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Cisco ME 3400 EthernetAccess Switch SoftwareConfiguration Guide
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Chapter13 Configuring IEEE 802.1Q and L ayer 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 reco mmend using ISL trunks
for connecting switches in the core layer. The Cisco ME switch does not support ISL trunks.
•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 th e nat ive VLAN, a re tagg ed . If t he swi tch
is configured to tag native VLAN packets on all IEEE 802.1Q trunks, the swit ch ac cept s unt agg ed
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.
Figure13-3 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
to support frames larger than 1500 bytes by using the system mtu global configurati on 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 IEEE 802.1Q tunneling feature incr ea ses the frame
size by 4 bytes when the metro tag is added, you must configure all switc hes in the serv ice -provide r
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 1546 bytes.
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
QQ
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