Content

| 2.4 |  | Configuring VLANs and VMPS |
| 2.4.6 |  | Access links and trunk links |

Static switch ports run in either the access or trunk mode. Accordingly, the links connected to these switch ports are referred to as access links or trunk links. In the access mode, the port belongs to one and only one VLAN. An access port is a switch port that connects to an end-user device or a server; the frames transmitted on an access link look like any other Ethernet frames.

A trunk link differs from an access link in that it is capable of supporting multiple VLANs. The VLANs are multiplexed over the link with a trunking protocol. The trunk link also does not belong to a specific VLAN. Trunk links are typically used to connect switches to other switches, routers, or a server that uses an IEEE 802.1q-enabled network card to participate in multiple VLANs.

Trunks can extend VLANs across an entire network. A trunk link can be configured to transport all VLANs or a restricted set of VLANs. Trunk links are typically configured on ports that support the greatest bandwidth for a given switch. Catalyst switches support trunk links on Fast Ethernet, Gigabit Ethernet, and 10Gigabit Ethernet ports.

To multiplex VLAN traffic, special protocols exist that encapsulate or tag the frames so that the receiving device can determine the frame’s VLAN membership. The Cisco proprietary Inter-Switch Link (ISL) protocol lets Cisco devices multiplex VLANs between Cisco devices. In multi-vendor environments, switches must use IEEE 802.1Q, an industry standard protocol that permits multiplexing of VLANs over trunk links. The Catalyst 3550 switches support both ISL and 802.1Q, while the Catalyst 2950 switch only supports 802.1Q.

Without trunk links, multiple access links would have to be installed to support multiple VLANs between switches (one link per VLAN). This is clearly neither a cost-effective nor scalable solution. Trunking is absolutely essential for interconnecting switches in a campus network.

Port A and Port B on Switch Y have been defined as access links on the same VLAN. By definition, they can belong to only VLAN 200 and cannot receive frames with a VLAN identifier. As Switch Y receives traffic from Port A destined for Port B, Switch Y will not add an ISL header to the frame.

Port C is also an access link that has been defined as a member of VLAN 200. If Port A sends a frame destined for Port C, the switch does the following:

1. Switch Y receives the frame and identifies it as traffic destined for VLAN 200 by the VLAN and port number association.
2. Switch Y encapsulates the frame with an ISL header identifying VLAN 200 and sends the frame through the intermediate switch on a trunk link.
3. This process is repeated for every switch that the frame must transit as it moves to its final destination of Port C.
4. Switch Z receives the frame, removes the ISL header, and forwards the frame to Port C.