switchport nonegotiate Sets trunking on and disables DTP?

[vincehgov]

Hi,  I have a few questions regarding switchport trunking.

1. I'm reading this article http://www.ciscopress.com/articles/article.asp?p=29803&seqNum=3  and it says that issuing "switchport nonegotiate" sets trunking on and disables DTP, which is confusing me.  Could someone please clarify that statement for me?  I thought that nonegotiate just disables the sending of DTP messages and whether or not the port is trunking depends on whether the mode is trunk or access.

2. Please take a look at my list below and let me know if my understanding is correct.

2.1.  mode trunk = Tags outgoing packets with vlan information.  Sends DTP messages.

2.2. mode trunk + nonegotiate = Tags outgoing packets with vlan info.  Do not send DTP messages.

2.3. mode access = Does not tag outgoing packets.  Sends DTP messages.

2.4. mode access + nonegotiate = Does not tag outgoign packets. Do not send DTP messages.

2.5. mode dynamic desirable = Tags outgoing packets with vlan information if receives DTP messages from other switch indicating its mode is "trunk", "dynamic desirable", or "dynamic auto".  Sends DTP messages.

2.6. mode dynamic auto = Tags outgoing packets with vlan information if receives DTP messages from other switch indicating its mode is "trunk" or "dynamic desirable".  Sends DTP messages.

2.7. mode dynamic desirable + nonegotiate = Tags outgoing packets with vlan information if receives DTP messages from other switch indicating its mode is "trunk", "dynamic desirable", or "dynamic auto".  Does not send DTP messages.

3. Is DTP required for both sides in order for the mode dynamic options to work (see 2.7) ?  As I understand it, nonegotiate merely disables the sending of DTP messages, but the switch can still receive them.  Is this right?

[Shashank Singh · Accepted solution]

Hi Vincent,

Switchport nonegotiate puts the LAN port into permanent trunking mode but prevents the port from generating DTP frames. You must configure the neighboring port manually as a trunk port to establish a trunk link.

The port with switchport nonego does not generate DTP frames and is not expected to receive also as the other end has to be a hardcoded trunk for the link to come up. In case it does receive DTP frames, they will be trivial.

Cheers,

Shashank

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[vincehgov]

Hi Shashank, thanks for replying.

If nonegotiate is set and puts the port into permanent trunking mode, what do we do if we want to configure the port as an access port but not send DTP messages?

[Shashank Singh]

Hi Vincent,

Negotiation is a concept which is valid only for trunking. To configure a port as access port, "switchport mode access" is enough and no negotiation is required / possible. An access port cannot send DTP messages (Dynamic Trunking Protocol).

Even when a port is configured with "switchport mode access", 'switchport nonego' command is accepted under the interface. But as long as the port mode is access, switchport nonego is trivial and does not make sense.

Cheers,

Shashank

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[vincehgov]

I see what you're saying now.  By setting "switchport mode access", the switch will automatically not send DTP messages.

[Shashank Singh]

Exactly.. It is no more a trunk so Dynamic Trunking Protocol (DTP) is not even active on the port.

Cheers,

Shashank