NetworkShaw: Configure, verify, and troubleshoot interswitch connectivity

Trunk ports-

Trunk ports are used to connect layer 2 devices together (when you need to expand your network)
This video shows you how to initially configure trunk ports.

It's probably a good sign when your network needs to expand
It's also a good thing to limit broadcast traffic and control bandwidth utilization across your layer 2 links
Manually pruning the VLANs on trunk ports will help control that excess traffic

Dynamic Trunking Protocol is how switchports automatically negotiate their mode before you ever configure them
A switchport can be set to one of four modes - Dynamic Auto (default), Dynamic Desirable, Trunk or Access
The chart below shows which mode a switchport will negotiate to (in white) given the starting mode of both switches (gray)

It's good to know how it works, but I recommend just turning it off completely to maintain more control over your switches and what is connecting to your network. By turning DTP off completely, you avoid switch spoofing and VLAN hopping.

VTP is responsible for synchronizing VLAN information within a VTP domain and reduces the need to configure the same VLAN information on each switch thereby minimizing the possibility of configuration inconsistencies that arise when changes are made
Cisco proprietary protocol
Propagates vlan information to other layer 2 devices on the local area network
Switches must be in the same domain to receive updates
Switches can be in one of four modes

server - maintains vlan database and sends out updates
client - receives updates from server, modifies its vlan database, forwards on VTP messages
transparent - receives updates from server, does not modify its own vlan database, forwards on VTP messages
off - turns off VTP (added in 12.2(52)SE)

Switches by default are configured as VTP servers, but with no VTP domain or password
Switches in server mode with the highest revision number take precedence
VTP provides the following benefits:

The potential exists for a switch that is being added to the network to wipe out existing VLAN databases if the new switch is in server mode, is configured for the proper VTP domain and has a higher revision number than other servers
Switches in different domains do not share VTP information. Non-matching VTP settings might result in issues in negotiating VLAN trunks, port-channels or Virtual Port Channels.

802.1Q, or dot1q for short, is a way to tag vlans on an Ethernet frame
It is an open standard and more popularly used than Cisco's proprietary VLAN encapsulation protocol called Inter-Switch Link (ISL)
ISL is not even mentioned on the CCNA exam topics so focus on 802.1Q!
Network devices that are vlan aware can add a vlan tag to the Ethernet frame that represents the vlan membership of the frame.
Each frame must belong to only one vlan
The vlan tag is 4 bytes in length (32 bits)

Tag protocol identifier (TPID) - 16 bits - set to 0x8100 to identify the frame as IEEE 802.1Q-tagged frame
Tag control information (TCI) - 16 bits

Priority code point (PCP) - 3 bits - 802.1p class of service stating the frame's priority
Drop eligible indicator (DEI) - 1 bit - indicate frames eligible to be dropped
VLAN identifier (VID) - 12 bits - 4,094 vlans available from 0x001 to 0xFFE

On a trunk link, each frame must be distinguishable as being within exactly one VLAN
If a frame does not carry an 802.1Q VLAN tag, it is considered to be in the Native VLAN
By default the Native VLAN is 1
You should change the native vlan to an unused vlan - read about double tagging.