Reference Guide

VLANs 337
4 The egress rules define whether the frame is to be sent as tagged or
untagged.
Special-case VLANs
VLAN#1 and VLAN#4095 are special-case VLANs:
• VLAN1 — Defined as the default VLAN, and may only be used as a Ports
Default VLAN ID (PVID). This means that if the VLAN, whose VID is the
current port's PVID, is deleted from the port (or from the system), that
port's PVID is set to 1. VLAN#1 cannot be deleted from the system.
•
VLAN #4095 — Defined (according to standard and industry practice) as
the "discard" VLAN. A frame classified to this VLAN is silently dropped.
QinQ Tagging
QinQ tagging enables you to add an additional tag to previously-tagged
packets. The added tag provides a VLAN ID to each customer, which ensures
private and segregated network traffic. The VLAN ID tag is assigned to a
customer port in the service provider network. The designated port then
provides additional services to the packets with the double-tags. This enables
administrators to expand service to VLAN users.
Port Modes
Ports participating in Layer 2 switching may be classified as:
• Access Ports
P
orts set to Access mode belong to a single VLAN, whose VID is
the
curr
ently set PVID (default =1). These ports accept all untagged frames
,
and all
frames tagged with the VID, currently set
as the port's PVID. All
traffic is se
nt untagged. If the VLAN, whose VID is set as the curr
ent PVID
of
the port, is deleted from the system, or deleted from the port,
the port's
PVID
will be set to 1, meaning that the port will be made a member
of
VLAN#1,
the default VLAN.
Ingress filtering is always enabled for ports in Access mode.
Setting an Access port’s PVID to 4095 effectively shuts it down, as
no
frames will be tr
ansferred in either direction.
Access mode ports are intended to connect end-stations to the syste
m,
especi
ally when the end-stations are incapable of generating VLAN tags.