802.1Q Tunneling Configuration Guidelines - Cisco WS-X6066-SLB-APC - Content Switching Module Software Manual

802.1Q Tunneling Configuration Guidelines

802.1Q Tunneling Configuration Guidelines
Follow these guidelines when configuring 802.1Q tunneling in your network:
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Catalyst 6000 Family Software Configuration Guide—Releases 6.3 and 6.4
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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 only tunnel ports to VLANs 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 non-tunnel port.
Ensure that the native VLAN of the 802.1Q trunk port in an asymmetrical link carries no traffic.
Because traffic in the native VLAN is untagged, it cannot be tunneled correctly. You must enter the
global set dot1q-all-tagged enable command to ensure that egress traffic in the native VLAN is
tagged with 802.1Q tags.
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.
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– 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 QoS for tunnel traffic.
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QoS cannot detect the received CoS value in the 802.1Q 2-byte Tag Control Information field.
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Asymmetrical links do not support the Dynamic Trunking Protocol (DTP), because only one port
on the link is a trunk. Configure the 802.1Q trunk port on an asymmetrical link with the nonegotiate
dot1q trunking keywords.
On an asymmetrical link, the Cisco Discovery Protocol (CDP) reports a native VLAN mismatch if
the VLAN of the tunnel port does not match the native VLAN of the 802.1Q trunk. The 802.1Q
tunnel feature does not require that the VLANs match. Ignore the messages if your configuration
requires nonmatching VLANs.
Jumbo frames can be tunneled as long as the jumbo frame length combined with the 802.1Q tag does
not exceed the maximum frame size.
The 802.1Q tunneling feature cannot be configured on ports configured to support:
Private VLANs
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Voice over IP (Cisco IP Phone 7960)
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The following Layer 2 protocols work between devices connected by an asymmetrical link:
CDP
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UniDirectional Link Detection (UDLD)
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Port Aggregation Protocol (PAgP)
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Chapter 7 Configuring IEEE 802.1Q Tunneling
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