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Catalyst 2950 and Catalyst 2955 Switch Software Configuration Guide
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Chapter 10 Configuring Interface Characteristics
Understanding Interface Types
These sections describes these types of interfaces:
• Access Ports, page 10-2
• Trunk Ports, page 10-2
• Port-Based VLANs, page 10-3
• EtherChannel Port Groups, page 10-3
• Connecting Interfaces, page 10-4
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
IEEE 802.1p- or 802.1Q-tagged packet for the VLAN assigned to the port, the packet is forwarded. If
the port receives an IEEE 802.1p- or IEEE 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 2950 or Catalyst 2955 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 2950 or Catalyst 2955 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 15, “Configuring Voice VLAN.”
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 IEEE 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.
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