Tips about configuring Ethernet port on Cisco switch

At first glance there is nothing special about configuring Ethernet port on Cisco switch.

However I always obey some tips to keep my network more stable and predictive.

First configure your port explicit as trunk or access. It is done mainly for security reasons (in dynamic mode anyone could connect his machine to the port and negotiate it to become trunk). On the other hand, if I connect the port to another switch I always want it to be a trunk port.

Secondly turn off DTP. If you configure your ports explicit as access/trunk then you don’t need negotiation process anymore. The behavior of DTP could be also difficult to predict. For example if you have two switches configured in different VTP domains (for any reason…) the trunk negotiation with DPT always fails.

4w2d: %DTP-SP-5-DOMAINMISMATCH: Unable to perform trunk negotiation on port Fa3/
3 because of VTP domain mismatch.

This error will occur even if your switch is in VTP transparent mode. I don’t wanna remember about such rules…

That is why I always configure my port explicitly for access/trunk mode and disable DTP.

I already mention about VTP. It is not directly related to port configuration. However it manages your VLANs. Every port is configured to operate in some VLAN(s) so in these sense VTP is related with port configuration. By default this feature is turned on (the switch is in VTP server mode) and in some cases your VLAN configuration can be changed accidentally (https://supportforums.cisco.com/discussion/12171036/stay-away-vtp). If you are not going to use VTP, then put your switch in VTP transparent mode.

Disable VTP, configure interface as access port:

win-xserw001(config)#vtp mode transparent
Device mode already VTP TRANSPARENT.
win-xserw001(config)#interface fastEthernet 0/15
win-xserw001(config-if)#switchport mode access
win-xserw001(config-if)#switchport nonegotiate

Disable VTP, configure interface as trunk port:

win-xserw001(config)#vtp mode transparent
Device mode already VTP TRANSPARENT.
win-xserw001(config)#interface fastEthernet 0/15
win-xserw001(config-if)#switchport trunk encapsulation dot1q
win-xserw001(config-if)#switchport nonegotiate
win-xserw001(config-if)#switchport mode trunk

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