Cisco 7604 Configuration Manual page 238

802.1Q Tunneling Configuration Guidelines and Restrictions
• Dedicate one VLAN for each tunnel.
Assign only tunnel ports to VLANs used for tunneling.
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Trunks require no special configuration to carry tunnel VLANs.
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Tunnel ports are not trunks. Any commands to configure trunking are inactive while the port is
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configured as a tunnel port.
Tunnel ports learn customer MAC addresses.
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We recommend that you use ISL trunks to carry tunnel traffic between devices that do not have
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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.
• By default, the native VLAN traffic of a dot1q trunk is sent untagged, which cannot be
double-tagged in the service provider network. Because of this situation, the native VLAN traffic
might not be tunneled correctly. Be sure that the native VLAN traffic is always sent tagged in an
asymmetrical link. To tag the native VLAN egress traffic and drop all untagged ingress traffic, enter
the global vlan dot1q tag native command.
Configure jumbo frame support on tunnel ports:
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Jumbo frames can be tunneled as long as the jumbo frame length combined with the 802.1Q tag does
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not exceed the maximum frame size.
Because tunnel traffic has the added ethertype and length field and retains the 802.1Q tag within the
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router, the following restrictions exist:
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On an asymmetrical link, the Cisco Discovery Protocol (CDP) reports a native VLAN mismatch if
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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.
Asymmetrical links do not support the Dynamic Trunking Protocol (DTP) because only one port on
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the link is a trunk. Configure the 802.1Q trunk port on an asymmetrical link to trunk unconditionally.
The 802.1Q tunneling feature cannot be configured on ports configured to support private VLANs.
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The following Layer 2 protocols work between devices connected by an asymmetrical link:
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Cisco 7600 Series Router Cisco IOS Software Configuration Guide, Release 12.2SX
17-4
See the "Configuring Jumbo Frame Support" section on page
Take note of the modules listed in the "Configuring Jumbo Frame Support" section that do not
support jumbo frames.
The Layer 3 packet within the Layer 2 frame cannot be identified in tunnel traffic.
Layer 3 and higher parameters cannot be identified in tunnel traffic (for example, Layer 3
destination and source addresses).
Because the Layer 3 addresses cannot be identified within the packet, tunnel traffic cannot be
routed.
The router can provide only MAC-layer filtering for tunnel traffic (VLAN IDs and source and
destination MAC addresses).
The router can provide only MAC-layer access control and QoS for tunnel traffic.
QoS cannot detect the received CoS value in the 802.1Q 2-byte Tag Control Information field.
CDP
UniDirectional Link Detection (UDLD)
Port Aggregation Protocol (PAgP)
Link Aggregation Control Protocol (LACP)
Chapter 17 Configuring IEEE 802.1Q Tunneling
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