3Com Switch 7750 Configuration Guide

90 CHAPTER 10: VLAN OVERVIEW
You can configure all the three types of ports on the same device. However, note
that you cannot directly switch a port between trunk and hybrid and you must set
the port as access before the switching. For example, to change a trunk port to
hybrid, you must first set it as access and then hybrid.
Adding an Ethernet Port
to Specified VLANs
You can add the specified Ethernet port to a specified VLAN. After that, the
Ethernet port can forward the packets of the specified VLAN, so that the VLAN on
this switch can intercommunicate with the same VLAN on the peer switch.
An access port can only be added to one VLAN, while hybrid and trunk ports can
be added to multiple VLANs.
n
The access ports or hybrid ports must be added to an existing VLAN.
Configuring the Default
VLAN ID for an Ethernet
Port
An Access port can belong to only one VLAN. Therefore, the VLAN an Access port
belongs to is also the default VLAN of the Access port. A Hybrid/Trunk port can
belong to several VLANs, and so a default VLAN ID for the port is required.
After a port is added to a VLAN and configured with a default VLAN, the port
receives and sends packets in a way related to its link type. For detailed
description, refer to Table 42, Table 43, and Table 44:
Table 42 Packet processing of an Access 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
Receive the packet and add
the default VLAN tag to the
packet.
If the VLAN ID is just the
default VLAN ID, receive the
packet.
If the VLAN ID is not the
default VLAN ID, discard the
packet.
Deprive the tag from the
packet and send the packet.
Table 43 Packet processing of a Trunk 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.
If the VLAN ID is just the
default VLAN ID, deprive the
tag and send the packet.
If the VLAN ID is not the
default VLAN ID, keep the
original tag unchanged and
send the packet.