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Cisco Catalyst Blade Switch 3130 for Dell Software Configuration Guide
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Chapter16 Configuring IEEE 802.1Q and L ayer 2 Protocol Tunneling Understanding IEEE 802.1Q Tunneling
Figure16-2 Original (Normal), IEEE 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 th e c ustom er net work. The pa cket i s sent as a
normal IEEE 802.1Q-tagged frame to preserve the origina l V LAN numb er s i n t he c ust omer n etwor k.
In Figure16-1, Customer A was assigne d VLAN 30, an d Cus tome r B was assi gned V LAN 40 . Packets
entering the edge switch 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 Cu stome rs 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 c ustomer’s network are recovered. It is
possible to have multiple levels of tunneling and tagging, but the switch supports only on e level in thi s
release.
If traffic coming from a customer network is not tagged (nati ve VLAN fr ames), these pack ets are bridge d
or routed as normal packets. All packets entering the servi ce-provider network through a tunnel port on
an edge switch are treated as untagged packets, whether they are untagged or already tagged with IEEE
802.1Q headers. The packets are encapsulated with the metro tag VLAN ID (set to the access VLAN of
the tunnel port) when they are sent through the service-provide r network on an IEEE 802.1Q trunk port.
The priority field on the metro tag is set to the interface class of service (CoS) pri ority conf igu red on t he
tunnel port. (The default is zero if none is configured.)
Because 802.1Q tunneling is configured on a per-port basis, it does not matt er w het her t he sw itch is a
standalone switch or a stack member. All configuration is done on the stac k ma ster.
Double-tagged
frame in servic
e
provider
infrastructure
IEE 802.1Q frame fro
m
customer network
Original Ethernet frame
D
estination
address Length/
EtherType Frame Check
Sequence
Source
address
SADA Len/Etype Data FCS
SADA Len/Etype DataEtype Ta g FCS
SADA Len/Etype DataEtype Tag Etype Tag FCS
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