Inter Switch Link ISL

This is the Cisco proprietary protocol for connecting multiple switches and maintaining VLAN information as traffic travels between switches. It is a method of multiplexing bridge groups over a high-speed backbone. With ISL, an Ethernet frame is encapsulated with a header that transports the VLAN ID between switches and routers. This VLAN ID is added to the frame only if the frame is destined to a non-local interface.

Examine Figure 16-4. If switch 1 receives traffic from segment A and wants to forward it to segment B, no ISL header is attached. If switch 1 must forward a frame to switch 3 on VLAN 200, the ISL header is added at switch 1 and is passed through switch 2. The packet is then forwarded to switch 3. When switch 3 receives the packet, it removes the ISL header and forwards it to the appropriate port for VLAN 200.

Figure 16-4. Frame Tagging for VLANs

Switch 3

Figure 16-4. Frame Tagging for VLANs

Switch 3

Switch 4

VLAN 100

This IEEE standard provides a method for secure bridging of data across a shared backbone. The 802.10 standard defines a single frame type known as the Secure Data Exchange (SDE) frame, which is a MAC-layer frame with an 802.10 header inserted between the MAC header and the frame data, as shown in Figure 16-5. The VLAN ID is carried in a four-byte Security Association Identifier (SAID) field.

Figure 16-5. IEEE 802.10 Frame Header

Cisco routers and switches can use 802.10 to switch traffic across high-speed backbones (FDDI, Fast Ethernet, Token Ring, and serial links). As mentioned earlier, the VLAN ID is inside the 802.10 header. When the switch receives the frame from a source station, it inserts the VLAN ID. On the receiving switch, the frame is stripped, and the 16-byte 802.10 header is removed. The frame is then forwarded to the interface that matches the VLAN ID.

As Figure 16-6 shows, all ports on switches 1, 2, and 3 are VLAN interfaces. To prevent traffic from segment C destined for segment B from flooding the entire switched network, switch 3 tags the frames with the IEEE 802.10 header when the frame leaves switch 3. Switch 2 recognizes the color and knows that it must forward these frames onto segment B. Switch 1 does not pass the frame to its segments.

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