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Many Cisco switches support Dynamic Trunking Protocol or DTP

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this allows switches to dynamically negotiate the forming of trunks

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DTP is a Cisco propriety protocol

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and hence other vendors don’t support it

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a lot of switches have this configured by default

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which means that they can  automatically negotiate

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trunking with neighboring switches.

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It’s, however, best practice to disable DTP and use manual configuration

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2 Cisco switches can dynamically through DTP negotiate to form a trunk

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and permit VLANs across the trunk automatically

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it can also negotiate the protocol used such as the ISL or 802.1Q

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I’ll show you demonstration of DTP in a moment

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but DTP supports 2 main modes, dynamic auto and dynamic desirable.

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Desirable means that the switch initiates trunking with the remote end.

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Auto means that it doesn’t initiate

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but if the other side initiates the trunking, trunking will be formed.

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So as an example if you have 2 switches configured for dynamic desirable

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a trunk will be formed between the 2 switches.

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If 1 side is set up with dynamic desirable

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and the other side is using dynamic auto

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trunking will be formed but if both sides

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using dynamic auto neither side initiates the trunking.

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So the port becomes an access port.

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It is better practice, however, to configure both sides manually

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for trunking or configure both sides as access ports.

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DTP packets are sent using CLAN 1 when ISL is used.

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When 8021.Q trunking is used, DTP packets are sent on the native VLAN

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this is the case even if the native VLAN is cleared from the trunk

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on ISL trunks, DTP packet are sent on VLAN 1

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even if VLAN 1 is cleared from the trunk.