ISL trunk encapsulation is designed for trunking over a point-to-point connection between two Catalysts using Ethernet. Only two Catalysts connect to the link. This contrasts with connectivity over an FDDI system. FDDI operates as a shared network media (half duplex) and can have more than two participants on the network. A different encapsulation scheme, therefore, is used when trunking over an FDDI network. Cisco adapted an IEEE standard for secure bridging over an 802-based network and applied it to FDDI trunking between Catalysts. IEEE 802.10 devised the standard to facilitate the transport of multiple traffic sources over shared local and metropolitan networks and yet retain logical isolation between the source networks at the receiver.
You can create interconnections between Catalysts where all Catalyst FDDI interfaces belong to the same VLAN. Only one VLAN transports over the FDDI, however. You can do this if you have a simple VLAN design and have an existing FDDI segment that you need to continue to use. The legacy network components might not support 802.10, forcing you to configure your Catalysts so they can share the FDDI network. A more typical use, however, might allow for multiple VLANs to share the backbone, as in Figure 8-10.
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