12-3
Catalyst 2928 Switch Software Configuration Guide
OL-23389-01
Chapter 12 Configuring Interface Characteristics
Understanding Interface Types
Two types of access ports are supported:
•
Static access ports are manually assigned to a VLAN (or through a RADIUS server for use with
IEEE 802.1x. For more information, see the
“Using IEEE 802.1x Authentication with VLAN
Assignment” section on page 9-9
.)
•
VLAN membership of dynamic access ports is learned through incoming packets. By default, a
dynamic access port is not a member of any 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 6500 series switch; the Catalyst 2928 switch cannot be a VMPS server.
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. For 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. By default, all possible VLANs (VLAN ID 1
to 4094) are in the allowed list. A trunk port can become a member of a VLAN only if VTP knows of
the VLAN and if 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.
For more information about trunk ports, see
Chapter 13, “Configuring VLANs.”
EtherChannel Port Groups
EtherChannel port groups treat multiple switch ports as one switch port. These port groups act as a single
logical port for high-bandwidth connections between switches or between switches and servers. An
EtherChannel balances the traffic load across the links in the channel. If a link within the EtherChannel
fails, traffic previously carried over the failed link changes to the remaining links. You can group
multiple trunk ports into one logical trunk port or multiple access ports into one logical access port. Most
protocols operate over either single ports or aggregated switch ports and do not recognize the physical
ports within the port group. Exceptions are the DTP, the Cisco Discovery Protocol (CDP), and the Port
Aggregation Protocol (PAgP), which operate only on physical ports.
When you configure an EtherChannel, you create a port-channel logical interface and assign an interface
to the EtherChannel. Use the
channel-group
interface configuration command to dynamically create the
port-channel logical interface. This command binds the physical and logical ports together. For more
information, see
