8-2
Catalyst 6500 Series Switch Software Configuration Guide—Release 8.7
OL-8978-04
Chapter 8 Configuring IEEE 802.1Q Tunneling and Layer 2 Protocol Tunneling
802.1Q Tunneling Configuration Guidelines
When a tunnel port receives the tagged customer traffic from an 802.1Q trunk port, it does not strip the
received 802.1Q tag from the frame header; instead, the tunnel port leaves the 802.1Q tag intact, adds a
2-byte EtherType field (0x8100) and a 2-byte length field, and puts the received customer traffic into the
VLAN to which the tunnel port is assigned. This EtherType 0x8100 traffic, with the received 802.1Q tag
intact, is called tunnel traffic.
A VLAN that carries tunnel traffic is an 802.1Q tunnel. The tunnel ports in the VLAN are the tunnel’s
ingress and egress points.
The tunnel ports do not have to be on the same network device. The tunnel can cross the other network
links and the other network devices before reaching the egress tunnel port. A tunnel can have as many
tunnel ports as required to support the customer devices that need to communicate through the tunnel.
An egress tunnel port strips the 2-byte EtherType field (0x8100) and the 2-byte length field and transmits
the traffic with the 802.1Q tag still intact to an 802.1Q trunk port on a customer device. The 802.1Q trunk
port on the customer device strips the 802.1Q tag and puts the traffic into the appropriate customer
VLAN.
Not all switches support the standard 2-byte EtherType field (0x8100). If your switch does not support
the 2-byte EtherType field, you can connect the switch to a Gigabit Interface Converter (GBIC) or
10-Gigabit port and separate untagged IP traffic from the IP management traffic with a specified
EtherType. The untagged IP traffic is automatically assigned to the native VLAN, and the traffic with
the specified EtherType is switched to a specified VLAN.
802.1Q Tunneling Configuration Guidelines
This section provides the guidelines for configuring 802.1Q tunneling in your network:
•
Use asymmetrical links to put traffic into a tunnel or to remove traffic from a tunnel.
•
Configure tunnel ports only to form an asymmetrical link.
•
Dedicate one VLAN for each tunnel.
•
Assign tunnel ports only to VLANs that are used for tunneling.
•
Trunks require no special configuration to carry tunnel VLANs.
•
We recommend that you use ISL trunks to carry tunnel traffic between devices that do not have
tunnel ports. Because of the 802.1Q native VLAN feature, using 802.1Q trunks requires that you be
very careful when you configure tunneling. A mistake might direct tunnel traffic to a nontunnel port.
•
Because tunnel traffic retains the 802.1Q tag within the switch, the Layer 2 frame header length
imposes the following restrictions:
–
The Layer 3 packet within the Layer 2 frame cannot be identified.
–
Layer 3 and higher parameters are not identifiable in tunnel traffic (for example, Layer 3
destination and source addresses).
–
Tunnel traffic cannot be routed.
–
The switch can filter tunnel traffic using only Layer 2 parameters (VLANs and source and
destination MAC addresses).
–
The switch can provide only MAC-layer quality of service (QoS) for tunnel traffic.
–
QoS cannot detect the received class of service (CoS) value in the 802.1Q 2-byte Tag Control
Information field.
