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Catalyst 2950 and Catalyst 2955 Switch Software Configuration Guide
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Chapter 10 Configuring Interface Characteristics
Understanding Interface Types
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.
For more information about trunk ports, see Chapter 16, “Configuring VLANs.”
Port-Based VLANs
A VLAN is a switched network that is logically segmented by function, team, or application, without
regard to the physical location of the users. For more information about VLANs, see Chapter 16,
“Configuring VLANs.” Packets received on a port are forwarded only to ports that belong to the same
VLAN as the rece iving port. Network devices in different VLANs cannot communicate with one another
without a Layer 3 device to route traffic between the VLANs.
VLAN partitions provide hard firewalls for traffic in the VLAN, and each VLAN has its own MAC
address table. A VLAN comes into existence when a local port is configured to be associated with the
VLAN, when the VLAN Trunking Protocol (VTP) learns of its existence from a neighbor on a trunk, or
when a user creates a VLAN.
To configure normal-range VLANs (VLAN IDs 1 to 1005), use the vlan vlan-id global configuration
command to enter config-vlan mode or the vlan database privileged EXEC command to enter VLAN
configuration mode. The VLAN configurations for VLAN IDs 1 to 1005 are saved in the VLAN
database. To configure extended-range VLANs (VLAN IDs 1006 to 4094) ) when the EI is installed, you
must use config-vlan mode with VTP mode set to transparent. Extended-range VLANs are not added to
the VLAN database. When VTP mode is transparent, the VTP and VLAN configuration is saved in the
switch running configuration, and you can save it in the switch startup configuration file by entering the
copy running-config startup-config privileged EXEC command.
Add ports to a VLAN by using the switchport interface configuration commands:
• Identify the interface.
• For a trunk port, set trunk characteristics, and if desired, define the VLANs to which it can belong.
• For an access port, set and define the VLAN to which it belongs.
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), the Port Aggregation Protocol (PAgP), and Link Aggregation Control Protocol (LACP) which
operate only on physical ports.