Redundancy could be subject to switching loops.

A switching loops form when a frame is transmitted, and ends up going back and forwards between switches.

A corrupt MAC address table could cause a switching loop.

STP is enabled by default.

STP determines a loop free path for frames, and all other ports are placed in a blocking mode.

Do not assume that the physically shortest path, is the path that STP will consider best.

Always lock up your switches!

By default, switch switch ports are on by default.

By default, switch ports are actively attempting to trunk.

All ports are in VLAN 1 by default and everyone knows that.

The first thing the switch does when it receives a frame is look at the MAC Address and adds it to the MAC Address table.

Port security only allows a “secure” source MAC Address to use the port.

To enable port security use “switchport port-security” on the interface.

If a port can possibly trunk, you cannot configure port-security on it.

When port security is in shutdown mode, it will shutdown the port when it detects an invalid MAC Address and send a message to the log and drops all violating frames. Restrict mode drops the violating frames and transmits a message to the log indicating an issue but does not shut the port down. Protect mode simply drops the violating frames.

If you configure port security with MAC address sticky, then the first source MAC address learned is the MAC address it will consider secure on that port.
Err-Disabled ports will have the LED go dark, and they must be manually re-opened with no shutdown.

=======================Spanning Tree Protocol=======================
Always try avoid a single point of failure

STP is strictly concerned with switching loops and is defined by IEEE 802.1d. It prevents switching loops from occuring by placing ports along the most desirable paths into forwarding mode, whilst ports  along less desirable paths are placed into blocking mode.

Once STP converges, every port on these paths is in either forwarding or blocking mode, making only one path available between any two destinations. Therefore a switching loop cannot occur.

STP is strictly a Layer 2 protocol and is used to prevent switching loops.

If a problem arises with the available path, STP will run the spanning-tree algorithm to recalculate the available paths and determine the best path. POrts along the enw best path will be brought out of blocking mode and into forwarding mode whilst ports along less-desirable paths are placed into blocking mode. And again, there will only be one available path.