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Catalyst 2950 Desktop Switch Software Configuration Guide
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Chapter 14 Configuring VLA Ns Configurin g VLAN Trunks
•Disabling span ning tree on the na ti v e VLAN of an 802.1 Q trunk without d isabling spann ing tree on
every VLAN in the network can po tentially cause spann ing-tr ee loops. We recomm end that you
leave spanning tre e e nab led on t he na tive VLAN of an 802. 1Q tr unk or d isabl e sp an ning tre e on
every VLAN i n the ne twork. Ma ke sure y our networ k is l oop-fr ee bef or e di sablin g spanni ng t ree .

Default Layer 2 Ethernet Interface VLAN Configuration

Table 14-5 sh ows the de fault L ayer 2 E ther net int erfac e V LAN co nfigurati on.

Configuring an Ethernet Interface as a Trunk Port

Because trunk ports send and receive VTP advertisements, to use VTP you must ensure that at least one
trunk port is configure d on the switch and that this trunk port is connected to the trunk port of a sec ond
switch. Otherwise, the switch cannot receive any VTP advertisements.
This section includes these procedures for conf iguring an Ethernet interface as a trunk port on the switch:
•Interac tio n wi th Oth er Fe atu re s, page 14 -17
•Defining the Al lowed VLAN s on a Trunk, page 14 -19
•Changing the Pruning-Eligible List, page 14-20
•Config uring the Native VLAN for Untagged Traffic, page 14-20
Note The default mode for Layer 2 interfaces is switchport mode dynamic desirable. If the neig hbo ring
interface supp orts trun king and i s configured to allow trunking, the lin k is a Layer 2 trunk.

Interaction with Other Features

Tr unking i nteract s with other fe atures in these way s:
•A trunk por t c an not be a sec ure port .
•T runk ports can be grouped into EtherChannel port groups, but all trunks in the group must ha ve the
same configuration. When a group is first created, all ports follow the parameters set for the first
port to be added to the group. If you change the configuration of one of these parameters, the switch
propagate s the setti ng you e nte red to all po rts in the g rou p:
–
allowed-VLAN list
–
STP port priority for each VLAN
Table 14-5 Default Layer 2 Ethernet Interface VLAN Configuration
Feature Default Setting
Inte rface m ode switchport mode dynamic desirable
Allowed VLAN range VLANs 1 to 4094 when the EI is installed and 1 to
1005 whe n the SI i s ins tal led
VLA N ra ng e el igi ble f or pr uning VLANs 2 t o 1 001
Default VLAN (for access ports) VLAN 1
Native VLAN ( for 80 2.1Q tru nks) VLAN 1