G.shdsl Router User Manual 152
14
Yours VLAN (Virtual Local Area Network)
Virtual LAN (VLAN) is defined as a group of devices on one or more LANs that are configured so
that they can communicate as if they were attached to the same wire, when in fact they are located
on a number of different LAN segments. Because VLAN is based on logical instead of physical
connections, it is extremely flexible.
The IEEE 802.1Q defines the operation of VLAN bridges that permit the definition, operation, and
administration of VLAN topologies within a bridged LAN infrastructure. VLAN architecture benefits
include:
1. Increased performance
2. Improved manageability
3. Network tuning and simplification of software configurations
4. Physical topology independence
5. Increased security options
As DSL (over ATM) links are deployed more and more extensively and popularly, it is rising
progressively to implement VLAN (VLAN-to-PVC) over DSL links and, hence, it is possible to be a
requirement of ISPs.
We discuss the implementation of VLAN-to-PVC only for bridge mode operation, i.e., the VLAN
spreads over both the COE and CPE sides, where there is no layer 3 routing involved.
Specification
1. The unit supports up to 8 active VLANs with shared VLAN learning (SVL) bridge out of 4096
possible VLANs specified in IEEE 802.1Q.
2. Each port always belongs to a default VLAN with its port VID (PVID) as an untagged member.
Also, a port can belong to multiple VLANs and be tagged members of these VLANs.
3. A port must not be a tagged member of its default VLAN.
4. If a non-tagged or null-VID tagged packet is received, it will be assigned with the default PVID
of the ingress port.
5. If the packet is tagged with non-null VID, the VID in the tag will be used.
6. The look up process starts with VLAN look up to determine whether the VID is valid. If the VID
is not valid, the packet will be dropped and its address will not be learned. If the VID is valid,
the VID, destination address, and source address lookups are performed.
7. The VID and destination address lookup determines the forwarding ports. If it fails, the packet
will be broadcasted to all members of the VLAN, except the ingress port.
8. Frames are sent out tagged or untagged depend on if the egress port is a tagged or untagged
member of the VLAN that frames belong to.
9. If VID and source address look up fails, the source address will be learned.
Frame Specification
An untagged frame or a priority-tagged frame does not carry any identification of the VLAN to
which it belongs. Such frames are classified as belonging to a particular VLAN based on
parameters associated with the receiving port. Also, priority tagged frames, which, by definition,
carry no VLAN identification information, are treated the same as untagged frames.
A VLAN-tagged frame carries an explicit identification of the VLAN to which it belongs; i.e., it
carries a tag header that carries a non-null VID. This results in a minimum tagged frame length of
68 octets. Such a frame is classified as belonging to a particular VLAN based on the value of the
VID that is included in the tag header. The presence of the tag header carrying a non-null VID
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