System Mtu - Cisco Catalyst 3550 series Software Configuration Manual

Chapter 15 Configuring 802.1Q and Layer 2 Protocol Tunneling
•
Figure 15-3 Potential Problem with 802.1Q Tunneling and Native VLANs
Switch 1
Customer A

System MTU

The default system MTU for traffic on the Catalyst 3550 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 SP 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 3550 Gigabit
Ethernet switches is 2000 bytes; the maximum system MTU for Fast Ethernet switches is 1546 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.
•
•
78-11194-09
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.
Switch 2
Packet tagged
for VLAN 30
Tunnel port
Access VLAN 40
802.1Q
trunk port
VLANs 30-40
Native VLAN 40
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.
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 2
Q = 802.1Q trunk ports
Catalyst 3550 Multilayer Switch Software Configuration Guide
Configuring 802.1Q Tunneling
Switch 4
Customer A
Tag
VLANs 30-40
removed
Native VLAN 40
Tunnel port
Switch 3
VLAN 40
Tunnel port
Access VLAN 30
Switch 5
Customer B
15-5