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Catalyst2950 Desktop Switch Software Configuration Guide
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Chapter9 Configuring Interface Cha rac teristics 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. 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 default PVID. All untagged traffic and tagg ed traf f ic
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 o ther tr affic is sen t w ith a V LA N t ag.
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 4094 when the enhanced software image is installed or VLAN ID 1 to 1005 when the standard
software image is installed) 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 ne w , 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 Chapter13, “Configuring 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 be tween 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 lin k changes to the re maining 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-channe l logical interface and assign an interface
to the EtherChannel. For Layer 2 interfaces, the logical interface is dynamically created. You manually
assign an interface to the EtherChannel by using the channel-group interface configuration command.
This command binds the physical and logical ports togeth er. For more inf orm atio n, se e Chapt er 26,
“Configuring EtherChannels.”
Connecting Interfaces
Devices within a single VLAN can communicate directly through any switch. Ports in d ifferent VLANs
cannot exchange data without going through a routing device or routed interface.
With a standard Layer 2 switch, ports in dif fer ent VLANs ha ve to ex change inf ormation thr ough a ro uter .
In the configuration shown in Figure 9-1, when Host A in VLAN 20 sends data to Host B in VLAN 30,
it must go from Host A to the switch, to the router, back to the switch, and then to Host B.