No DTP but states are changed automatically

[pochtaliot]

Hi, we have a Cisco SG250-26P. This vendor, does not support DTP. But when we were doing some configuration and tests, we noticed that sometimes VLAN ports changed it's states from access to trunk. How it can be possible?

[Deepak Kumar]

Hi,

Cisco SG250-26P is supporting to the DTP and another vendor may also be understanding the DTP protocol or sending DTP packet. Share another device make and model for more information.  

[pochtaliot]

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@Deepak Kumar wrote:

Hi,

Cisco SG250-26P is supporting to the DTP and another vendor may also be understanding the DTP protocol or sending DTP packet. Share another device make and model for more information.  

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Another device is Draytek Vigor 3900 configured as access, and on another cisco switch port (also configured as access) - Ubiquiti NanoStation Loco M5 

[pochtaliot]

Can we disable any DTP on this Cisco switch?

[Deepak Kumar]

Hi,

Yes, We can disable DTP on the Cisco switch as run a command under the trunk port "switchport nonegotion".

As per my understating, If you Puts the interface (access port) into permanent nontrunking mode and negotiates to convert the link into a nontrunk link. The interface becomes a nontrunk interface, regardless of whether the neighboring interface is a trunk interface. Means if access port will receive a DTP then it will convert to the trunk port.

[paul driver]

Helllo

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@Deepak Kumar wrote:

Hi,

Yes, We can disable DTP on the Cisco switch as run a command under the trunk port "switchport nonegotion".

As per my understating, If you Puts the interface (access port) into permanent nontrunking mode and negotiates to convert the link into a nontrunk link. The interface becomes a nontrunk interface, regardless of whether the neighboring interface is a trunk interface. Means if access port will receive a DTP then it will convert to the trunk port.

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This isnt correct @Deepak Kumar switchport nonegotiate DOESN'T put a trunk port into a access mode state, it however does disable dtp on a trunkport and turns off negotiation of a trunk

Disabling DTP on a edge port is done by putting the port in a administrative mode of access mode which also truns off negotiation of a trunk

switchport mode access

[Deepak Kumar]

Hi @paul driver 

I think you didn't get my point or I failed to explain due to English :). I mentioned that he can disable DTP on a trunk port with "switchport nonegotion" command. 

Spoiler

DTP is a Cisco propriety as such other vendors may not support it - However DTP is automatically disabled when you put a port access mode 

My second paragraph was on the comment where someone mentioned that you can put switchport in access mode and it will disable the DTP. It is not correct, it will not send any DTP packet but if access port will receive a DTP then it will change from access port to trunk port.

[paul driver]

Hello

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@Deepak Kumar wrote:

My second paragraph was on the comment where someone mentioned that you can put switchport in access mode and it will disable the DTP. It is not correct, it will not send any DTP packet but if access port will receive a DTP then it will change from access port to trunk port.

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Can you elaborate on the above as my understanding this isn't correct statement !

As far as I am aware a port in an administrative mode of access (switchport mode access)- cannot go into a trunk and it will disable DTP.

[pochtaliot]

Well the problem is that I can't disable autonegotiation

(config)#int gi2

(config-if)#switchport mode access
(config-if)#switchport nonegotiate
% Wrong number of parameters or invalid range, size or characters entered

[paul driver]

Hello

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@pochtaliot wrote:

Well the problem is that I can't disable autonegotiation

(config)#int gi2

(config-if)#switchport mode access
(config-if)#switchport nonegotiate
% Wrong number of parameters or invalid range, size or characters entered

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You only need switchport mode access on a edge port to disable dtp

int x/x
switchport mode access
exit

sh int x/x switchport | in Neg

[paul driver]

Hello
DTP is a Cisco propriety as such other vendors may not support it - However DTP is automatically disabled when you put a port access mode or you apply switchport  nonegociate - So it seems to suggest your port is neither set to access mode or doesn’t have nonegociate applied.

Also nonegociate further disables dtp on trunk ports so not to advertise the vtp domain name of the switch’s  so to enable establishing a trunk between two switches in different vtp domains 

So edge ports it is suggested to disable DTP altogether so no port can possibly ever become a trunk by applying the below:
int x/x
description edge port
switchport mode access

Allow trunk port connecting to different vtp domain by disabling DTP

int x/x
description- trunkport
switchport mode trunk
switchport nonegociate

[josedelpino]

Hi pochtaliot,

I have a similar experience with switches of the small business line. The problem was caused by the "Smartport" functionality.  I will suggest that you look into it.

https://www.cisco.com/c/en/us/support/docs/smb/switches/cisco-small-business-200-series-smart-switches/smb991-smartport-properties-configuration-on-the-200-300-series-man.html

[pochtaliot]

Hi, josedelpino

That is interesting. I saw this functionality, but didn't analyse it deeply. I'll try to disable it and write you back. Thank you.