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Chapter 19 VLAN

IES-1248-51V User’s Guide

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across the network. A tagged frame is four bytes longer than an untagged frame

and contains two bytes of TPID (Tag Protocol Identifier, residing within the type/

length field of the Ethernet frame) and two bytes of TCI (Tag Control Information,

starts after the source address field of the Ethernet frame).

The CFI (Canonical Format Indicator) is a single-bit flag, always set to zero for

Ethernet switches. If a frame received at an Ethernet port has a CFI set to 1, then

that frame should not be forwarded as it is to an untagged port. The remaining

twelve bits define the VLAN ID, giving a possible maximum number of 4,096 (212)

VLANs. Note that user priority and VLAN ID are independent of each other. A

frame with VID (VLAN Identifier) of null (0) is called a priority frame, meaning that

only the priority level is significant and the default VID of the ingress port is given

as the VID of the frame. Of the 4096 possible VIDs, a VID of 0 is used to identify

priority frames and value 4095 (FFF) is reserved, so the maximum possible VLAN

configurations are 4,094.

The IES-1248-51V handles up to 4094 VLANs (VIDs 1-4094). The device accepts

incoming frames with VIDs 1-4094.

19.2.1 Forwarding Tagged and Untagged Frames

Each port on the device is capable of passing tagged or untagged frames. To

forward a frame from an 802.1Q VLAN-aware switch to an 802.1Q VLAN-unaware

switch, the IES-1248-51V first decides where to forward the frame and then strips

off the VLAN tag. To forward a frame from an 802.1Q VLAN-unaware switch to an

802.1Q VLAN-aware switch, the IES-1248-51V first decides where to forward the

frame, and then inserts a VLAN tag reflecting the ingress port's default VID. The

default PVID is VLAN 1 for all ports, but this can be changed.

The egress (outgoing) port(s) of a frame is determined on the combination of the

destination MAC address and the VID of the frame. For a unicast frame, the egress

port (based on the destination MAC address) must be a member of the VID, also;

otherwise, the frame is blocked. For a broadcast frame, it is duplicated only on

ports (except the ingress port itself) that are members of the VID, thus confining

the broadcast to a specific domain.

Whether to tag an outgoing frame depends on the setting of the egress port on a

per-VLAN, per-port basis (recall that a port can belong to multiple VLANs). If the

tagging on the egress port is enabled for the VID of a frame, then the frame is

transmitted as a tagged frame; otherwise, it is transmitted as an untagged frame.

TPID

Bytes

User Priority

3 Bits

CFI

1 Bit

VLAN ID

12 bits

1 2 ... 159 160 161 162 163 164 165 166 167 168 169 ... 653 654

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