8-20
Catalyst 2950 Desktop Switch Software Configuration Guide
78-11380-03
Chapter8 Configuring VLANs
How VLAN Trunks Work
How VLAN Trunks Work
A trunk is a point-to-point link that transmits and receives traffic between switches or between switches
and routers. Trunks carry the traffic of multiple VLANs and can extend VLANs across an entire network.
Figure 8-3 shows a network of switches that are connected by 802.1Q trunks.
Figure 8-3 Catalyst 2950, 2900 XL, and 3500 XL Switches in a 802.1Q Trunking Environment
Note DTP is a point-to-point protocol. However, some internetworking devices might forward DTP frames
improperly. To avoid this, ensure that interfaces connected to devices that do not support DTP are
configured with the access keyword if you do not intend to trunk across those links. To enable trunking
to a device that does not support DTP, use the nonegotiate keyword to cause the interface to become a
trunk but to not generate DTP frames. See Table 8-7 for more details.
Table 8-7 Interface Modes
Mode Function
switchport mode access Puts the interface (access port) into permanent nontrunking mode and negotiates to
convert the link into a nontrunk link. The interface becomes a nontrunk interface even if
the neighboring interface is not a trunk interface.
switchport mode dynamic
desirable
Makes the interface actively attempt to convert the link to a trunk link. The interface
becomes a trunk interface if the neighboring interface is set to trunk, desirable,orauto
mode. The default mode for all Ethernet interfaces is dynamic desirable.
switchport mode dynamic auto Makes the interface able to convert the link to a trunk link. The interface becomes a trunk
interface if the neighboring interface is set to trunk or desirable mode.
Catalyst 5000 series
switch
Catalyst
2900 XL
switch
Catalyst
3500 XL
switch
Catalyst
2950
switch
Catalyst
3500 XL
switch
VLAN2
VLAN3VLAN1
VLAN1
VLAN2
VLAN3
802.1Q
trunk
802.1Q
trunk
802.1Q
trunk
802.1Q
trunk
44962
To Next Page
Loading...
