Cisco Catalyst 4500 Series Command Reference Manual page 1129

Chapter 2 Cisco IOS Commands for the Catalyst 4500 Series Switches
Usage Guidelines The vlan_list format is all | none | [add | remove | except] vlan_atom[,vlan_atom...], where:
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The no form of the native vlan command resets the native mode VLAN to the appropriate default VLAN
for the device.
The no form of the allowed vlan command resets the list to the default list, which allows all VLANs.
The no form of the pruning vlan command resets the list to the default list, which enables all VLANs
for VTP pruning.
These configuration guidelines and restrictions apply when using 802.1Q trunks and impose some
limitations on the trunking strategy for a network:
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OL-27596 -01
all specifies all VLANs from 1 to 4094. This keyword is not supported on commands that do not
permit all VLANs in the list to be set at the same time.
none indicates an empty list. This keyword is not supported on commands that require certain
VLANs to be set or at least one VLAN to be set.
add adds the defined list of VLANs to those currently set, instead of replacing the list.
remove removes the defined list of VLANs from those currently set, instead of replacing the list.
except lists the VLANs that should be calculated by inverting the defined list of VLANs.
vlan_atom is either a single VLAN number from 1 to 4094 or a continuous range of VLANs
described by two VLAN numbers (the lesser one first, separated by a hyphen).
When connecting Cisco switches through an 802.1Q trunk, make sure that the native VLAN for an
802.1Q trunk is the same on both ends of the trunk link. If the native VLAN on one end of the trunk
is different from the native VLAN on the other end, spanning-tree loops might result.
Disabling spanning tree on the native VLAN of an 802.1Q trunk without disabling spanning tree on
every VLAN in the network can cause spanning-tree loops. We recommend that you leave spanning
tree enabled on the native VLAN of an 802.1Q trunk. If this is not possible, disable spanning tree
on every VLAN in the network. Make sure that your network is free of physical loops before
disabling spanning tree.
When you connect two Cisco switches through 802.1Q trunks, the switches exchange spanning-tree
BPDUs on each VLAN that is allowed on the trunks. The BPDUs on the native VLAN of the trunk
are sent untagged to the reserved 802.1d spanning-tree multicast MAC address
(01-80-C2-00-00-00). The BPDUs on all other VLANs on the trunk are sent tagged to the reserved
SSTP multicast MAC address (01-00-0c-cc-cc-cd).
Non-Cisco 802.1Q switches maintain only a single instance of spanning tree (MST) that defines the
spanning-tree topology for all VLANs. When you connect a Cisco switch to a non-Cisco switch
through an 802.1Q trunk, the MST of the non-Cisco switch and the native VLAN spanning tree of
the Cisco switch combine to form a single spanning-tree topology known as the CST.
Because Cisco switches transmit BPDUs to the SSTP multicast MAC address on the VLANs other
than the native VLAN of the trunk, non-Cisco switches do not recognize these frames as BPDUs and
flood them on all ports in the corresponding VLAN. Cisco switches connected to the non-Cisco
802.1Q network receive these flooded BPDUs. Because Cisco switches receive the flooded BPDUs,
the switches can maintain a per-VLAN spanning-tree topology across a network of non-Cisco
802.1Q switches. The non-Cisco 802.1Q network separating the Cisco switches is treated as a single
broadcast segment between all switches that are connected to the non-Cisco 802.1Q network
through the 802.1Q trunks.
Ensure that the native VLAN is the same on all of the 802.1Q trunks connecting the Cisco switches
to the non-Cisco 802.1Q network.
Catalyst 4500 Series Switch Cisco IOS Command Reference—Release IOS XE 3.4.0SG and IOS 15.1(2)SG)
switchport trunk
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