Dell PowerConnect 55xx Systems User Guide 469
FILE LOCATION: C:\Users\gina\Desktop\Checkout_new\Maintenance Projects\Dell
Contax\CxUGSwitching_VLAN.fm
DELL CONFIDENTIAL â PRELIMINARY 9/11/12 - FOR PROOF ONLY
Frame Flow
Figure 14-1 describes the flow of VLAN frames from the Ingress port to the
Egress port:
Figure 14-1. Frame Flow Through a VLAN
When a frame is received, it must be assigned a VLAN. VLAN assignment is
accomplished by the following steps:
1
If the frame contains a VLAN tag, that tag is used, otherwise the frame is
classified by the port's default VLAN (PVID), if it is defined.
2
After classification, the frame may pass (if enabled) through ingress
filtering, where the frame is dropped if the frame's VLAN ID is not one of
the VLANs to which the ingress port belongs.
3
A forwarding decision is made, as a function of the VLAN ID and the
destination MAC address.
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:
â˘
VLAN1
â 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.
â˘
VLAN #4095
â Defined (according to standard and industry practice) as
the "discard" VLAN. A frame classified to this VLAN is silently dropped.
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