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Cisco ME 3400 EthernetAccess Switch SoftwareConfiguration Guide
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Chapter11 Configuring VLANs Configuring VLAN Trunks
•To enable trunking, use the switchport mode trunk interface configuration command to change the
interface to a trunk.
IEEE 802.1Q Configuration Considerations
The IEEE 802.1Q trunks impose these limitations on the trun ki ng st rat egy for a n etwork:
•In a network of Cisco switches connected through IEEE 802.1Q trun ks, the swi tches mai nta in o ne
spanning-tree instance for each VLAN allowed on the trunks. Non-Cisco devices might suppo rt one
spanning-tree instance for all VLANs.
When you connect a Cisco switch to a non-Cisco device through an IEEE 80 2. 1Q tru nk, the C isco
switch combines the spanning-tree instance o f the V LAN o f the tr unk wit h the s pann ing -tree
instance of the non-Cisco IEEE 802.1Q switch. However, spanning-tree information for each VLAN
is maintained by Cisco switches separated by a cloud of non-Cisco IEE E 80 2. 1Q sw itch es. T he
non-Cisco IEEE 802.1Q cloud separating the Cisco switches is tr eated as a single trunk link between
the switches.
•Make sure that the native VLAN for an IEEE 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 IEEE 802.1Q trunk without disabling spanning
tree on every VLAN in the network can potentially cause spanning-tree loops. We recommend that
you leave spanning tree enabled on the native VLAN of an IEEE 802.1Q trunk or d isable spanning
tree on every VLAN in the network. Make sure that your network is loop-free before disabling
spanning tree.
Table11-4 Layer 2 Interface Modes
Mode Function
switchport mode access Puts the interface (access port) into permanent nontrunking mode an d negot ia tes to
convert the link into a nontrunk link. The interface becomes a nontrunk interface
regardless of whether or not the neighboring interface is a trunk interface. This is the
default mode.
switchport mode trunk Puts the interface into permanent trunking mode and negotiates to convert the
neighboring link into a trunk link. The interface becomes a trunk interface even if the
neighboring interface is not a trunk interface.
switchport mode dot1q-tunnel Configures the interface as a tunnel (nontrunking) port to be connecte d in an asymmetric
link with an IEEE 802.1Q trunk port. The IEEE 802.1Q tunneling is used to maintain
customer VLAN integrity across a service provider network. See Chapter 13,
“Configuring IEEE 802.1Q and Layer 2 Protocol Tunneling,” for more infor ma tion on
tunnel ports.
switchport mode private-vlan Configure the interface as a private VLAN host or promiscuous port (only NNIs can be
configured as promiscuous ports). For information about private VLANs, see Chapter 12,
“Configuring Private VLANs.”