H3C Technologies H3C S3100 Series Switches - Manual (Page 606)

Operation Manual – VLAN VPN
H3C S3100-52P Ethernet Switch Chapter 1 VLAN-VPN Configuration
1-2
VLAN tag. If the packet already carries a VLAN tag, the packet becomes a dual-tagged
packet. Otherwise, the packet becomes a packet carrying the default VLAN tag of the
port.
1.1.3 Adjusting the TPID Values of VLAN-VPN Packets
Tag protocol identifier (TPID) is a field of the VLAN tag. IEEE 802.1Q specifies the
value of TPID to be 0x8100.
Figure 1-3 illustrates the structure of the Tag packet of an Ethernet frame defined by
IEEE 802.1Q.
DA SA Tag Frame Load FCS
TPID User Priority CFI VLAN ID
2 bytes 3 bits 1bit 12 bits
DA SA Tag Frame Load FCS
TPID User Priority CFI VLAN ID
2 bytes 3 bits 1bit 12 bits
DA SA Tag Frame Load
6 bytes 6 bytes 4 bytes 46 to 1,500 bytes
FCS
TPID User Priority CFI VLAN ID
2 bytes 3 bits 1bit 12 bits
DA SA Tag Frame Load FCS
4 bytes
TPID User Priority CFI VLAN ID
2 bytes 3 bits 1bit 12 bits
DA SA Tag Frame Load FCS
TPID User Priority CFI VLAN ID
2 bytes 3 bits 1bit 12 bits
DA SA Tag Frame Load FCS
TPID User Priority CFI VLAN ID
2 bytes 3 bits 1bit 12 bits
DA SA Tag Frame Load
6 bytes 6 bytes 4 bytes 46 to 1,500 bytes
FCS
TPID User Priority CFI VLAN ID
2 bytes 3 bits 1bit 12 bits
DA SA Tag Frame Load FCS
4 bytes
TPID User Priority CFI VLAN ID
2 bytes 3 bits 1bit 12 bits
Figure 1-3
The structure of the Tag packet of an Ethernet frame
H3C S3100-52P switch adopt the protocol default TPID value (0x8100). Other vendors
use other TPID values (such as 0x9100 or 0x9200) in the outer tags of VLAN-VPN
packets.
To be compatible with devices coming from other vendors, S3100-52P switch can
adjust the TPID values of VLAN-VPN packets based on ports. You can configure the
TPID value of a port connecting to the public network side by yourself. When a packet is
forwarded through the port, the port replaces the TPID value in the outer VLAN tag of
this packet with the user-defined value. Thus, the VLAN-VPN packets sent to the public
network can be recognized by devices of other vendors.
As the position of the TPID field in an Ethernet packet is the same as that of the protocol
type field in a packet without VLAN Tag, to avoid confusion in the process of
receiving/forwarding a packet, the TPID value cannot be any of the protocol type value
listed in Table 1-1
Table 1-1
Commonly used protocol type values in Ethernet frames
Protocol type Value
ARP 0x0806
IP 0x0800
MPLS 0x8847/0x8848