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Catalyst 3750-X and 3560-X Switch Software Configuration Guide
OL-25303-03
Chapter 1 Configuring IEEE 802.1Q and Layer 2 Protocol Tunneling
Configuring IEEE 802.1Q Tunneling
For example, the switch supports a maximum frame size of 1496 bytes with one of these configurations:
• The switch has a system jumbo MTU value of 1500 bytes, and the switchport mode dot1q tunnel
interface configuration command is configured on a 10-Gigabit or Gigabit Ethernet switch port.
• The Catalyst 3750 member has a system MTU value of 1500 bytes, and the switchport mode dot1q
tunnel interface configuration command is configured on a Fast Ethernet port of the member.
For information about the maximum allowable system MTU on 10-Gigabit and Gigabit Ethernet
interfaces, see Table 1-5 on page 1-44.
IEEE 802.1Q Tunneling and Other Features
Although IEEE 802.1Q tunneling works well for Layer 2 packet switching, there are incompatibilities
between some Layer 2 features and Layer 3 switching.
• A tunnel port cannot be a routed port.
• IP routing is not supported on a VLAN that includes IEEE 802.1Q ports. Packets received from a
tunnel port are forwarded based only on Layer 2 information. If routing is enabled on a switch
virtual interface (SVI) that includes tunnel ports, untagged IP packets received from the tunnel port
are recognized and routed by the switch. Customer can access the internet through its native VLAN.
If this access is not needed, you should not configure SVIs on VLANs that include tunnel ports.
• Fallback bridging is not supported on tunnel ports. Because all IEEE 802.1Q-tagged packets
received from a tunnel port are treated as non-IP packets, if fallback bridging is enabled on VLANs
that have tunnel ports configured, IP packets would be improperly bridged across VLANs.
Therefore, you must not enable fallback bridging on VLANs with tunnel ports.
• Tunnel ports do not support IP access control lists (ACLs).
• Layer 3 quality of service (QoS) ACLs and other QoS features related to Layer 3 information are
not supported on tunnel ports. MAC-based QoS is supported on tunnel ports.
• EtherChannel port groups are compatible with tunnel ports as long as the IEEE 802.1Q
configuration is consistent within an EtherChannel port group.
• Port Aggregation Protocol (PAgP), Link Aggregation Control Protocol (LACP), and UniDirectional
Link Detection (UDLD) are supported on IEEE 802.1Q tunnel ports.
• Dynamic Trunking Protocol (DTP) is not compatible with IEEE 802.1Q tunneling because you must
manually configure asymmetric links with tunnel ports and trunk ports.
• VLAN Trunking Protocol (VTP) does not work between devices that are connected by an
asymmetrical link or devices that communicate through a tunnel.
• Loopback detection is supported on IEEE 802.1Q tunnel ports.
• When a port is configured as an IEEE 802.1Q tunnel port, spanning-tree bridge protocol data unit
(BPDU) filtering is automatically enabled on the interface. Cisco Discovery Protocol (CDP) and the
Layer Link Discovery Protocol (LLDP) are automatically disabled on the interface.
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