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Catalyst2950 and Catalyst2955 Switch Software Configuration Guide
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Chapter11 Configuring Interface Characteristics
Understanding Interface Types
These sections describes these types of interfaces:
•Access Ports, page 11-2
•Trunk Ports, page 11-2
•Port-Based VLANs, page 11-3
•EtherChannel Port Groups, page 11-3
•Connecting Interfaces, page 11-4
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 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 dr opped , t he sour ce
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 i ncomi ng pac kets. By de fau lt, a
dynamic access port is a member of no VLAN, and forwarding to and from the p or t is en ab led o nl y
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. Fr om m ore i n forma tio n abo ut
voice VLAN ports, see Chapter19, “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 I EEE 80 2.1Q tru nk por t sup port s
simultaneous tagged and untagged traffic. An 802.1Q trunk port is assig ned a d efault Por t VLA N ID
(PVID), and all untagged traffic travels on the port def a ult PVI D. All untagged traffic and tagged traffic
with a NULL VLAN ID are assumed to belong to the port default PVID. A pa cket with a VLAN ID equal
to the outgoing port default PVID is sent untagged. All ot her t raffic is sen t wi th a VL AN 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 (VL AN ID1
to 1005 when the standard software image [SI] is installed or VLAN ID 1 to 4094 when the enhanced
software image [EI] is installed) are in the allowed list. A trunk port can only become a membe r 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