Specifications

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Catalyst 4500 Se ries S wit ch C is co IO S C om mand R efer ence —Re lease I OS XE 3 .3.0 XO(1 5.1 (1)XO)
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Chapter 2 Cisco IOS Commands for the Catalyst 4500 Series Switches
switchport trunk
• excep t 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 lesse r one first, separate d by a hyphen).
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 guidelin es and restrictions apply when using 802.1Q trunks a nd impose some
limitations on the trunking strate gy for a network:
• When connecting Cisco switc hes through an 802.1Q trunk, make sure that the native VLAN for an
802.1Q trunk is the same on b oth ends of the t runk link . If the na tive VL AN on one end of the trunk
is different from the native VLAN on the other end, spanning-tree loops might result.
• Disabling spanni ng tree on the na tive VLAN of an 802. 1Q trunk without disa bling spa nning tre e on
every VLAN in the netwo rk can ca use spanning -tree loop s. We reco mmend tha t 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 t hrough 802. 1Q trunks , the switc hes exchan ge 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 BPDU s on all other VLANs on the trunk are sent tagged to t he 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 switc h
through an 802.1Q trunk, the MST of th e non-Cisco switch and the na tive VLAN spanning tree of
the Cisco switch combine to form a singl e spanning-tree topolo gy 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 tr unk, non-C isco switche s do not reco gnize th ese frames as B PDUs and
flood them on all ports in the corresponding VLAN. Cisco switches connected to the non-Cisco
802.1Q network receive the se flooded BPDUs. Be cause Cisc o switches r eceive the f looded BPDU s,
the switches can maintain a per-VLA N spanning-tree topology a cross a network of non- Cisco
802.1Q switches. The non- Cisco 802. 1Q network separating t he Cisco swi tches is t reated as a si ngle
broadcast segment between all swit ches that are connect ed to the non-Cisco 802.1 Q network
through the 802.1Q trunks.
• Ensure that the native VLAN is the same on all of the 802.1Q tr unks co nnecting the Cisco sw itches
to the non-Cisco 802.1Q network.
• If you are connect ing multiple Cisco switches to a non-Cisco 802.1Q network, all of the connections
must be through the 802.1Q trunks. You cannot c onnect Cisco switche s to a non-Cisco 802.1Q
network through the ISL trunks or through the access ports. This action causes the switch to place
the ISL trunk port or access port into the spanning-tree “port inconsistent” state and no traffic will
pass through the port.
Follow these guidelines for native VLAN tagging:
• The no switchport trunk native vlan tag command disables th e native VLA N tagging operation on
a port. This overrides the global taggin g configuration.
• The switchport trunk native vlan tag command can be used to reenable tagging on a disabled port.