30-5
Catalyst 4500 Series Switch, Cisco IOS Software Configuration Guide - Cisco IOS XE 3.9.xE and IOS 15.2(5)Ex
Chapter 30 Configuring 802.1Q Tunneling, VLAN Mapping, and Layer 2 Protocol Tunneling
Configuring 802.1Q Tunneling
System MTU
The default system MTU for traffic on the switch is 1500 bytes. You can configure the switch to support
larger frames by using the system mtu global configuration command. Because the 802.1Q tunneling
feature increases the frame size by 4 bytes when the metro tag is added, you must configure all switches
in the service provider network to be able to process larger frames by increasing the switch system MTU
size to at least 1504 bytes. The maximum allowable system MTU for Catalyst 4500 Gigabit Ethernet
switches is 9198 bytes; the maximum system MTU for Fast Ethernet switches is 1552 bytes.
802.1Q Tunneling and Other Features
Although 802.1Q tunneling works well for Layer 2 packet switching, incompatibilities exist 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 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. Customers can access the Internet through the native VLAN.
If this access is not needed, you should not configure SVIs on VLANs that include 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 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 802.1Q tunnel ports.
• Dynamic Trunking Protocol (DTP) is not compatible with 802.1Q tunneling because you must
manually configure asymmetric links with tunnel ports and trunk ports.
• Loopback detection is supported on 802.1Q tunnel ports.
• When a port is configured as an 802.1Q tunnel port, spanning-tree bridge protocol data unit (BPDU)
filtering is automatically enabled on the interface. Cisco Discovery Protocol (CDP) is automatically
disabled on the interface.
Configuring an 802.1Q Tunneling Port
To configure a port as an 802.1Q tunnel port, perform this task:
Command Purpose
Step 1
Switch# configure terminal
Enters global configuration mode.
Step 2
Switch(config)# interface
interface-id
Enters interface configuration mode and the interface to be configured as
a tunnel port. This should be the edge port in the service provider network
that connects to the customer switch. Valid interfaces include physical
interfaces and port-channel logical interfaces (port channels 1 to 64).
Step 3
Switch(config-if)# switchport
access vlan vlan-id
Specifies the default VLAN, which is used if the interface stops trunking.
This VLAN ID is specific to the particular customer.
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