Specifications

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Cisco MWR 2941 Mobile Wireless Edge Router Release 3.5 Software Configuration Guide, Cisco IOS Release 15.1(3)MR
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Chapter 8 Configuring IEEE 802.1Q Tunneling and Layer 2 Protocol Tunneling
Understanding 802.1Q Tunneling
Note Remove the Layer 2 protocol configuration from a trunk port because incoming encapsulated packets
change that trunk port to error disabled. The outgoing encapsulated VTP (CDP and STP) packets are
dropped on that trunk.
Figure 8-2 Original (Normal), 802.1Q, and Double-Tagged Ethernet Packet Formats
When the packet enters the trunk port of the service-provider egress switch, the outer tag is again
stripped as the switch internally processes the packet. However, the metro tag is not added when the
packet is sent out the tunnel port on the edge switch into the customer network. The packet is sent as a
normal 802.1Q-tagged frame to preserve the original VLAN numbers in the customer network.
In Figure 8-1, Customer A was assigned VLAN 30, and Customer B was assigned VLAN 40. Packets
entering the edge switch tunnel ports with 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. 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. It is
possible to have multiple levels of tunneling and tagging.
Note The Cisco MWR 2941 currently supports only one level of tagging.
If traffic coming from a customer network is not tagged (native VLAN frames), these packets are bridged
or routed as normal packets. All packets entering the service-provider network through a tunnel port on
an edge switch are treated as untagged packets, whether they are untagged or already tagged with 802.1Q
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
SADA Len/Etype Data FCS
SADA Len/Etype DataEtype Ta g FCS
SADA Len/Etype DataEtype Ta g Etype Ta g FCS
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