Network Protocol Concepts

Ethernet Frame Struncture
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Note: minimal octets for payload is 42 when 802.1Q tag exists, otherwise it is 46 octets.

Dynamic Trunking Protocol (DTP)
DTP is used by interface to automatically negotiate trunking with neighboring interface who resides in the same VTP domain. Switch interface support below trunk configurations.
Form ISL or 802.1Q trunk automatically, DTP Frame is send with DST MAC (0100.0ccc.cccc) and SNAP Protocol (0x 2004). 
swtichport mode access: interface configured to permanent nontrunking mode, however, DTP is still generated on that interface.
swtichport mode trunk: interface configured to permanent trunk mode, DTP Frames are still generated on that interface.
switchport mode dynamic desirable: configure the interface proactively negotiate with neighboring interface through DTP to convert link to Trunk mode. If the neighboring interface is configured as trunk, desirable or auto mode, link becomes trunk. if neighboring interface is configured as access or non-negotiate, link becomes non-trunking link. Dynamic desirable is the default mode for all Ethernet interfaces.
switchport mode dynamic auto: trunk link formed if neighboring interface is in trunk or desirable mode, otherwise non-trunk link will be formed.
switchport nonegotiate: DTP frames are not generated.

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To be continue