3Com Switch 7754, Configuration Manual

Page: 91 / 940

Protocol-Based VLAN 91
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CAUTION: You are recommended to set the default VLAN ID of the local Hybrid
or Trunk ports to the same value as that of the Hybrid or Trunk ports on the peer
switch. Otherwise, packet forwarding may fail on the ports.
Protocol-Based VLAN
Introduction to
Protocol-Based VLAN
Protocol-based VLAN is also known as protocol VLAN, which is another way to
classify VLANs besides port-based VLAN. Through the protocol-based VLANs, the
switch can analyze the received un-VLAN-tagged packets on the port and match
the packets with the user-defined protocol template automatically according to
different encapsulation formats and the values of the special fields. If a packet is
matched, the switch will add a corresponding VLAN tag to it automatically. Thus,
the data of the specific protocol is assigned automatically to the corresponding
VLAN for transmission.
This feature is used for binding the ToS provided in the network to VLAN to
facilitate management and maintenance.
Encapsulation Format of
Ethernet Data
This section introduces the common encapsulation formats of Ethernet data for
you to understand well the procedure for the switch to identify the packet
protocols.
Ethernet II and 802.2/802.3 encapsulation
In the link layer, there are two main packet encapsulation types: Ethernet II and
802.2/802.3, whose encapsulation formats are described in the following figures.
Ethernet II packet:
Figure 28 Ethernet II encapsulation format
802.2/802.3 standard packet:
Figure 29 802.2/802.3 standard encapsulation format
Table 44 Packet processing of a Hybrid port
Processing of an incoming packet
Processing of an outgoing
packet If the packet does not
carry a VLAN tag If the packet carries a VLAN
tag
If the port is already added
to its default VLAN, add the
default VLAN tag to the
packet and then forward the
packet.
If the port is not added to its
default VLAN, discard the
packet.
If the VLAN ID is one of the
VLAN IDs allowed to pass
through the port, receive the
packet.
If the VLAN ID is not one of
the VLAN IDs allowed to pass
through the port, discard the
packet.
Send the packet if the VLAN
ID is allowed to pass through
the port. Use the
port hybrid
vlan command to configure
whether the port keeps or
strips the tags when sending
packets of a VLAN (including
the default VLAN).
DA&SA(12) Type(2) Data
DA&SA(12) Length(2) Data DSAP(1) SSAP(1) Control(1) OUI(3) PID(2)