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Default trunking mode

What will the default trunking protocol be if negotiated by dtp ? e.g. ISL or 802.1q


Re: Default trunking mode


I really appreciate your question.

Though I couldnt verify any Cisco Document saying any default protocol for the Trunk Port, though I'm certianly able to find documentation (Cisco Student Guide for BCMSN) which states that:-

1. Switchport mode trunk: The switch will use DTP (Dynamic Trunking Protocol) to determine which Trunking Protocol (i.e. ISL / Dot 1 Q) to be used for trunk.

2. Switchport mode trunk nonegotiate: Will NOT use DTP and would bring the port up only in the Trunk Mode provided the opposite end is also in the same mode.

3. Switchport mode trunk encap dot1q / isl: Will bring the port using the respective Trunking Protocol.

I hope that I have been able to answer your query, please feel to revert back if you any more queries.

Kind Regards,

Wilson Samuel

PS: Please rate if it helps.

Cisco Employee

Re: Default trunking mode

Hi Friend,

When you configure command "switchport trunk encapsulation negotiate" the interface negotiate with the neighboring interface to become an ISL (preferred) or 802.1Q trunk, depending on the configuration and capabilities of the neighboring interface.

So if you have a neighboring switch like 2950 who only supports dot1q then DTP will negotiate to dot1q but if your neighbor switch supports both ISL and Dot1q and you have not configured manually something on the neighbor switch then ISL is preferred.

I hope I answered your question.



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