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Catalyst 2950 Desktop Switch Software Configuration Guide
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Chapter 9 Configuri ng th e Switch Interfaces Understanding Interface Types

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 informa tion ab out VLAN s, see Chapter 14,
“Configuring V LANs.” Packets received on a port are forwarded only to ports that belong to the same
VLA N as the receiving por t. 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 confi g ure n or mal- rang e VLA Ns ( VLA N ID s 1 to 1 005) , use the vlan vlan-id global configurat ion
command to enter config-vlan mode or the vlan database privileged EXEC command to enter VLAN
configurati on mode . Th e VLAN c onfigurat ions for VLA N ID s 1 t o 100 5 ar e saved in the VLA N
database . To configure extended-range VL ANs (VLAN IDs 1006 to 4094) whe n 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 con figuration 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 por ts to a VLA N by us ing t he 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 a ccess p ort , se t a nd de fine t he V LAN to whic h it be long s.

EtherChannel Port Groups

EtherChann el port groups provid e the ability to treat multiple switch ports as one swit ch port. These port
groups act as a single logical port for high-bandwidth connections between switches or between switches
and servers. An EtherCh annel balances the traffic load across the links in the ch annel. If a link within
the Ether Channel fail s, traf f ic pre viously car ried ov er the f ailed link changes to the rema ining links. You
can group mu ltiple trun k ports into one logical trunk por t or group m ultiple a ccess ports into one logica l
access port. Most protoc ols opera te over either single port s or aggregated sw itch ports and do no t
recognize the physical ports within the port group. Exceptions are the DTP , the Cisco Discovery Protocol
(CDP), an d the Port Aggregat ion Pro toc ol ( PAgP), w hich ope rat e on ly on p hysi cal p ort s.
When you configure an EtherChannel, you create a port-channel logical interface and assign an interface
to the EtherCh annel. For Lay er 2 interfaces, the logical interface is dynamically created. You manually
assign an interface to the EtherChannel by using the channel-group inte rface co nfigurat ion c om ma nd .
This c omm and bi nds the p hysi cal a nd logi ca l po rt s tog eth er. For mo re inf orm a tio n, se e Cha pter 27,
“Configuring Et herCha nnels.”

Connecting Interfaces

Devices within a sin gl e V LAN ca n co mmunic ate d ire ctly through any sw itch . Po rts in d ifferent VLA Ns
cannot exchange data withou t going throu gh a rout ing device or rout ed inte rface.
W ith a s tandard Layer 2 sw itch, po rts 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.