two switches connected with a trunk. Switch A Port 8 Port 9 Port 10 Port 11 Port 12 Port 13 Port 14 Port 15 VLAN 20 VLAN 30 VLAN 40 VLAN 10 VLAN 10 VLAN 20 Port 0 Port 1 Port 2 Port 3 Port 4 Port 5 Port 6 Port 7 Trunk Port 8 Port 9 Port 10 Port 11 Port 12 Port 13 Port 14 Port 15 Diane VLAN 20 VLAN 30 VLAN 40 VLAN 10 VLAN 10 VLAN 20 Port 0 Port 1 Port 2 Port 3 Port 4 Port 5 Port 6 Port 7 Switch B Figure 5-1. A trunk connecting two switches Trunking is generally related to switches, but a router can connect to a trunk as well. The router on a stick scenario described in Chapter 4 requires a router to communicate with a trunk port on a switch. How Trunks Work Figure 5-2 shows a visual representation of a trunk. VLANs 10, 20, 30, and 40 exist on both sides of the trunk. Any traffic from VLAN 10 on Switch-1 that is destined for VLAN 10 on Switch-2 must traverse the trunk. (Of course, the reverse is true as well.) Switch 1 Switch 2 VLAN 10 VLAN 10 VLAN 20 VLAN 20 Trunk VLAN 30 VLAN 30 VLAN 40 VLAN 40 Figure 5-2. Visual representation of a trunk For the remote switch to know how to forward the frame, the frame must contain a reference to the VLAN to which it belongs. IP packets have no concept of VLANs, though, and nor does TCP, UDP, ICMP, or any other protocol above layer two. Remember that a VLAN is a layer-two concept, SO if there were to be any mention of a VLAN, it would happen at the data-link layer. Ethernet was invented before VLANs, SO there is no mention of VLANs in any Ethernet protocols, either. To accomplish the marking or tagging of frames to be sent over a trunk, both sides must agree to a protocol. Currently, the protocols for trunking supported on Cisco switches are Cisco's Inter-Switch Link (ISL) and the IEEE standard 802.1Q. Not all Cisco switches support both protocols. For example, the Cisco 2950 and 4000 switches only support 802.1Q. To determine whether a switch can use a specific trunking protocol, use the IOS command show interface capabilities, or the CatOS command show port capabilities: Switc