Allied Telesis SwitchBlade x3100 Series - Hvlan (Port Based and Vlan Based); Port Based Hvlan

Introduction Port Based HVLAN
Software Reference for SwitchBlade x3100 Series Switches (Layer Two Switching)
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4.9 HVLAN (Port Based and VLAN Based)
4.9.1 Port Based HVLAN
A VLAN allows broadcast traffic to flood only ports that are members of that VLAN. Moreover, ports can be
tagged or untagged, with a tagged Ethernet frame including the VID field that uniquely identifies the VLAN of
the frame. The number of VLANs that can be configured across the network operator network is restricted to
the 12-bit VID field (1 to 4094).
To help overcome the VLAN addressing limitation, an additional or outer tag can be added on top of the
802.1q tagged. The use of the additional tag creates a hierarchical VLAN (HVLAN).
At the port, incoming customer frames are wrapped with an outer tag that is used to switch the traffic across
the network. At the port for the outgoing traffic, the outer tag is removed and the frame is delivered to the
customer’s VLAN.
By using this outer tag, Allied Telesis system users can expand service to customers in the following ways:
• Two VLAN tags are used to identify the customer VLAN, in theory expanding the number range of cus-
tomer VLAN tags to 4094 * 4094.
• Since the inner tag is used by each customer, the VLAN ID for different customers may be the same (over-
lap). Thus, the customer VLAN ID is preserved and unchanged as it crosses the network.
By using this outer tag, network operators can tunnel the VLANs of each customer into a single VLAN (the
VLAN ID of the outer tag) and send them across the network, allowing businesses to interconnect devices
from multiple locations in a network operator area.
To understand the HVLAN feature, the 802.1q tagged ethernet frame and the fields it contains must be fully
understood. These are listed in Table 4-26
.
TABLE 4-26 VLAN Tag Fields
Field Name Length Description
Tag Protocol Identifier
(TPID)
2 octets The TPID is used to identify the frame as a tagged frame.The value
of the TPID for an 802.1q ethernet tagged frame is 0x8100
User Priority 3 bits The User Priority field can represent up to eight priority levels. (This
field is explained in greater detail when discussing traffic manage-
ment, in Priority Queuing.
Canonical Format Indicator
(CFI)
1 bit The CFI is a flag to indicate whether all MAC address information
that may be present in the MAC data carried by the frame is in
canonical format.