Switchport Configurations Explained

Filed in CCIE, Switching by on November 27, 2007 6 Comments

It always helps me to think of the English translation when trying to memorize and understand some the Cisco IOS settings I think are important.

Here are some Cisco IOS switchport configurations translated into English:

switchport mode trunk‘ says: “Always trunk on this end, and I will send DTP to attempt to negotiate a trunk on the other end.”

switchport nonegotiate‘ says: “Do not send or respond to DTP from this end. Disable all DTP on this port.” (Best used on user access ports, when trunking to non-Cisco switches, when trunking to a router1, or if you are paranoid about fast convergence2)

switchport mode dynamic desirable‘ says: “Ask the other end to trunk using DTP and trunk if the negotiation succeeds. If DTP negotiation fails then become an access port.”

switchport mode dynamic auto‘ says: “If the other end asks me to be a trunk with DTP, then become a trunk, but I wont initiate any negotitation from this end. If no one asks me to become a trunk then I will become an access port.”

switchport mode access‘ says: “Never trunk on this end, and I will send out DTP to help my link partner reach the same conclusion.”

switchport trunk encapsulation‘ says: “Do not negotiate the trunk protocol with DTP. Only use the trunk protocol specified in this command (isl or dot1q).

 

[1] Cisco routers do not talk DTP

[2] The process of DTP message exchange adds some delay in negotiating and bringing up a trunk. Use ‘switchport mode trunk‘ + ‘switchport nonegotiate‘ + ‘switchport trunk encapsulation‘ for the fastest possible formation of a trunk.

 

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About the Author ()

Brad Hedlund (CCIE Emeritus #5530) is an Engineering Architect in the CTO office of VMware’s Networking and Security Business Unit (NSBU). Brad’s background in data center networking begins in the mid-1990s with a variety of experience in roles such as IT customer, value added reseller, and vendor, including Cisco and Dell. Brad also writes at the VMware corporate networking virtualization blog at blogs.vmware.com/networkvirtualization

Comments (6)

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  1. sarah says:

    Very good article ! I just want to clear one confusion , when port is configured as access port, doesn’t it disable the dtp?
    thanks a lot!

  2. Brad Hedlund says:

    Sarah,

    DTP remains on even when you configure ‘switchport mode access’, only now the DTP messages are saying “I am not a trunk, don’t even bother”.
    DTP will be disabled entirely if you add ‘switchport nonegotiate’.

    Brad

  3. Sai says:

    If i dont have switchport nonegotiate configured and if i try to configure it on a live trunk, will the trunk formation start all over again or will this be seamless?

  4. Plaridel says:

    Always a great read Brad

  5. Chennai CCNA says:

    Dear Brad,

    What is the difference between “switchport mode access” and “switchport nonegotiate” ?
    Look like both are almost similar?

    Chennai.

    • Matt says:

      “switchport mode access” means the port cannot go into trunk mode, but it will still inform the remote side, using, DTP. “switchport nonegotiate” means don’t talk DTP at all. If you have both enabled, it means “don’t become a trunk and don’t talk about it either!” So the commands serve different purposes, because you could have “switchport mode trunk” and “switchport nonegotiate” just as easily.

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