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Embed code for: 20.4 Trunking with ISL and 802.1Q
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20.4 Trunking with ISL and 802.1Q
Trunking with ISL and 802.1Q Section 20: Implementing VLANs and Trunks For VLANs in networks that have multiple interconnected switches, the switches must use VLAN trunking on the segments between the switches. VLAN trunking causes the switches to use a process called "VLAN tagging," so that the sending switch adds another header to the frame before sending it over the trunk. This extra VLAN header includes a VLAN identifier (VLAN ID field) so that the sending switch can list the VLAN ID and the receiving switch can identify the VLAN that each frame belongs in. The figure shows the basic idea. VLAN Trunking Between Two Switches Trunking allows switches to pass frames from multiple VLANs over a single physical connection. For example, the figure shows Switch 1 receiving a broadcast frame on interface Fa0/1, which is a member of VLAN1. In a broadcast, the frame must be forwarded to all ports in VLAN1. Since there are ports on Switch 2 that are members of VLAN1 switch, the frame must be forwarded to Switch 2. Before forwarding the frame, Switch 1 adds a header that identifies the frame as belonging to VLAN1. This header tells Switch 2 that the frame should be forwarded to VLAN1 ports. Switch 2 removes the header and then forwards the frame for all ports that are part of VLAN1. As another example, the device on the Switch 1 Fa0/5 interface sends a broadcast. Switch 1 sends the broadcast out port Fa0/6 (because that port is in VLAN 2) and out Fa0/23 (because it is a trunk, meaning that it supports multiple VLANs). Switch 1 adds a trunking header to the frame, listing a VLAN ID of 2. Switch 2 strips off the trunking header because the frame is part of VLAN 2, so Switch 2 knows to forward the frame out only ports Fa0/5 and Fa0/6, and not ports Fa0/1 and Fa0/2. Cisco switches support two trunking protocols: ISL and IEEE 802.1Q. Trunking protocols provide several features. Most importantly, they define headers, which identify the VLAN ID, as shown in the figure. They do have some other differences as well. Cisco created ISL many years before the IEEE created the 802.1Q standard VLAN trunking protocol. Because ISL is Cisco proprietary, it can be used only between two Cisco switches that support ISL. ISL is not often used. Some newer Cisco switches do not even support ISL. IEEE 802.1Q The IEEE standardizes many of the protocols that relate to LANs today. Years after Cisco created ISL, the IEEE completed work on the 802.1Q standard, which defines a different way to do trunking. Today, 802.1Q has become the more popular trunking protocol. 802.1Q uses a different style of header than ISL to tag frames with a VLAN number. In fact, 802.1Q does not actually encapsulate the original frame in another Ethernet header and trailer. Instead, 802.1Q inserts an extra 4byte VLAN header into the Ethernet header of the original frame. As a result, the frame still has the original source and destination MAC addresses. Also, because the original header has been expanded, 802.1Q encapsulation forces a recalculation of the original FCS field in the Ethernet trailer, because the FCS is based on the contents of the entire frame. The figure shows the 802.1Q header and framing of the revised Ethernet header. 802.1Q Trunking Header The table summarizes the key features and points of comparison of ISL and 802.1Q. ISL and 802.1Q Compared ISL 802.1Q ISL 802.1Q Defined by Cisco IEEE Inserts another 4byte header instead of completely encapsulating the original frame No Yes Supports normalrange (1–1005) and extendedrange (1006–4094) VLANs Yes Yes Allows multiple spanning trees Yes Yes Uses a native VLAN No Yes Up Next: VLAN Trunking Configuration