Ciscoswitches separated by a cloud of non-Cisco IEEE 802.1Q switches. The non-Cisco IEEE 802.1Q
cloudseparating the Cisco switches is treated as a single trunk link between the switches.
•Makesure the native VLAN for an IEEE 802.1Q trunk is the same on both ends of the trunk link. If the
nativeVLAN on one end of the trunk is different from the native VLAN on the other end, spanning-tree
loopsmight result.
•Disablingspanning tree on the native VLAN of an IEEE 802.1Q trunk without disabling spanning tree
onevery VLAN in the network can potentially cause spanning-tree loops. We recommend that you leave
spanningtree enabled on the nativeVLAN of anIEEE 802.1Q trunkor disablespanning tree on every
VLANin the network. Make sure your network is loop-free before disablingspanning tree.
Restrictions for VLAN Trunks
DynamicTrunking Protocol (DTP) is not supported on private-VLAN ports or tunnel ports.
Theswitch does not support Layer 3 trunks; you cannot configure subinterfaces or use the encapsulation
keywordon Layer 3 interfaces. The switch does support Layer 2 trunks and Layer 3 VLAN interfaces, which
provideequivalent capabilities.
Information About VLAN Trunks

Trunking Overview

Atrunk is a point-to-point link between one or more Ethernet switch interfaces and another networking device
suchas a router or a switch. Ethernet trunks carry the traffic of multiple VLANs over a single link, and you
canextend the VLANsacross an entirenetwork.
Thefollowing trunkingencapsulations are availableon all Ethernet interfaces:
•IEEE802.1Q—Industry-standard trunking encapsulation.

Trunking Modes

Ethernettrunk interfaces support different trunking modes. You can set an interface as trunking or nontrunking
orto negotiatetrunking with the neighboring interface. To autonegotiate trunking, the interfaces must be in
thesame VTP domain.
Trunknegotiation is managed by the Dynamic Trunking Protocol (DTP), which is a Point-to-Point Protocol
(PPP).However, some internetworking devices might forward DTP frames improperly, which could cause
misconfigurations.
Toavoid this, you should configure interfaces connected to devices that do not support DTP to not forward
DTPframes, that is, toturn offDTP.
•Ifyou do not intend to trunk across those links, use the switchport mode access interface configuration
commandto disable trunking.
•Toenable trunking to a device that does not support DTP,use the switchport mode trunk and switchport
nonegotiateinterface configuration commands to cause the interface to become a trunk but to not
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Configuring VLAN Trunks
Restrictions for VLAN Trunks