Core issue
This issue occurs in a scenario where two Cisco Catalyst switches in two different VTP domains are connected together via a cross-over cable and you see a link light, but no traffic passes at the Layer 2 level between the switches.
A trunk link can be negotiated between two switches only if these switches belong to the same VTP management domain or if one or both switches have not defined their VTP domain (that is the NULL domain).
Dynamic Trunking Protocol (DTP) sends the VTP domain name in a DTP packet. Therefore, if you have two ends of a link that belong to different VTP domains, the trunk does not come up if you use DTP.
Note: The VTP domain name is case sensitive. 'CISCO' and 'cisco' are two different domain names.
Resolution
If two switches are in different VTP domains and trunking is desired between them, you must configure the trunk mode as ON or NONEGOTIATE on both sides in order to allow the trunk to come up without DTP negotiation agreement. In other words, this setting forces the link to be a trunk link.
The switchport nonegotiate command specifies that the device does not engage in a negotiation protocol on an interface.
If the switch is running CatOS, then issue the set trunk nonegotiate command.
For more information, refer to the Data Traffic Blocked between VTP Domains section of Understanding and Configuring VLAN Trunk Protocol (VTP).
