Figure 2: Port-Based Vlan Assignment; Figure 3: 802.1Q Tagging (After Port-Based Vlan Assignment); Figure 4: 802.1Q Tag Assignment - Juniper EX2500 Configuration Manual

Figure 2: Port-Based VLAN Assignment

CRC
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)

PVID = 2
Untagged memeber
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

Tagged packet
CRC Data
PVID = 2
Untagged packet
Data SA DA
B efore
Port 1 Port 2
802.1Q Switch
Port 6 Port 7
CRC
of VLAN 2
Data
SA
Outgoing
untagged packet
DA
(unchanged)
PVID = 2
Tag SA DA
B efore
Port 1 Port 2 Port 3
802.1Q Switch
Port 6 Port 7 Port 8
Untagged member
of VLAN 2
Tagged member
Port 3
of VLAN 2
CRC* Data
(*Recalculated)
Port 8
8100
16 bits
Key
Priority - User_priority
CFI - Canonical format indicator
VID - VLAN identifier
Port 1 Port 2 Port 3
802.1Q Switch
Port 6 Port 7 Port 8
Untagged member
of VLAN 2
Chapter 2: VLANs
Tagged member
of VLAN 2
BS45011A
Tag SA DA
Priority CFI VID = 2
3 bits 1 bits 12 bits
After
BS45012A
Tagged member
of VLAN 2
BS45013A
VLAN Tagging
25