Vlan Configuration; Shared Vlan Overview - 3Com 7757 Configuration Manual

21
Shared VLAN
Overview
Generation of Shared
VLAN
S VLAN C
HARED
Shared VLAN is special VLAN which is created based on I/O Modules of the device.
It is designed to avoid packet broadcast in the applications of selective QinQ.
Like a QinQ-enabled port, a port with the selective QinQ enabled also learns the
source MAC addresses of user packets to the MAC address table of the default
VLAN of the port. However, the port with selective QinQ enabled can insert an
outer VLAN tag besides the default VLAN tag to the packets. Thus, when packets
from the service provider to customers are forwarded, broadcast arises because
each of these packets fails to find its destination MAC address in the MAC table of
its outer VLAN.
Figure 48 Learn MAC addresses of selective QinQ frames
Public
Network
VLAN4
MAC-A
PVID=2 VLAN3
MAC-A
Private
Network
Receives the data that the
private network sends to the
service provider s network
As shown in Figure 48, when user packets are received, the default VLAN of the
incoming port is VLAN 2, and the incoming port is specified to receive packets of
VLAN 3, with outer tag of VLAN 4. When a packet is received, its source MAC
address MAC-A is learned into the MAC address table of the default VLAN (VLAN
2) of the port.
When a response packet is returned to the device from VLAN 4 of the service
provider network, the device will search the outgoing port for MAC-A in the MAC
address table of VLAN 4. However, because the corresponding entry is not learned
into the MAC address table of VLAN 4, this packet is considered to be a unicast
packet with unknown destination MAC address. As a result, this packet will be
broadcast to all the ports in VLAN 4, which wastes the network resources and
endangers the network.
The problem above can be solved by using the shared VLAN feature, which
summarizes the MAC address tables of all the VLANs. The switch can find the
outgoing port for a packet according to the MAC address table of the shared
VLAN and unicast the packet.
ONFIGURATION
Public
Network
VLAN4
MAC-A
PVID=2
Private
Network
Receives the data that the
service provider s network
sends to the private network