Cisco Catalyst 3560-X Series, Software Configuration Manual

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1-5
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
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 1-3 Potential Problem with IEEE 802.1Q Tunneling and Native VLANs
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.
802.1Q
trunk port
VLANs 30-40
Native VLAN 40
Tunnel port
Access VLAN 30
Tunnel port
Service
provider
Tag not added
for VLAN 40
Tag
removed
VLANs 5-50
Switch D
Customer X
VLANs 30-40
Native VLAN 40
Switch B Switch C
Q Q
Switch E
Customer Y
Switch A
Customer X
Native
VLAN 40
101820
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
Tunnel port
Access VLAN 40
Packet tagged
for VLAN 30
VLAN 40