Native Vlan Ids For Trunk Ports; Tagging Native Vlan Traffic; Allowed Vlans - Cisco 7604-RSP720C-R Configuration Manual

Information About Access and Trunk Interfaces
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Native VLAN IDs for Trunk Ports

A trunk port can carry nontagged packets simultaneously with the 802.1Q tagged packets. When you
assign a default port VLAN ID to the trunk port, all untagged traffic travels on the default port VLAN
ID for the trunk port, and all untagged traffic is assumed to belong to this VLAN. This VLAN is referred
to as the native VLAN ID for a trunk port. That is, the native VLAN ID is the VLAN that carries
untagged traffic on trunk ports.
Native VLAN ID numbers must match on both ends of the trunk.
Note
The trunk port sends an egressing packet with a VLAN that is equal to the default port VLAN ID as
untagged; all the other egressing packets are tagged by the trunk port. If you do not configure a native
VLAN ID, the trunk port uses the default VLAN.
You cannot use an FCoE VLAN as a native VLAN for an Ethernet trunk switchport.
Note

Tagging Native VLAN Traffic

The Cisco software supports the IEEE 802.1Q standard on trunk ports. In order to pass untagged traffic
through the trunk ports, you must create a VLAN that does not tag any packets (or you can use the default
VLAN). Untagged packets can pass through trunk ports and access ports.
However, all packets that enter the device with an 802.1Q tag that matches the value of the native VLAN
on the trunk are stripped of any tagging and egress the trunk port as untagged packets. This situation can
cause problems because you may want to retain the tagging on packets on the native VLAN for the trunk
port.
You can configure the device to drop all untagged packets on the trunk ports and to retain the tagging of
packets entering the device with 802.1Q values that are equal to that of the native VLAN ID. All control
traffic still passes on the native VLAN. This configuration is global; trunk ports on the device either do
or do not retain the tagging for the native VLAN.

Allowed VLANs

By default, a trunk port sends traffic to and receives traffic from all VLANs. All VLAN IDs are allowed
on each trunk. However, you can remove VLANs from this inclusive list to prevent traffic from the
specified VLANs from passing over the trunk. Later, you can add any specific VLANs that you may want
the trunk to carry traffic for back to the list.
To partition the Spanning Tree Protocol (STP) topology for the default VLAN, you can remove VLAN1
from the list of allowed VLANs. Otherwise, VLAN1, which is enabled on all ports by default, will have
a very big STP topology, which can result in problems during STP convergence. When you remove
VLAN1, all data traffic for VLAN1 on this port is blocked, but the control traffic continues to move on
the port.
See the Cisco Nexus 7000 Series NX-OS Layer 2 Switching Configuration Guide, Release 5.x, for more
Note
information about STP.
Cisco Nexus 7000 Series NX-OS Interfaces Configuration Guide, Release 5.x
3-6
Chapter 3 Configuring Layer 2 Interfaces
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