Ieee 802.1Q Standard - Cabletron Systems SmartSTACK ELS10-27TX Supplement Manual

Introduction

1.2.1 IEEE 802.1Q Standard

IEEE 802.1Q is a standard for virtual bridged local area networks (VLANs). It
provides an alternate method for forwarding packets through a switch. In an
802.1D (Spanning Tree) bridge, packets are forwarded in accordance with the
spanning tree as dynamically created by the 802.1D protocol (Bridge Protocol
Data Units [BPDUs]), and the spanning tree state of each port. In 802.1Q, in
addition to spanning tree, packets are forwarded in accordance with a VLAN tag
that is embedded in the packet, and the set of ports registered for that VLAN. A
set of rules is used for ingress (receipt), forwarding, and egress (transmit).
The SmartSTACK Ethernet ELS10-27TX provides support for port based
VLANs.
The ingress rules deal with the reception of tagged and untagged packets and the
decision to either drop them, or forward them to the forwarding process.
The forwarding rules deal with forwarding packets using topology restrictions,
the filtering database, queue frames (not supported), map priorities (not
supported), and recalculation of the FCS. The forward/filtering database can
contain three types of entries:
• Permanent entries
• Static entries
• Dynamic entries
Dynamic entries are created using the protocols supported by the switch (GVRP).
There will be no learning of VLANs outside of this protocol.
There is no support for GVRP with ELS10-27TX Version 1.01.00.
NOTE
GVRP will be supported in a subsequent release.
The egress rules deal with whether the destination port(s) are members of the
VLAN, and whether the VLAN tag is to be stripped. There is also an issue of
address translation on a packet that is having the VLAN tag stripped. This only
applies to multi-protocol environments, such as a combination of Ethernet,
FDDI, and Token Ring protocols and does not apply to the SmartSTACK
Ethernet ELS10-27TX.
The 802.1Q standard does not replace 802.1D. It limits the relaying of packets on
the spanning tree to a subset: the subset being the members of a particular
VLAN. This is determined by comparing the VLAN ID, which is a field within
the tag, to the ports which are members of that VLAN.
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