Appendix D. Extending VLANs Within Layer 3 SwitchesSome Layer 3 switches will perform Layer 3 processing only at the interface level in hardware and have no concepts of VLANs and VLAN databases. Routers also fall into this category. This appendix explains how to set up a broadcast domain between ports on a device with only Layer 3 interfaces. This appendix also explains how to map these broadcast domains to ISL or 802.1Q trunks to extend a switch VLAN into a Layer 3 device. A Layer 3-only device, such as a router or a 2948G-L3 switch, is specialized to process packets based on Layer 3 header information. Within a Layer 3 device, bridging software can be enabled and ports can be assigned to bridge groups. All ports in a common bridge group are in the same broadcast domain. By default a Layer 3 device will still attempt to route an IP, IPX, or AppleTalk packet unless otherwise directed. Layer 3 devices have no knowledge of VLANs that exist in switches but can understand 802.1Q or ISL tags coming from a switch. By mapping a bridge group to an ISL or 802.1Q VLAN, the broadcast domain of a VLAN can be extended into a Layer 3 switch.
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