Trunking with ISL and 8021q

When using VLANs in networks that have multiple interconnected switches, you need to use VLAN trunking between the switches. When sending a frame to another switch, the switches need a way to identify the VLAN from which the frame was sent. With VLAN trunking, the switches tag each frame sent between switches so that the receiving switch knows which VLAN the frame belongs to. Figure 10-3 outlines the basic idea.

Figure 10-3 VLAN Trunking Between Two Switches

Switch1

1 VLAN1

VLAN2

Figure 10-3 VLAN Trunking Between Two Switches

Switch2

Trunk

Switch2

VLAN1

VLAN2

VLAN ID Ethernet Frame

With trunking, you can support multiple VLANs that have members on more than one switch. For instance, when Switch1 receives a broadcast from a device in VLAN1, it needs to forward the broadcast to Switch2. Before sending the frame, Switch1 adds another header to the original Ethernet frame; that new header has the VLAN number in it. When Switch2 receives the frame, it sees that the frame was from a device in VLAN1, so Switch2 knows that it should forward the broadcast only out its own interfaces in VLAN1.

Cisco switches support two different trunking protocols, Inter-Switch Link (ISL) and IEEE 802.1q. They both provide basic trunking, as shown in Figure 10-3. They do have some differences, as will be covered next.

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