3Com 4200G 24-Port - Security Mode of Voice VLAN

1-6
Table 1-3 Matching relationship between port types and voice devices acquiring voice VLAN through
manual configuration
Voice VLAN assignment
mode
Port type Supported or not
Access Not supported
Trunk
Supported
Make sure the default VLAN of
the port exists and is not a
voice VLAN, and the access
port permits the traffic of the
default VLAN.
Automatic
Hybrid
Supported
Make sure the default VLAN of
the port exists and is not a
voice VLAN, and the default
VLAN is in the list of the tagged
VLANs whose traffic is
permitted by the access port.
Access Not supported
Trunk
Supported
Make sure the default VLAN of
the port exists and is not a
voice VLAN, and the access
port permits the traffic of the
default VLAN.
Manual
Hybrid
Supported
Make sure the default VLAN of
the port exists and is not a
voice VLAN, and the default
VLAN and the voice VLAN is in
the list of the tagged VLANs
whose traffic is permitted by the
access port.
Security Mode of Voice VLAN
The automatic mode and manual mode described earlier only apply to the process of assigning a port to
the voice VLAN. After a port is assigned to the voice VLAN, the switch receives and forwards all voice
VLAN-tagged traffic without matching the source MAC address of each received packet against its OUI
list. For a port in the manual mode with the default VLAN as the voice VLAN, any untagged packet can
be transmitted in the voice VLAN. This makes the voice VLAN vulnerable to flow attacks, because
malicious users can create a large amount of voice VLAN-tagged packets to consume the voice VLAN
bandwidth, affecting normal voice communication.
Switch 4200G series switches provide the security mode for voice VLAN to address this problem. When
the voice VLAN works in security mode, the switch checks the source MAC address of each packet to
enter the voice VLAN and drops the packets whose source MAC addresses do not match the OUI list.
However, checking packets occupies lots of system resources. Therefore, in a relatively safe network,
you can configure the voice VLAN to operate in normal mode.