109
allows Spanning Tree to be enabled on all ports and work normally.
Some relevant terms:
-
Tagging
- The act of putting 802.1Q VLAN information into the header of a packet.
-
Untagging
- The act of stripping 802.1Q VLAN information out of the packet header.
■
802.1Q VLAN Tags
The figure below shows the 802.1Q VLAN tag. There are four additional octets inserted after the source MAC address. Their
presence is indicated by a value of
0x8100
in the Ether Type field. When a packet's Ether Type field is equal to 0x8100, the packet
carries the IEEE 802.1Q/802.1p tag. The tag is contained in the following two octets and consists of 3 bits of user priority, 1 bit of
Canonical Format Identifier (CFI - used for encapsulating Token Ring packets so they can be carried across Ethernet backbones),
and 12 bits of
VLAN ID (VID)
. The 3 bits of user priority are used by 802.1p. The VID is the VLAN identifier and is used by the
802.1Q standard. Because the VID is 12 bits long, 4094 unique VLAN can be identified.
The tag is inserted into the packet header making the entire packet longer by 4 octets. All of the information originally contained in
the packet is retained.
802.1Q Tag
User Priority
CFI
VLAN ID (VID)
3 bits
1 bit
12 bits
TPID (Tag Protocol Identifier)
TCI (Tag Control Information)
2
bytes 2
bytes
Preamble Destination
Address
Source
Address
VLAN TAG
Ethernet
Type
Data FCS
6 bytes
6 bytes
4 bytes
2 bytes
46-1500 bytes
4 bytes
The Ether Type and VLAN ID are inserted after the MAC source address, but before the original Ether Type/Length or Logical Link
Control. Because the packet is now a bit longer than it was originally, the Cyclic Redundancy Check (CRC) must be recalculated.
Adding an IEEE802.1Q Tag
Dest. Addr.
Src. Addr.
Length/E. type
Data
Old CRC
Dest. Addr.
Src. Addr.
E. type
Tag
Length/E. type
Data
New CRC
Priority
CFI
VLAN ID
Original Ethernet
New Tagged Packet
Summary of Contents for NS3702-24P-4S
Page 1: ...NS3702 24P 4S User Manual P N 1072832 REV 00 01 ISS 14JUL14 ...
Page 102: ...102 Figure 4 5 4 LACP Port Configuration Page Screenshot ...
Page 119: ...119 Figure 4 6 4 VLAN Membership Status for Static User Page Screenshot ...
Page 124: ...124 Figure 4 6 6 Private VLAN Membership Configuration page screenshot ...
Page 140: ...140 Figure 4 6 21 Group Name to VLAN Mapping Table Page Screenshot ...
Page 164: ...164 Figure 4 8 2 Multicast Flooding ...
Page 184: ...184 Figure 4 8 15 MLD Snooping Port Group Filtering Configuration Page Screenshot ...
Page 204: ...204 Figure 4 9 6 QoS Egress Port Tag Remarking Page Screenshot ...
Page 209: ...209 QoS Class QoS Class value can be any of 0 7 DPL Drop Precedence Level 0 1 ...
Page 251: ...251 Figure 4 11 3 Authentication Method Configuration Page Screenshot ...
Page 286: ...286 Figure 4 11 11 RADIUS Server Configuration Screenshot ...
Page 290: ...290 Figure 4 11 17 Add User Properties Screen Figure 4 11 18 Add User Properties Screen ...
Page 298: ...298 non committed changes will be lost ...
Page 349: ...349 Figure 4 16 2 PoE Configuration Screenshot ...