8-3
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
• 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.
• 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. Alternatively, you
can enter the global set dot1q-all-tagged enable command to ensure that egress traffic in the native
VLAN is tagged with 802.1Q tags.
Note See Chapter 5, “Configuring Ethernet VLAN Trunks,” for information on using the global
set dot1q-all-tagged enable command.
• 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.
• You can tunnel jumbo frames if the jumbo frame length combined with the 802.1Q tag does not
exceed the maximum frame size.
Note To set the correct maximum transmission unit (MTU) size, you must enable jumbo frames on all ports
that carry 802.1 tunnel traffic.
• You cannot configure 802.1Q tunneling on ports that are configured to support the following:
–
Private VLANs
–
Voice over IP (Cisco IP Phone 7960)
• The following Layer 2 protocols work between devices that are connected by an asymmetrical link:
–
CDP
–
UniDirectional Link Detection (UDLD)
–
Port Aggregation Protocol (PAgP)
• VLAN Trunking Protocol (VTP) does not work between the following devices:
–
Devices that are connected by an asymmetrical link
–
Devices communicating through a tunnel
Note To configure an EtherChannel as an asymmetrical link, all ports in the EtherChannel
must have the same tunneling configuration. Because the Layer 3 packet within the
Layer 2 frame cannot be identified, configure the EtherChannel to use
MAC-address-based frame distribution.
• Generic Attribute Registration Protocol (GARP) VLAN Registration Protocol (GVRP) works
between devices that communicate through a tunnel but does not work between devices that are
connected by an asymmetrical link.
• An interconnected network cannot have redundant paths to two different edge switches in an ISP.
An interconnected network can have redundant paths to the same edge switch in an ISP, but the
customer network must use Per VLAN Spanning Tree + (PVST+); it cannot be configured for
Multi-Instance Spanning Tree Protocol (MISTP) or Multiple Spanning Tree (MST). The ISP
infrastructure must use either PVST+, MISTP-PVST+, or MST-PVST+.
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