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9-2
Catalyst 2940 Switch Software Configuration Guide
78-15507-02
Chapter 9 Configuring the Switch Interfaces
Understanding Interface Types
Access Ports
An access port belongs to and carries the traffic of only one VLAN (unless it is configured as a voice
VLAN port). Traffic is received and sent in native formats with no VLAN tagging. Traffic arriving on
an access port is assumed to belong to the VLAN assigned to the port. If an access port receives
an 802.1P- or 802.1Q-tagged packet for the VLAN assigned to the port, the packet is forwarded. If the
port receives an 802.1P- or 802.1Q-tagged packet for another VLAN, the packet is dropped, the source
address is not learned, and the frame is counted in the No destination statistic.
The Catalyst 2940 switch does not support ISL-tagged packets. If the switch receives an ISL-tagged
packet, the packet is flooded in the native VLAN of the port on which it was received because the MAC
destination address in the ISL-tagged packet is a multicast address.
Two types of access ports are supported:
• Static access ports are manually assigned to a VLAN.
• VLAN membership of dynamic access ports is learned through incoming packets. By default, a
dynamic access port is a member of no VLAN, and forwarding to and from the port is enabled only
when the VLAN membership of the port is discovered. Dynamic access ports on the switch are
assigned to a VLAN by a VLAN Membership Policy Server (VMPS). The VMPS can be a
Catalyst 6000 series switch; the Catalyst 2940 switch does not support the function of a VMPS.
You can also configure an access port with an attached Cisco IP Phone to use one VLAN for voice traffic
and another VLAN for data traffic from a device attached to the phone. From more information about
voice VLAN ports, see Chapter 13, “Configuring VLANs.”
Trunk Ports
A trunk port carries the traffic of multiple VLANs and by default is a member of all VLANs in the VLAN
database. Only IEEE 802.1Q trunk ports are supported. An IEEE 802.1Q trunk port supports
simultaneous tagged and untagged traffic. An 802.1Q trunk port is assigned a default Port VLAN ID
(PVID), and all untagged traffic travels on the port default PVID. All untagged traffic and tagged traffic
with a NULL VLAN ID are assumed to belong to the port default PVID. A packet with a VLAN ID equal
to the outgoing port default PVID is sent untagged. All other traffic is sent with a VLAN tag.
Although by default, a trunk port is a member of every VLAN known to the VTP, you can limit VLAN
membership by configuring an allowed list of VLANs for each trunk port. The list of allowed VLANs
does not affect any other port but the associated trunk port. By default, all possible VLANs (VLAN ID 1
to 1005) are in the allowed list. A trunk port can only become a member of a VLAN if VTP knows of
the VLAN and the VLAN is in the enabled state. If VTP learns of a new, enabled VLAN and the VLAN
is in the allowed list for a trunk port, the trunk port automatically becomes a member of that VLAN and
traffic is forwarded to and from the trunk port for that VLAN. If VTP learns of a new, enabled VLAN
that is not in the allowed list for a trunk port, the port does not become a member of the VLAN, and no
traffic for the VLAN is forwarded to or from the port.
Note VLAN 1 cannot be excluded from the allowed list.
For more information about trunk ports, see Chapter 13, “Configuring VLANs.”