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Webcast-Catalyst9k
New Member

Question about trunk mode and port channel mode

Hi Expert,

A couple of question about the CatOS on Catalyst 4000:

(1) How do I decide the best setting for the trunk mode: on, desirable, auto, nonegotiate?

(2) Can you explain a little bit about the port channel mode desirable silent vs auto silent vs on?

Thanks a million.

3 REPLIES
Hall of Fame Super Bronze

Re: Question about trunk mode and port channel mode

1)

trunk mode on:

Puts the port into permanent trunking mode and negotiates to convert the link into a trunk link. The port becomes a trunk port even if the neighboring port does not agree to the change.

trunk mode desirable:

Makes the port actively attempt to convert the link to a trunk link. The port becomes a trunk port if the neighboring port is set to on, desirable, or auto mode.

trunk mode auto:

Enables the port to convert the link to a trunk link. The port becomes a trunk port if the neighboring port is set to on or desirable mode. This is the default mode for Fast and Gigabit Ethernet ports.

trunk mode nonegotiate:

Puts the 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.

In short, the mode selection depends on the neighboring device. They both must have compatible trunk modes.

2)

Etherchanneling desirable:

Places a port into an active negotiating state, in which the port initiates negotiations with other ports by sending PAgP packets. A channel is formed with another port group in either desirable or auto mode.

Use the silent keyword when you are connecting to a "silent partner" (a device that is not generating BPDUs or other traffic). An example of a silent partner is a traffic generator that is not transmitting packets. Use this keyword with the auto or desirable mode. If you do not specify silent or non-silent, silent is assumed.

Use the non-silent keyword when you are connecting to a device that will transmit BPDUs or other traffic. Use this keyword with the auto or desirable mode.

Etherchanneling auto:

Places a port into a passive negotiating state, in which the port responds to PAgP packets it receives but does not initiate PAgP packet negotiation. A channel is formed only with another port group in desirable mode. (Default)

Etherchanneling On:

Forces the port to channel without negotiation. PAgP packets are not exchanged. The port is channeling regardless of how the peer port is configured. If the peer port is in on mode, a channel is formed. In any other mode, the peer port is placed in the errdisable state due to a channel misconfiguration.

New Member

Re: Question about trunk mode and port channel mode

Thank you for the detailed explanation.

Here is my neighboring port setting:

switchport trunk encapsulation dot1q

switchport mode trunk

If I know for sure I will only only the port for trunking, should I just put the trunk mode on?

Hall of Fame Super Bronze

Re: Question about trunk mode and port channel mode

With IOS, once you enter

switchport mode trunk

the mode On is implied.

You can verify the mode by typing

show interface [interface_id] switchport.

HTH,

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