3Com 4500 PWR 26-Port Configuration Manual page 793

Provides simple Layer 2 VPN solutions for small-sized MANs or intranets.
Imple mentation of VLAN-VPN
With the VLAN-VPN feature enabled, no matter whether or not a received packet already carries a
VLAN tag, the switch will ta
the source MAC address to the MAC address table of the default VLAN. When a packet re
VLAN-VPN-enabled port:
If the packet already carries a VLAN tag, the packet becomes a dual-tagged packet.
Otherwise, the packet becomes a packet carryi
Conf iguring the TPID for VLAN-VPN Packets
A VLAN tag uses the tag protocol identifier (TPID) field to identify the protocol type of the t
of this field is 0x8100 for IEEE 802.1Q.
Figure 65-2 illustrates the structure of the IEEE 802.1Q VLAN tag in an Ethernet frame.
Figure 65-2 The structure of the VLAN tag in an Ethernet frame
A Switch 4500 switch determines whether a received frame is VLAN tagged by comparing its own TPID
with the TPID field in the received frame. If they match, the frame is considered as a VLAN tagged
frame. If not, the switch tags the frame with the default VLAN tag of the receiving port.
By default, Switch 4500 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 Switch 4500
series switches allow you to ch
frame as needed. When doing that, you should set the same TPID on both the customer-side port and
the service provider-side port.
The TPID in an Ethernet frame h
VLAN tag. To avoid problems in packet forwarding and handling, you cannot set the TPID value to any
of the values in the table below.
Table 65-1 Comm
ARP
IP
MPLS
IPX
IS-IS
LACP
802.1x
g the received packet with the default VLAN tag of the receiving port and add
ange the TPID that a port uses when tagging a received VLAN-VPN
as the same position with the protocol type field in a frame without a
only used proto col type values in Ethernet frames
Protocol type
ng the default VLAN tag of the port.
0x0806
0x0800
0x8847/0x8848
0x8137
0x8000
0x8809
0x888E
65-2
aches a
ag. The value
Value