3Com Switch 8800 Advanced Software V5 Configuration Guide

182 CHAPTER 18: QINQ CONFIGURATION
■ Saves the public network VLAN ID resources.
■ Enables customers to plan their own private network VLAN IDs, without
running into conflicts with public network VLAN IDs.
■ Provides a simple Layer 2 VPN solution for small-sized MANs or Enterprise
networks.
Implementations of
QinQ
For Switch 8800s, the QinQ feature is implemented through enabling the basic
QinQ feature on ports.
With the basic QinQ feature enabled on a port, when a frame arrives at the port,
the port will tag it with the port’s default VLAN tag, regardless of whether the
frame is tagged or untagged. If the received frame is already tagged, this frame
becomes a double-tagged frame; if it is an untagged frame, it is tagged with the
port’s default VLAN tag.
Adjustable TPID Value of
QinQ Frames
A VLAN tag uses the tag protocol identifier (TPID) field to identify the protocol
type of the tag. The value of this field, as defined in IEEE 802.1Q, is 0x8100.
Figure 54 shows the structure of a VLAN-tagged Ethernet frame defined in IEEE
802.1Q.
Figure 54 Structure of a VLAN-tagged Ethernet frame
n
Refer to “VLAN Configuration” on page 155 for the description on the TPID,
Priority, CFI, and VLAN ID fields.
On devices of different vendors, the TPID of the outer VLAN tag of QinQ frames
may have different default values. You can set and/or modify this TPID value, so
that the QinQ frames, when arriving at the public network, carries the TPID value
of a specific vendor to allow interoperation with devices of that vendor.
The TPID in an Ethernet frame has the same position with the protocol type field in
a frame without a VLAN tag. To avoid chaotic packet forwarding and receiving,
you cannot set the TPID value to any of the values in the table below.
User Priority CFI
DA SA VLAN Tag Etype DATA FCS
TPID VLAN ID
6 bytes 6 bytes 4 bytes
2 bytes 3bits 1 bit 12 bits
4bytes2 bytes 46 to 1500 bytes
Tabl e 17 Protocol type values
Protocol type Value
ARP 0x0806
PUP 0x0200
RARP 0x8035
IP 0x0800
IPv6 0x86DD