Many Cisco switches support Dynamic Trunking Protocol or DTP
this allows switches to dynamically negotiate the forming of trunks
DTP is a Cisco propriety protocol
and hence other vendors don’t support it
a lot of switches have this configured by default
which means that they can automatically negotiate
trunking with neighboring switches.
It’s, however, best practice to disable DTP and use manual configuration
2 Cisco switches can dynamically through DTP negotiate to form a trunk
and permit VLANs across the trunk automatically
it can also negotiate the protocol used such as the ISL or 802.1Q
I’ll show you demonstration of DTP in a moment
but DTP supports 2 main modes, dynamic auto and dynamic desirable.
Desirable means that the switch initiates trunking with the remote end.
Auto means that it doesn’t initiate
but if the other side initiates the trunking, trunking will be formed.
So as an example if you have 2 switches configured for dynamic desirable
a trunk will be formed between the 2 switches.
If 1 side is set up with dynamic desirable
and the other side is using dynamic auto
trunking will be formed but if both sides
using dynamic auto neither side initiates the trunking.
So the port becomes an access port.
It is better practice, however, to configure both sides manually
for trunking or configure both sides as access ports.
DTP packets are sent using CLAN 1 when ISL is used.
When 8021.Q trunking is used, DTP packets are sent on the native VLAN
this is the case even if the native VLAN is cleared from the trunk
on ISL trunks, DTP packet are sent on VLAN 1
even if VLAN 1 is cleared from the trunk.