Ethernet Virtual Circuits (EVCs) allow us to use existing 802.1q VLAN tags in a brand new way. Traditionally the VLAN tag defined both classification (which VLAN) and forwarding (which CAM table to do a MAC lookup in). Now, with EVCs we can separate these concepts the VLAN tag is used for classification and the Service Instance defines the forwarding action. e.g. we could allocate VLAN 100 to different customers on every switchport and forward each customer’s traffic across different MPLS Pseudowires, but never actually configure VLAN 100 globally
On conventional switches whenever we have a trunk interface we use the VLAN tag to demultiplex the VLANs. The switch needs to find out which MAC Address table to look in for a forwarding decision. To do this we require the switch to do two things:
1.) Have the VLAN’s configured globally
2.) Perform MAC learning in this VLAN
The challenge with this strategy is that it requires us to use finite resources, perhaps without reason. Since the 802.1q VLAN tag is only 12-bits wide we can only configure a maximum of 4096 VLANs. Furthermore switches have a finite amount of CAM space for MAC Learning limiting the number of hosts we can support.