1-2Implementation of VLAN-VPNWith the VLAN-VPN feature enabled, no matter whether or not a received packet already carries aVLAN tag, the switch will tag the received packet with the default VLAN tag of the receiving port and addthe source MAC address to the MAC address table of the default VLAN. When a packet reaches aVLAN-VPN-enabled port:z If the packet already carries a VLAN tag, the packet becomes a dual-tagged packet.z Otherwise, the packet becomes a packet carrying the default VLAN tag of the port.Configuring the TPID for VLAN-VPN PacketsThe contents of this section are only applicable to the S3100-EI series among S3100 series switches.A VLAN tag uses the tag protocol identifier (TPID) field to identify the protocol type of the tag. The valueof this field is 0x8100 for IEEE 802.1Q.Figure 1-3 illustrates the structure of the IEEE 802.1Q VLAN tag in an Ethernet frame.Figure 1-3 The structure of the VLAN tag in an Ethernet frame0 3115TPID Priority VLAN IDCFIAn S3100 switch determines whether a received frame is VLAN tagged by comparing its own TPID withthe TPID field in the received frame. If they match, the frame is considered as a VLAN tagged frame. Ifnot, the switch tags the frame with the default VLAN tag of the receiving port.By default, S3100 series switches adopt the IEEE 802.1Q TPID value 0x8100. Some vendors, however,use other TPID values such as 0x9100. For compatibility with these systems, the S3100 series switchesallow you to change the TPID that a port uses when tagging a received VLAN-VPN frame as needed.When doing that, you should set the same TPID on both the customer-side port and the serviceprovider-side port.The TPID in an Ethernet frame has the same position with the protocol type field in a frame without aVLAN tag. To avoid problems in packet forwarding and handling, you cannot set the TPID value to anyof the values in the table below.Table 1-1 Commonly used protocol type values in Ethernet framesProtocol type ValueARP 0x0806IP 0x0800MPLS 0x8847/0x8848IPX 0x8137
