Native Vlans - Cisco Catalyst 4500 series Administration Manual

Configuring 802.1Q Tunneling

Native VLANs

When configuring 802.1Q tunneling on an edge switch, you must use 802.1Q trunk ports for sending
packets into the service provider network. However, packets going through the core of the service
provider network can be carried through 802.1Q trunks, ISL trunks, or nontrunking links. When 802.1Q
trunks are used in these core switches, the native VLANs of the 802.1Q trunks must not match any native
VLAN of the nontrunking (tunneling) port on the same switch because traffic on the native VLAN is not
tagged on the 802.1Q sending trunk port
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:
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Software Configuration Guide—Release IOS XE 3.6.0E and IOS 15.2(2)E
29-4
Chapter 29
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.
Configuring 802.1Q Tunneling, VLAN Mapping, and Layer 2 Protocol Tunneling
(Figure 29-3).
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