System Mtu; 802.1Q Tunneling And Other Features - Cisco ME 3400 Software Configuration Manual

Chapter 13 Configuring IEEE 802.1Q Tunneling and Layer 2 Protocol Tunneling
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Figure 13-3
Switch A
Customer X

System MTU

The default system MTU for traffic on the switch is 1500 bytes. You can configure Fast Ethernet ports
to support frames larger than 1500 bytes by using the system mtu global configuration command. You
can configure Gigabit Ethernet ports to support frames larger than 1500 bytes by using the system mtu
jumbo 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 maximum frames by increasing the switch system MTU size to at least 1504 bytes.
The maximum allowable system MTU for Gigabit Ethernet interfaces is 9000 bytes; the maximum
system MTU for Fast Ethernet interfaces is 1998 bytes.

802.1Q Tunneling and Other Features

Although 802.1Q tunneling works well for Layer 2 packet switching, there are incompatibilities between
some Layer 2 features and Layer 3 switching.
Layer 3 switching is supported only when the metro IP access image is running on the switch.
Note
OL-9639-07
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.
Potential Problem with 802.1Q Tunneling and Native VLANs
Switch B
Packet tagged
for VLAN 30
Tunnel port
Access VLAN 40
802.1Q
trunk port
VLANs 30-40
Native VLAN 40
Tag not added
for VLAN 40
Service
provider
VLANs 5-50
Native
Q
VLAN 40
Trunk
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
Cisco ME 3400 Ethernet Access Switch Software Configuration Guide
Configuring 802.1Q Tunneling
Switch D
Customer X
Tag
VLANs 30-40
removed
Native VLAN 40
Tunnel port
Switch C
VLAN 40
Q
Tunnel port
Access VLAN 30
Switch E
Customer Y
13-5