Specifications

Configuring Routing Between VLANs
Information About Routing Between VLANs
6
Figure 75 Tagging Scheme
The EtherType and VLAN ID are inserted after the MAC source address, but before the original
Ethertype/Length or Logical Link Control (LLC). The 1-bit CFI included a T-R Encapsulation bit so that
Token Ring frames can be carried across Ethernet backbones without using 802.1H translation.
Frame Control Sequence Recomputation
Figure 76 shows how adding a tag in a frame recomputes the Frame Control Sequence. 802.1p and
802.1Q share the same tag.
Figure 76 Adding a Tag Recomputes the Frame Control Sequence
Native VLAN
Each physical port has a parameter called PVID. Every 802.1Q port is assigned a PVID value that is of
its native VLAN ID (default is VLAN 1). All untagged frames are assigned to the LAN specified in the
PVID parameter. When a tagged frame is received by a port, the tag is respected. If the frame is untagged,
the value contained in the PVID is considered as a tag. Because the frame is untagged and the PVID is
tagged to allow the coexistence, as shown in Figure 77, on the same pieces of cable of VLAN-aware
bridge/stations and of VLAN-unaware bridges/stations. Consider, for example, the two stations
54712
Destination address
Source address
EtherType = 0x8100
Tag control information
MAC length/type
Data
VID (VLAN ID) - 12 bits
User
priority
CFI
PA D
FCS
6
6
2
2
2
Variable
4
54711
Dest FCSSrc Len/Etype Data
Dest
PRI VLAN ID
Token ring encapsulation flag
Tagged
frame
Original
frame
(VLAN ID and
TR encapsulations
are 802.1Q,
not 802.1p)
Src Etype Len/EtypeTa g FCSData