Cisco Systems 3750E manual System MTU, 17-5

Chapter 17 Configuring IEEE 802.1Q and Layer 2 Protocol Tunneling

Configuring IEEE 802.1Q Tunneling

These are some ways to solve this problem:

•Use ISL trunks between core switches in the service-provider network. Although customer interfaces connected to edge switches must be IEEE 802.1Q trunks, we recommend using ISL trunks for connecting switches in the core layer.

•Use the vlan dot1q tag native global configuration command to configure the edge switch so that all packets going out an IEEE 802.1Q trunk, including the native VLAN, are tagged. If the switch is configured to tag native VLAN packets on all IEEE 802.1Q trunks, the switch accepts untagged packets, but sends only tagged packets.

•Ensure that the native VLAN ID on the edge-switch trunk port is not within the customer VLAN range. For example, if the trunk port carries traffic of VLANs 100 to 200, assign the native VLAN a number outside that range.

Figure 17-3 Potential Problem with IEEE 802.1Q Tunneling and Native VLANs

										Switch D
		Tag not added				Tag			Customer X
									VLANs 30-40
		for VLAN 40			removed
									Native VLAN 40
				Service
				provider
							Tunnel port
		Switch B		VLANs 5-50			Switch C
Packet	tagged							VL	AN	40
			Q	Native		Q
for VLAN 30
Switch A		Tunnel port		VLAN 40		Tunnel port
		Access VLAN 40			Access VLAN 30
Customer X
802.1Q
trunk port
VLANs 30-40										Switch E
Native VLAN 40
				Trunk					Customer Y

Asymmetric link Correct path for traffic

Incorrect path for traffic due to misconfiguration of native VLAN by sending port on Switch B

Q = 802.1Q trunk ports

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System MTU

The default system MTU for traffic on the switch is 1500 bytes. You can configure Fast Ethernet ports on the Catalyst 3750 members in the mixed hardware switch stack to support frames larger than 1500 bytes by using the system mtu global configuration command.

You can configure 10-Gigabit and Gigabit Ethernet ports to support frames larger than 1500 bytes by using the system mtu jumbo global configuration command.

The system MTU and system jumbo MTU values do not include the IEEE 802.1Q header. Because the IEEE 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 maximum frames by adding 4 bytes to the system MTU and system jumbo MTU sizes.

Catalyst 3750-E and 3560-E Switch Software Configuration Guide

OL-9775-02 17-5