TelePresence codecs are assigned to the Voice VLAN, whereas most endpoint devices operate within the Data VLAN. It would be inefficient, costly, and administratively complex to use dedicated Ethernet ports and cables for each VLAN. Therefore, a logical separation of VLANs over a physical link is more efficient, cost-effective, and simpler to administer.
A trunk is a point-to-point link between two networking devices (switches and routers) capable of carrying traffic from multiple VLANs over a single link. VLAN frames are encapsulated with trunking protocols to preserve logical separation of traffic while transiting the trunk.
There are two trunking encapsulations available to Cisco devices:
- Inter-Switch Link (ISL): A Cisco-proprietary trunking encapsulation
- IEEE 802.1Q: An industry-standard trunking encapsulation and the trunking protocol used by TelePresence codecs
You can configure trunks on individual links or on EtherChannel bundles (discussed in the following section).
ISL encapsulates the original Ethernet frame with both a header and a Field Check Sequence (FCS) trailer, for a total of 30 bytes of encapsulation.
You can configure ISL trunking on a switch port interface, as demonstrated in Example 1. The trunking mode is set to ISL, and the VLANs permitted to traverse the trunk are explicitly identified. In this example VLANs 2 and 102 are permitted over the ISL trunk.
Example 1: ISL Trunk Example
Switch(config)#interface GigabitEthernet8/3 Switch(config-if)# switchport Switch(config-if)# switchport trunk encapsulation isl Switch(config-if)# switchport trunk allowed 2, 102
In contrast with ISL, 801.1Q doesn’t actually encapsulate the Ethernet frame, but rather inserts a 4-byte tag after the Source Address field and recomputes a new FCS, as shown in Figure 1. This tag not only preserves VLAN information, but also includes a 3-bit field for Class of Service (CoS) priority
IEEE 802.1Q also supports the concept of a native VLAN. Traffic sourced from the native VLAN is not tagged but is rather simply forwarded over the trunk. As such, only a single native VLAN can be configured for an 802.1Q trunk, to preserve logical separation.
| Note |
IEEE 802.1Q trunking is likewise configured on a switch port interface, as demonstrated in Example 2. The trunking mode is set to 802.1Q, and the VLANs permitted to traverse the trunk are explicitly identified. (In this example VLANs 3 and 103 are permitted over the 802.1Q trunk.) Additionally, VLAN 103 is specified as the native VLAN.
Example 2: IEEE 802.1Q Trunk Example
Switch(config)# interface GigabitEthernet8/4 Switch(config-if)# switchport Switch(config-if)# switchport trunk encapsulation dot1q Switch(config-if)# switchport trunk allowed 3, 103 Switch(config-if)# switchport trunk native vlan 103
Trunks are typically (but not always) configured in conjunction with EtherChannels, which allow for network link redundancy
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