How Vlan Trunks Work - Cisco Catalyst 2950 Software Manual

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
Figure 8-3
DTP is a point-to-point protocol. However, some internetworking devices might forward DTP frames
Note
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 Interface Modes
Mode
switchport mode access
switchport mode dynamic
desirable
switchport mode dynamic auto
Catalyst 2950 Desktop Switch Software Configuration Guide
8-20
shows a network of switches that are connected by 802.1Q trunks.
Catalyst 2950, 2900 XL, and 3500 XL Switches in a 802.1Q Trunking Environment
802.1Q 802.1Q
trunk trunk
Catalyst
2900 XL
switch
Catalyst
3500 XL
switch
VLAN1
VLAN2
Function
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.
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, or auto
mode. The default mode for all Ethernet interfaces is dynamic desirable.
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
802.1Q
trunk
Catalyst
2950
switch
VLAN3 VLAN2
VLAN1
Table 8-7 for more details.
Chapter 8 Configuring VLANs
802.1Q
trunk
Catalyst
3500 XL
switch
VLAN3
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