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New Member

Dynamic Trunking Protocol modes, please answer.

why trunk cannot be negotiated with both ends set to auto?

can you explain why? and how does it work,

also the no negotiate command, explain how does it disable dtp.

thank you for your answers.

9 REPLIES
Cisco Employee

Dynamic Trunking Protocol modes, please answer.

Hello,

Auto is kind of Passive trunking mode - it is waiting for DTP from other side and keep silent itself. So it kinda says "If anyone on the other side actively wanna become a trunk - I'm fine". So it is all about if

- Interface is trying to become trunk by DTP messages

- if that is not trying but fine to become trunk if other side is actively trying to

- If that does not want to become trunk at all

There are different flavors of these modes and all are dependant on the connection type. DTP is Cisco protocol so 3rd party will not support it so no sense to enable Auto or desirable on Cisco side when other is not supporting it - you can use no-negotiate and that will bring the trunk up unconditionally. No-negotiate just stops sending DTP packets and also switch is expecting not to receive any - if smth received then trunk is not formed. No-negotiate primarily for connection to switches which are not supporting DTP. If the other side is supporting it and trying to become a trunk - no-negotiate will block it.

Mode ON and Off are those which do not use DTP at all.

Check the step 12 in the following doc:

http://www.cisco.com/en/US/products/hw/switches/ps700/products_tech_note09186a00801429fb.shtml#catos

It explain all the states and various results on different negotiations.

Nik

Cisco Employee

Dynamic Trunking Protocol modes, please answer.

Hi Nik,

Mode ON and Off are those which do not use DTP at all.

A small correction here: the on mode does use DTP - in order to inform the neighboring switch that this port is statically set to trunk.

Best regards,

Peter

Cisco Employee

Re: Dynamic Trunking Protocol modes, please answer.

Correct Peter,

My bad, thx for pointing out. Indeed even though the

switchport mode trunk (mode ON) will not modify its own interface to be anything other than trunking, it will send out DTP frames to attempt to negotiate the other end of the link to be a trunking interface.

Nik

Silver

Re: Dynamic Trunking Protocol modes, please answer.

Hi David,

why trunk cannot be negotiated with both ends set to auto?

Please, see the table below. Both ends in Dynamic auto mode will not establish a trunk - in addition to form a trunk at least one end need to be at Dynamic desirable or in Trunk mode.

can you explain why? and how does it work,

With DTP enabled, both sides are periodically exchanging DTP messages. Messages are sent every second during negotiation, and every 30 seconds after that. In these messages is exchanged the information about the DTP mode of the sending port (Auto,Desirable,On).

Ports in the Dynamic auto mode are passively willing to form a trunk - that means that if only the other side is in Dynamic desirable or Trunk mode they will form a trunk.

also the no negotiate command, explain how does it disable dtp.

This command disable sending of DTP frames from a port thus preventing the link from forming a trunk dynamically.

These are DTP combinations results:

Dynamic

Auto

Dynamic

Desirable

Trunk (on)

Access (off)

Non-Negotiate (access)

Non-Negotiate (trunk)

Dynamic

Auto

Static access

Trunk

Trunk

Static access

Static access

Unexpected Results

Dynamic

Desirable

Trunk

Trunk

Trunk

Static access

Static access

Unexpected Results

Trunk (on)

Trunk

Trunk

Trunk

Unexpected Results

Unexpected Results

Trunk

Access (off)

Static access

Static access

Unexpected Results

Static access

Static access

Unexpected Results

Non-Negotiate (access)

Static access

Static access

Unexpected Results

Static access

Static access

Unexpected Results

Non-Negotiate (trunk)

Unexpected Results

Unexpected Results

Trunk

Unexpected Results

Unexpected Results

Trunk

Hope this helps. Feel free to ask!

Best regards,

Jan

New Member

Dynamic Trunking Protocol modes, please answer.

Thank you guys for answering those questions. I'm somehow enlightened by your answers.

Correct me if I'm wrong,

Before trunking occurs between two links, the two must agree or negotiate by sending/receiving DTP frames from each other. DTP frames is like a GO signal that the link is willing to form a trunk with its neighbor. So, in auto mode, the link just listens to its neighbor and waiting for DTP frames to be transmitted to form a trunk and if not, no trunking will happen. Moreover, in nonegotiate mode, since it disables the sending of DTP frames, there will be no trunking.

I have a follow up question, please explain also how the other modes work, in words that can be easily understood.

Also what is an ISL trunk?

I'm confused with these statements:

  • desirable: DTP is spoken to the neighbor switch to which you want to establish an ISL trunk. The switch with desirable configuration communicates that the switch is able to be an ISL trunk and wants the neighbor switch to also be an ISL trunk. Cisco Technical Support recommends the configuration of desirable trunking mode on the ports of the trunk link.
  • on: DTP is spoken to the neighbor switch. The on state automatically enables ISL trunking on the port, regardless of the state of the neighbor switch. The port remains an ISL trunk unless the port receives an ISL packet that explicitly disables the ISL trunk.
  • nonegotiate: DTP is not spoken to the neighbor switch. The nonegotiate state automatically enables ISL trunking on the port, regardless of the state of the neighbor switch.
  • off: There can be no use of ISL on this port, regardless of the configuration of DTP mode on the other switch port.

This is how i understand those statements,

ON mode, enables the link to trunk whatever the mode of the other side of the link. It's the boss. hehe

DESIRABLE mode is like ON mode but it's not the bossy type. It advertise itself to the neighboring switch that it wants to be a trunk but depends on the mode of the other.

I'm now confused with the statements about ISL trunk. Please help me.

Explain how other modes work thanks

Thanks a million guys,

Nik,Peter,Jan and the others who will answer my questions.

Silver

Re: Dynamic Trunking Protocol modes, please answer.

Hi David,

Thanks! I'm really glad that the information was useful .

Before trunking occurs between two links, the two must agree or  negotiate by sending/receiving DTP frames from each other. DTP frames is  like a GO signal that the link is willing to form a trunk with its  neighbor. So, in auto mode, the link just listens to its neighbor and  waiting for DTP frames to be transmitted to form a trunk and if not, no  trunking will happen. Moreover, in nonegotiate mode, since it disables  the sending of DTP frames, there will be no trunking.

Well not necessarily. DTP stands for DYNAMIC trunking protocol. But as you can see from the table of combinations trunk can be also formed without negotiation (without DTP). DTP can be off and you can still have trunks configured on your link. You just tell the port to be in trunk mode and that's it (non-negotiate trunk + non-negotiate trunk = trunk).

ISL (Inter-Switch Link Protocol) is older VLAN trunking protocol that is cisco proprietary. I have always used IEEE 802.1Q which is a standard. So this is question for the other guys . Have a nice day!

Best regards,

Jan

Cisco Employee

Re: Dynamic Trunking Protocol modes, please answer.

Jan is correct,

SO DTP is a way to exchange messages between ports to agree if the trunk should be formed or now. BUT ISL is the protocol to inform how to encapsulate the traffic whcih is being sent through the trunk. Other similar protocol is dot1q. ISL is Cisco proprietary and dot1q is standard as now mostly used as it allows to have a native VLAN and lower header thus less load on the link (plus some other differencies).

So When you bring up physical link - DTP starts to play and negotiate trunk. Once DTP finished- traffic start to flow through the link and ISL or dot1q now handles that traffic through this trunk.

Nik

Cisco Employee

Re: Dynamic Trunking Protocol modes, please answer.

Hi Jan,

Just to clarify a slightly convoluted terminology: the DTP is a protocol to negotiate a trunk connection between two switches, but it has absolutely nothing to do with encapsulating or tagging frames with their VLAN ID. That is the job of either ISL or 802.1Q once a trunk has been established - either via DTP or via static configuration (preferred).

I am glad to see you giving such informative and helpful answers here on CSC - keep up the good job you're doing!

Best regards,

Peter

EDIT: Oh, I just see Nik has already covered this. Sorry for duplicate thoughts.

Silver

Re: Dynamic Trunking Protocol modes, please answer.

Hi Nikolay and Peter,

Of course, thank you for pointing that out! I was  rather in a hurry so I didn't stress enough the fact that DTP has  absolutely nothing to do with encapsulation.

Thank you once again!

I am glad to see you giving such informative and helpful answers here on CSC - keep up the good job you're doing!

Thank you Peter! I'm honored by such words from expert in networking like you, I'll try to do my best!

Best regards,

Jan

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