3/2 VLAN Type Remote SPAN VLANs Primary Secondary Type Ports Chapter 4: VLANs CHAPTER 5 Trunking A trunk, using Cisco's terminology, is an interface or link that can carry frames for multiple VLANs at once. As you saw in the previous chapter, a trunk can connect two switches SO that devices in VLANs on one switch can communicate with devices in the same VLANs on another switch. Unless there is only one VLAN to be connected, switches are connected at Layer 2 using trunks. Figure 5-1 shows two switches con- nected 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 routers, firewalls, and all manner of de- vices can connect with trunks 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 is 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 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 determine 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, nor do TCP, UDP, ICMP, or any other protocol above Layer 2. Remember that a VLAN is a Layer-2 concept, SO any reference to a VLAN would happen at the data- link layer. Ethernet was invented before VLANs, SO there is no reference to VLANs in any Ethernet protocols, either. To mark, or tag, f