Cisco Switchport Modes

[kes166]

Good Afternoon,

I've been studying communications between switches and it seems relativly easy to understand with the exception of why they have so many modes. There's trunk, nonegotiate, dynamic desirable, dynamic auto, and access. (I think that's all of them). In addition to this, some switches can choose between dot1q or ISL encapsulation. So what's the purpose of all the different modes?

If the port is a trunk, set it as a trunk. If the port is access, set it to access. Is there benefits to using dynamic desirable or dynamic auto?

[rubberman]

Time for you to read the Cisco documentation. I'm sure they cover that in the installation/configuration docs for the switches in question.

[kes166]

I know what the modes are - I just don't see the point in what seems like redundancy.

• Switchport mode trunk – forces the interface to become a trunk and negotiation will occur to confirm match with the connecting device.
• Switchport mode nonegotiate – DTP is disabled on the interface. This can prevent useless network communication on access ports.
• Switchport mode dynamic desirable – asks the connecting device on the port using DTP if it wants to trunk. If it doesn’t, the port becomes an access port.
• Switchport mode dynamic auto – if a device asks this port to become a trunk, it will become a trunk. It will not ask other devices to trunk and will function as an access port until asked to trunk.
• Switchport mode access – this port will never trunk. DTP will be sent out to the connecting device to make them aware of this.

If it's an access, then do switchport mode access. If it's a trunk, do switchport mode trunk. Why waste time with the other ones other than maybe switchport mode nonegotiate to shut off DTP if it's an access port?

[CimmerianX]

It's all about automating how switches talk to each other. I'm not personally a 'fan' of letting networks do this type of thing. I mean, letting it auto detect duplexing and speed is one thing, but letting the switches determine what ports will trunk seems kindof useless and can be a bit dangerous.

For the novice, i suppose it can help with setting up trunking.... but seriously, If you can't design and manage your inter-switch trunks, you probably shouldn't be doing this job in the 1st place.

I might be a little old-school, but I like to set my networks up manually, and turn off this extra auto discovery stuff that cisco has inplemented. I like to know exactly where my switches trunk, over what ports, what VLANS are allowed over tunks, etc...