ZyXEL Communications OMC-2301, User Manual

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Chapter 8 VLAN Profile
OMC-2301 User’s Guide
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The egress (outgoing) port(s) of a frame is determined on the combination of the destination
MAC address and the VID of the frame. For a unicast frame, the egress port (based on the
destination MAC address) must be a member of the VID, also; otherwise, the frame is
blocked. A broadcast frame (or a multicast frame for a multicast group that is known by the
system) is duplicated only on ports that are members of the VID (except the ingress port
itself), thus confining the broadcast to a specific domain.
Whether to tag an outgoing frame depends on the setting of the egress port on an individual
VLAN and port basis (remember that a port can belong to multiple VLANs). If the tagging on
the egress port is enabled for the VID of a frame, then the frame is transmitted as a tagged
frame; otherwise, it is transmitted as an untagged frame.
8.2 VLAN Stacking
A service provider can use VLAN stacking to allow it to distinguish multiple customers
VLANs, even those with the same (customer-assigned) VLAN ID, within its network.
Use VLAN stacking to add an outer VLAN tag to the inner IEEE 802.1Q tagged frames that
enter the network. By tagging the tagged frames (“double-tagged” frames), the service
provider can manage up to 4,094 VLAN groups with each group containing up to 4,094
customer VLANs. This allows a service provider to provide different service, based on
specific VLANs, for many different customers.
A service provider’s customers may require a range of VLANs to handle multiple
applications. A service provider’s customers can assign their own inner VLAN tags on ports
for these applications. The service provider can assign an outer VLAN tag for each customer.
Therefore, there is no VLAN tag overlap among customers, so traffic from different customers
is kept separate.
8.2.1 VLAN Stacking Example
In the following example figure, both A and B are Service Provider’s Network (SPN)
customers with VPN tunnels between their head offices and branch offices respectively. Both
have an identical VLAN tag for their VLAN group. The service provider can separate these
two VLANs within its network by adding tag 37 to distinguish customer
A and tag 48 to
distinguish customer B at edge device 1 and then stripping those tags at edge device 2 as the
data frames leave the network.