Etherchannel Port Groups; Connecting Interfaces - Cisco Catalyst 2950 Software Configuration Manual

Chapter 9 Configuring Interface Characteristics
(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 when the enhanced software image is installed and 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 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.
VLAN 1 cannot be excluded from the allowed list.
Note
For more information about trunk ports, see

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. In Layer 2 interfaces, the logical interface is dynamically created. For Layer 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

Connecting Interfaces

Devices within a single VLAN can communicate directly through any switch. Ports in different VLANs
cannot exchange data without going through a routing device or interface.
With a standard Layer 2 switch, ports in different VLANs have to exchange information through a router.
In the configuration shown in
it must go from Host A to the switch, to the router, back to the switch, and then to Host B.
78-11380-04
Chapter 25, "Configuring EtherChannels."
Figure 9-1, when Host A in VLAN 20 sends data to Host B in VLAN 30,
Chapter 13, "Configuring VLANs."
Catalyst 2950 Desktop Switch Software Configuration Guide
Understanding Interface Types
9-3