31 Days Before Your CCNA Exam

74 31 Days Before Your CCNA ExamThe traffic from the PC5 attached to the IP Phone passes through the IP Phone untagged. The linkbetween S2 and the IP Phone act as a modified trunk to carry both the tagged voice traffic and theuntagged data traffic.Trunking VLANsA VLAN trunk is an Ethernet point-to-point link between an Ethernet switch interface and anEthernet interface on another networking device, such as a router or a switch, carrying the trafficof multiple VLANs over the singular link. A VLAN trunk allows you to extend the VLANs acrossan entire network. A VLAN trunk does not belong to a specific VLAN; rather, it serves as a conduitfor VLANs between switches. Figure 24-2 shows a small switched network with a trunk linkbetween S1 and S2 carrying multiple VLAN traffic.Figure 24-2Example of a VLAN TrunkVLAN 1 – Control Traffic - 172.17.1.0/24VLAN 10 – Faculty/Staff - 172.17.10.0/24VLAN 20 – Students - 172.17.20.0/24VLAN 30 – Guest (Default) - 172.17.30.0/24VLAN 99 – Management and Native - 172.17.99.0/24FacultyVLAN 10 -172.17.10.21PC1S11 Switch Port5 VLANs: 1, 10, 20, 30, 99StudentVLAN 20 -172.17.20.22PC2S2GuestVLAN 30 -172.17.30.23PC3When a frame is placed on a trunk link, information about the VLAN it belongs to must be added tothe frame. This is accomplished by using IEEE 802.1Q frame tagging. When a switch receives a frameon a port configured in access mode and destined for a remote device via a trunk link, the switch takesapart the frame and inserts a VLAN tag, recalculates the frame check sequence (FCS), and sends thetagged frame out the trunk port. Figure 24-3 shows the 802.1Q tag inserted in an Ethernet frame.The VLAN tag field consists of a 16-bit Type field called the EtherType field and a Tag controlinformation field. The EtherType field is set to the hexadecimal value of 0x8100. This value iscalled the tag protocol ID (TPID) value. With the EtherType field set to the TPID value, the switchreceiving the frame knows to look for information in the Tag control information field. The Tagcontrol information field contains the following:■■■3 bits of user priority: Used to provide expedited transmission of Layer 2 frames, such asvoice traffic.1 bit of Canonical Format Identifier (CFI): Enables Token Ring frames to be carried acrossEthernet links easily.12 bits of VLAN ID (VID): VLAN identification numbers.