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Chapter 9
Ethernet Operation
VLAN Support
9.4.1 Q-Tagging (IEEE 802.1Q)
IEEE 802.1Q allows the same physical port to host multiple 802.1Q VLANs. Each 802.1Q VLAN
represents a different logical network.
The ONS 15454 works with Ethernet devices that support IEEE 802.1Q and those that do not support
IEEE 802.1Q. If a device attached to an ONS 15454 Ethernet port does not support IEEE 802.1Q, the
ONS 15454 only uses Q-tags internally. The ONS 15454 associates these Q-tags with specific ports.
With Ethernet devices that do not support IEEE 802.1Q, the ONS 15454 takes non-tagged Ethernet
frames that enter the ONS network and uses a Q-tag to assign the packet to the VLAN associated with
the ONS network's ingress port. The receiving ONS node removes the Q-tag when the frame leaves the
ONS network (to prevent older Ethernet equipment from incorrectly identifying the 8021.Q packet as an
illegal frame). The ingress and egress ports on the ONS network must be set to Untag for the process to
occur. Untag is the default setting for ONS ports. Example #1 in
Figure 9-20
illustrates Q-tag use only
within an ONS network.
With Ethernet devices that support IEEE 802.1Q, the ONS 15454 uses the Q-tag attached by the external
Ethernet devices. Packets enter the ONS network with an existing Q-tag; the ONS 15454 uses this same
Q-tag to forward the packet within the ONS network and leaves the Q-tag attached when the packet
leaves the ONS network. Set both entry and egress ports on the ONS network to Tagged for this process
to occur. Example #2 in
Figure 9-20
illustrates the handling of packets that both enter and exit the ONS
network with a Q-tag.
For more information about setting ports to Tagged and Untag, see the
"Provision Ethernet Ports for
VLAN Membership" procedure on page
9-24.
Cisco ONS 15454 Installation and Operations Guide
9-22
November 2001
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