Trunk Connections

Under normal circumstances, a switch port can carry traffic for a single VLAN only. For VLANs to span multiple switches, a trunk connection must be created. This trunk connection transports data from multiple VLANs. Trunk connections allow VLANs to be used throughout the switching fabric of large networks.

Any Fast Ethernet or Asynchronous Transfer Mode (ATM) port on a switch can be designated as a trunk port. This port typically connects to another switch via a crossover 100BASE-T cable in the case of a Fast Ethernet trunk.

For the trunked port to transport multiple VLANs, it must understand frame tags.

Interswitch Link (ISL)

ISL is a technology developed by Cisco that allows a single Ethernet interface to participate in multiple VLANs. When a trunk connection is made on a Catalyst switch's Ethernet port, ISL is used. ISL is also available on Ethernet cards that can be used in servers or routers.

A device utilizing an ISL Ethernet card will appear to have many physical cards, each connected to a different segment. ISL allows this single Ethernet card to have many logical (virtual) addresses. The user must configure each logical interface with an address that reflects the VLAN to which it belongs.

ISL works by allowing the frame-tagging information to be passed along to the Ethernet card. The Ethernet card then reads the frame tag, which identifies the VLAN to which the frame belongs. Conversely, the ISL Ethernet card creates the frame tags when transmitting frames .

ISL is a technology proprietary to Cisco and, therefore, is not supported on equipment made by other vendors. However, in mid-1998, the Institute of Electrical and Electronics Engineers (IEEE) standardized a frame-tagging process similar to Cisco's ISL. The new standard is a protocol called 802.1Q. With 802.1Q, switches from multiple vendors can coexist in the same switching fabric.

VLAN Trunking Protocol (VTP)

VTP is a protocol used between switches to simplify the management of VLANs. Configuration changes that are made to a VTP server are propagated across trunks to all connected switches.

All switches that are to be managed in this way must be members of the same management domain . A VTP management domain is the entire group of switches that share configuration information.

For example, when you add a new VLAN to a member switch, the VLAN is available in all the network switches automatically. VTP allows switched networks to scale to larger environments; otherwise , VLAN configuration would have to be maintained manually on individual switches.

By default, Catalyst switches are set to a no-management-domain state. The switches remain in a no-management state until a user configures the management domain or the switches receive an advertisement for a domain over a trunk link.

VTP Modes

When it has a management domain, a switch operates in one of three VTP modes: server, client, or transparent. The default mode is server.

In VTP server mode, a switch can create, modify, or delete VLAN and other configuration parameters for the entire VTP domain. VTP messages are sent over all trunk links, and configuration changes are propagated to all switches in the management domain.

In VTP client mode, the switch receives VTP messages and applies configuration changes made from any VTP server. However, a client cannot create, change, or delete VLAN information.

In VTP transparent mode, the switch forwards all VTP messages to other switches in the domain but does not use the configuration from VTP advertisements. A VTP transparent switch can create, modify, or delete VLANs, but the changes apply only locally and are not transmitted to other switches.

VTP Pruning

VTP can detect whether a trunk connection is carrying unnecessary traffic. By default, all trunk connections carry traffic from all VLANs in the management domain. In many cases, however, a switch does not need a local port configured for each VLAN. In this event, it is not necessary to flood traffic from VLANs other than the ones supported by that switch (see Figure 6.7). VTP pruning enables the switching fabric to prevent flooding traffic on trunk ports that do not need it.

Figure 6.7. VTP pruning.