Carrier Ethernet Access Switches (CEAS) are switch components that connect the customer links to the Carrier Ethernet Switch Routers ( CESR) in the Carrier Ethernet ( CE) provider network.
To form a provider bridge( PB), they add additional bytes to the Ethernet frame they receive from the customer. The Provider Bridges can be used to tunnel data packets through the Provider Network.
Carrier Ethernet access switches add additional bytes to the Ethernet frame. Provider bridging adds a 4- byte service tag to the Ethernet frame. By using a second VLAN identifier in the Service VLAN Identifier (S- VID), different users can be supported with their own S-VIDs. This technique is also referred to as Q-in-Q.