Specifications

Configuring Routing Between VLANs
Information About Routing Between VLANs
5
Figure 74 Relaying Function
The Tagging Scheme
Figure 75 shows the tagging scheme proposed by the 802.3ac standard, that is, the addition of the four
octets after the source MAC address. Their presence is indicated by a particular value of the EtherType
field (called TPID), which has been fixed to be equal to 0x8100. When a frame has the EtherType equal
to 0x8100, this frame carries the tag IEEE 802.1Q/802.1p. The tag is stored in the following two octets
and it contains 3 bits of user priority, 1 bit of Canonical Format Identifier (CFI), and 12 bits of VLAN
ID (VID). The 3 bits of user priority are used by the 802.1p standard; the CFI is used for compatibility
reasons between Ethernet-type networks and Token Ring-type networks. The VID is the identification
of the VLAN, which is basically used by the 802.1Q standard; being on 12 bits, it allows the
identification of 4096 VLANs.
After the two octets of TPID and the two octets of the Tag Control Information field there are two octets
that originally would have been located after the Source Address field where there is the TPID. They
contain either the MAC length in the case of IEEE 802.3 or the EtherType in the case of Ethernet
version 2.
54713
Port state
information
Ingress
rules
Forwarding
process
Filtering
database
Port state
information
Egress
rules
Frame
reception
Frame
transmission