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In general, there are two port settings of connecting switches to some network device. The switch port settings can be either access or trunk port. When switch port is set as access port, then the switch considers the connected network device as non-switch, or to be specific is unable to understand BPDU (Bridge Protocol Data Unit). When switch port is set as trunk port, then the switch considers the connected network device as switch/bridge, or to be specific is able to speak and read BPDU.
BPDU was originally developed at times of LAN bridge introduction into networking. Back then, there was no concept of (switch) trunk port capable of carrying multiple VLAN information. The LAN bridge was developed to only deal with single broadcast domain (read: one VLAN), carried only Bridge ID and MAC addresses to determine Spanning Tree network setup.
At a time when switch trunk technology was introduced, a switch had to have certain mechanism to ensure the other end of connectivity had to have ability to acknowledge the multiple VLAN information passed through trunk port. BPDU was then used to also incorporate multiple VLAN information. In Cisco PVST for instance, switches that produce BPDU carrying multiple VLAN info could setup different Spanning Tree network design for each VLAN. Further info on this can be found in following links.
Inter-Switch Link and IEEE 802.1Q Frame Format
STP and MST
How BPDU is transmitted with Native VLAN for PVST and MSTP
As rough understanding, BPDU is now also what bridges (switches) use to pass VLAN information. In switch access port setting, the switch assumes no BPDU communication between the switch and the connected network device. In other words, the switch considers the connected network device as dumb device or regular host that need switch intelligence to determine VLAN information. By nature, the switch intelligence decides to set connected network device within the same VLAN. This nature behavior sets such access switch port to act as "old-school" bridge that carries just one VLAN.
In switch trunk setting, the switch assumes there is BPDU communication between the switch and the connected network device. This BPDU communication includes of pass multiple VLAN information. Through this trunk port, multiple VLAN can pass through simultaneously. This behavior sets such trunk port to act as multiple "old-school" bridge (or "modern" switch) that carries multiple VLAN.
In load balance or active-standby scenario, there is a need to connect a switch with some network device using multiple switch port. In a case of one connection breaks or non-function, there are still other connections to pass through traffic.
When the connected network device is non-BPDU speaking, such setup is generally set as active-standby scenario. As an illustration, a server with dual NIC without ability to understand BPDU may connect one NIC to one switch port and connect another NIC to another switch port. Server sets one NIC as active and another NIC as standby.
You may notice that such multiple connection in Layer-2 network potentially creates a loop (Spanning Tree loop) when the connected network device is BPDU speaking. To avoid the loop, those multiple connections are bundled into one virtual connection. In Cisco implementation, such virtual connection is called Port Channel.
Keep in mind that Port Channel can bundle either multiple access or multiple trunk ports. When multiple access ports are bundled into one Port Channel, this Port Channel pass through just one VLAN in active-standby or load-balance scenario. When multiple trunk ports are bundled into one Port Channel, this Port Channel has ability to pass through multiple VLAN in active-standby or load-balance scenario.
When the connected network device is BPDU-speaking, the setup of multiple connection might be set as load-balance scenario. As illustration, a server with dual NIC that understand BPDU connect both NIC into the same switch. Note that the NIC setting could be in access or trunk mode. Whichever the mode is set on the NIC, make sure that the switch ports are set the same.
More on Switching
Virtual LANs and VLAN Trunking
VLANs - Tagging ALL Traffic Implies ISL
ISL = No Native VLAN
»Cisco Forum FAQ »Switch and VLAN Management Best Practice
»[Config] Port Channels with different distance links
»Take 4 vlans in a port and pass each vlan to its own port
»Configuring Trunking Between ESXi 5 server and CISCO Switch
»VLANs from a Vswitch config Teamed Port
»2950 channel-protocol lacp pagp
»Ether Channel Help