Switch, Cisco IOS Software Configuration Guide - Cisco IOS XE 3.9.xE and IOS 15.2(5)Ex
Default Configuration
None
60 minutes
3
None configured
VLAN Membership Policy Server
"About VMPS" section on
17-23