When troubleshooting trunk links is important to check a few parameters such as: Trunk mode (negotiate, non-negotiate, etc), Trunk encapsulation (ISL or dot1q), Native VLAN and allowed VLANs through a trunk link. For more information refer to: http://www.cisco.com/c/en/us/td/docs/switches/datacenter/sw/42/nx-os/interfaces/configuration/guide/ifnxos/ifaccesstrunk.html#wp1407664 / - - - {enhanced explanation 20151221-zgz} - - make sure you know default port mode is DYNAMIC-AUTO, (neither hard-coded access-port or Trunk). Also remember that 2 interconnected Dynamic-Auto's will not create a Trunk-Link, - - - - - - - if one side is modified as "switchport mode dynamic desirable" this will automatically create a trunk link with an interconnected (crossover-cable) port with the default Dynamic-Auto. - - - And the default will be to trunk ALL vlans, with the default-vlan-1 automatically set as the "native" vlan which is passed without additional frame-tagging, - - - - - Also remember you could automatically create a trunk link with a default (dynamic-auto) partner by configuring one side with: "switchport mode trunk" which sets the mode to: ON (set trunking mode to TRUNK unconditionally)