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VLAN Tagging 25
Chapter 2: VLANs
Figure 2: Port-Based VLAN Assignment
As shown in Figure 3, the untagged packet is marked (tagged) as it leaves the
switch through port 5, which is configured as a tagged member of VLAN 2. The
untagged packet remains unchanged as it leaves the switch through port 7, which is
configured as an untagged member of VLAN 2.
Figure 3: 802.1Q Tagging (after Port-Based VLAN Assignment)
In Figure 4, tagged incoming packets are assigned directly to VLAN 2 because of the
tag assignment in the packet. Port 5 is configured as a tagged member of VLAN 2,
and port 7 is configured as an untagged member of VLAN 2.
Figure 4: 802.1Q Tag Assignment
Port 6
DASADataCRC
BS45011A
Port 7 Port 8
Port 1
Port 4
Port 5
Port 2 Port 3
802.1Q Switch
PVID = 2
Untagged packet
Untagged member
of VLAN 2
Tagged member
of VLAN 2
B
e
f
o
r
e
BS45012A
Port 6 Port 7 Port 8
Port 1
Port 4
Port 5
Port 2 Port 3
802.1Q Switch
Key
Priority
CFI
VID
- User_priority
- Canonical format indicator
- VLAN identifier
PVID = 2
Tagged member
of VLAN 2
Untagged memeber
of VLAN 2
After
DA
SA
Data
CRC
(*Recalculated)
Outgoing
untagged packet
(unchanged)
DASADataCRC* Tag
VID = 2Priority
16 bits 3 bits 1 bits 12 bits
8100 CFI
Port 6
DASATagDataCRC
Tagged packet
BS45013A
Port 7 Port 8
Port 1
Port 4
Port 5
Port 2 Port 3
802.1Q Switch
PVID = 2
Untagged member
of VLAN 2
Tagged member
of VLAN 2
B
e
f
o
r
e