ustrated in Figure 14.5. A variation on this theme that permits further scaling is to use both the inner and outer VLAN tags to create an extended 24-bit cross-connect identifier, allowing approximately 16 million VLANs per port. This extended mode is, however, not consistent with IEEE stan- dards, where only the outer tag is involved in the forwarding process. A further approach is to minimize changes to the Ethernet forwarding model (and hardware) by recognizing that VLANs are ultimately a scarce resource and that it is therefore Chapter 14 Next Generation Ethernet Ethernet Switch Cross-connections Frame Ethernet VLAN XC Tag Frame Fig. 14.5 VLAN cross-connection. of interest to examine alternative mechanisms to scale the connection space. Ethernet switches that employ Independent VLAN Learning (IVL) forward frames on the basis of a full 60-bit lookup of both the VLAN tag and the destination MAC address in each packet. It is clear that simply examining the entries of a forwarding table is not sufficient to describe the form of the forwarding-in Ethernet it is the MAC learning mechanism that results in connectionless behavior. Consequently, it is possible to provide a more deterministic form of forwarding with the same hardware simply by turning off some of the bridging functions. Bridging employs MAC learning and flooding to populate the forwarding tables of Ethernet switches, but these forwarding tables can also be configured by means of a control plane or via management configuration (e.g., via the command line interface). This allows for the VLAN tag and destination MAC address combination to be exploited in a different manner. There is a benefit to maintaining one of the primary features of Ethernet-namely, globally unique MAC addresses-but there is, as we have seen for VLAN cross-connects, no need to maintain the global uniqueness of the entire VLAN identifier range. The requirement for a global VLAN identifier comes not from any fundamental require- ment of the unicast forwarding behavior of an IVL switch 