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Chapter 8 Configuring Interface Characteristics
Understanding Interface Types
Trunk Ports
A trunk port carries the traffic of multiple VLANs and by default is a member of all VLANs in the VLAN
database. Two types of trunk ports are supported:
•
In an ISL trunk port, all received packets are expected to be encapsulated with an ISL header, and
all transmitted packets are sent with an ISL header. Native (non-tagged) frames received from an
ISL trunk port are dropped.
•
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 9, “Creating and Maintaining VLANs.”
EtherChannel Port Groups
EtherChannel port groups provide the ability to 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 group 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. For Layer 3 interfaces, you manually create the logical interface by using the
interface port-channel global configuration command. For Layer 2 interfaces, the logical interface is
dynamically created. For both Layer 3 and 2 interfaces, you manually assign an interface to the
EtherChannel by using the channel-group interface configuration command. This command binds the
physical and logical ports together. For more information, see
Chapter 21, “Configuring EtherChannel.”
