Ethernet trunks carry the traf?¬?c of multiple
VLANs over a single link and allow you to extend the VLANs across an entire network.
Cisco supports IEEE 802.1Q for FastEthernet and Gigabit Ethernet interfaces. In addition,
some Cisco switches support Cisco Inter-Switch Link (ISL) trunks, a prestandard trunking
technology. Figure 2-8 shows an example of trunks interconnecting Cisco Catalyst
switches.
Static VLAN Dynamic VLAN Voice VLAN
Fa0/1 Fa0/2 Fa0/3
MAC = 1111.1111.1111
VMPS
1111.1111.1111 = VLAN 10
VLAN 10
VLAN 55
VLAN 15
VLAN 5
IP
Implementing VLANs and Trunks 25
Figure 2-8 802.1Q Trunks
Ethernet trunk interfaces support different trunking modes. You can con?¬?gure an interface
as trunking or nontrunking, or you can have it negotiate trunking with the neighboring
interface.
Every 802.1Q port is assigned to a trunk, and all ports on a trunk are in a native VLAN. A
native VLAN is used in IEEE 802.1Q to send untagged frames to any non-802.1Q devices
that might exist on the segment. Every 802.1Q port is assigned an identi?¬?er value that is
based on the native VLAN ID (VID) of the port. (The default is VLAN 1.) All untagged
frames are assigned to the VLAN speci?¬?ed in this VID parameter.
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