Cisco 15327 - Manual (Page 137)

9-3
Ethernet Card Software Feature and Configuration Guide, R7.2
Chapter 9 Configuring IEEE 802.1Q Tunneling and Layer 2 Protocol Tunneling
Understanding IEEE 802.1Q Tunneling
Figure 9-2 Normal, IEEE 802.1Q, and IEEE 802.1Q-Tunneled Ethernet Packet Formats
When the packet enters the trunk port of the service-provider egress switch, the outer tag is again
stripped as the packet is processed internally on the switch. However, the metro tag is not added when it
is sent out the tunnel port on the edge switch into the customer network, and the packet is sent as a normal
IEEE 802.1Q-tagged frame to preserve the original VLAN numbers in the customer network.
In Figure 9-1 on page 9-2 , Customer A was assigned VLAN 30, and Customer B was assigned VLAN
40. Packets entering the ML-Series card tunnel ports with IEEE 802.1Q tags are double-tagged when
they enter the service-provider network, with the outer tag containing VLAN ID 30 or 40, appropriately,
and the inner tag containing the original VLAN number, for example, VLAN 100. Even if both
Customers A and B have VLAN 100 in their networks, the traffic remains segregated within the
service-provider network because the outer tag is different. With IEEE 802.1Q tunneling, each customer
controls its own VLAN numbering space, which is independent of the VLAN numbering space used by
other customers and the VLAN numbering space used by the service-provider network.
At the outbound tunnel port, the original VLAN numbers on the customer’s network are recovered. If
the traffic coming from a customer network is not tagged (native VLAN frames), these packets are
bridged or routed as if they were normal packets, and the metro tag is added (as a single-level tag) when
they exit toward the service provider network.
If the native VLAN (VLAN 1) is used in the service provider network as a metro tag, this tag must always
be added to the customer traffic, even though the native VLAN ID is not normally added to transmitted
frames. If the VLAN 1 metro tag is not added on frames entering the service provider network, then the
customer VLAN tag appears to be the metro tag, with disastrous results. The global configuration vlan
dot1q tag native command must be used to prevent this by forcing a tag to be added to VLAN 1.
Avoiding the use of VLAN 1 as a metro tag transporting customer traffic is recommended to reduce the
risk of misconfiguration. A best practice is to use VLAN 1 as a private management VLAN in the service
provider network.
The IEEE 802.1Q class of service (COS) priority field on the added metro tag is set to zero by default,
but can be modified by input or output policy maps.
Double-tagged
frame in service
provider
infrastructure
IEE 802.1Q frame from
customer network
Original Ethernet frame
Destination
address
Length/
EtherType
Frame Check
Sequence
Source
address
SA DA Len/Etype Data FCS
SA DA Len/Etype Data Etype Tag FCS
SA DA Len/Etype Data Etype Tag Etype Tag FCS
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