Cisco Catalyst 4500 Series Software Configuration Manual page 874

Configuring 802.1Q Tunneling
VLAN 40 is configured as the native VLAN for the 802.1Q trunk port from Customer A at the ingress
edge switch in the service provider network (Switch 2). Switch 1 of Customer A sends a tagged packet
on VLAN 30 to the ingress tunnel port of Switch 2 in the service provider network, which belongs to
access VLAN 40. Because the access VLAN of the tunnel port (VLAN 40) is the same as the native
VLAN of the edge-switch trunk port (VLAN 40), the metro tag is not added to tagged packets received
from the tunnel port. The packet carries only the VLAN 30 tag through the service provider network to
the trunk port of the egress-edge switch (Switch 3) and is misdirected through the egress switch tunnel
port to Customer B.
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 802.1Q trunks, we recommend using ISL trunks for
connecting switches in the core layer.
Use the switchport trunk native vlan tag per-port command and the vlan dot1q tag native global
•
configuration command to configure the edge switch so that all packets going out an 802.1Q trunk,
including the native VLAN, are tagged. If the switch is configured to tag native VLAN packets on
all 802.1Q trunks, the switch ensures that all packets exiting the trunk are tagged and prevents the
reception of untagged packets on the trunk port.
• 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 30-3
Switch 1
Customer A
Catalyst 4500 Series Switch, Cisco IOS Software Configuration Guide - Cisco IOS XE 3.9.xE and IOS 15.2(5)Ex
30-4
Chapter 30
Potential Problem with 802.1Q Tunneling and Native VLANs
Tag not added
for VLAN 40
Switch 2
Packet tagged
Q
for VLAN 30
Tunnel port
Access VLAN 40
802.1Q
trunk port
VLANs 30-40
Native VLAN 40
Configuring 802.1Q Tunneling, VLAN Mapping, and Layer 2 Protocol Tunneling
Tag
removed
Service
provider
Tunnel port
VLANs 5-50 Switch 3
Native
VLAN 40
Tunnel port
Access VLAN 30
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
Switch 4
Customer A
VLANs 30-40
Native VLAN 40
VLAN 40
Switch 5
Customer B