Q-Tagging (Ieee 802.1Q) - Cisco ONS 15327 User Documentation

Chapter 9 Ethernet Operation
VLAN Support
The ONS 15327 IEEE 802.1Q-based VLAN mechanism provides logical isolation of subscriber LAN
traffic over a common SONET transport infrastructure. Each subscriber has an Ethernet port at each site,
and each subscriber is assigned to a VLAN. Although the subscriber's VLAN data flows over shared
circuits, the service appears to the subscriber as a private data transport.

9.5.1 Q-Tagging (IEEE 802.1Q)

IEEE 802.1Q allows the same physical port to host multiple IEEE 802.1Q VLANs. Each IEEE 802.1Q
VLAN represents a different logical network.
The ONS 15327 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 15327 Ethernet port does not support IEEE 802.1Q, the
ONS 15327 only uses Q-tags internally. The ONS 15327 associates these Q-tags with specific ports.
With Ethernet devices that do not support IEEE 802.1Q, the ONS 15327 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 ingress port of the ONS network. The receiving ONS node removes the Q-tag when the frame leaves
the ONS network (to prevent older Ethernet equipment from incorrectly identifying the IEEE 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 15327 uses the Q-tag attached by the external
Ethernet devices. Packets enter the ONS network with an existing Q-tag; the ONS 15327 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 9-25.
Cisco ONS 15327 User Documentation, R3.3
9-22
June 2002