\section{Spanning Tree}

Spanning Tree exists for the \textbf{sole} reason to save "your" network and all the broadcast storms an network engineer having a bad day can by mistake create!

STP comes from the above desire where redundancy was wanted but no protocol existed before STP to help in this regard.

\begin{table}[h]
	\centering
	\caption{Spanning Tree standrds}
	\label{stpstandards}
	\resizebox{\columnwidth}{!}{%
		\begin{tabular}{|l|l|l|l|l|}
			\hline
			\textbf{} & \textbf{Standard} & \textbf{Ressource Usage} & \multicolumn{2}{l|}{\textbf{Convergence}} \\ \hline
			CST       & 802.1D            & Low                      & Slow              & All vlans             \\ \hline
			PVST+     & Cisco             & High                     & Slow              & Per vlan              \\ \hline
			RSTP      & 802.1w            & So-so (Med.)             & Fast              & All vlans             \\ \hline
			RPVST+    & Cisco             & On-the-double (V.High)   & Fast              & Per vlan              \\ \hline
			MST       & 802.1s            & Med. - High              & Fast              & Vlan list             \\ \hline
		\end{tabular}%
	}
\end{table}

\subsection{Port Roles}

When a switch is enabled for Spanning Tree. One of the following roles will have been assumed by any port on the switch in question.

\begin{itemize}
	\item \textbf{Root port:} Only 1 port on any switch (non-counting the root bridge!). Is always the port with the lowest metric (aka. best path) to the root bridge.
	\item \textbf{Designated port:} A designated port is the port on any segment closest to the root bridge and forwarding traffic.
	\item \textbf{\textit{Non}-designated port:} Put in blocking mode and not currently forwarding traffic.
	\item \textbf{Disabled port:} The port has been one-way-or-another shut down.
\end{itemize}

\subsection{Standards}

\begin{itemize}
    \item STP {\scriptsize Spanning Tree Protocol}
    \begin{itemize}
        \item Ieee 802.1D
        \item Was created in a time where bridged networks was the norm.
        \item Supports a single vlan/lan.
    \end{itemize}
    \item CST {\scriptsize Common Spanning Tree}
    \begin{itemize}
        \item An evolution of stp
        \item Cst still only supports one stp instance.
        \item But cst do thou in contrast to stp support \textit{multiple} vlans.
    \end{itemize}
    \item PVST {\scriptsize Per Vlan Spanning Tree}
    \begin{itemize}
        \item Now obsolute and succeded by pvst+
    \end{itemize}
    \item PVST+ {\scriptsize Per Vlan Spanning Tree Plus}
    \begin{itemize}
        \item Runs an instance of stp per vlan.
        \item Can guarante better utilization of available network bandwidth.
        \item Root bridge and port priorities can be configured per vlan.
    \end{itemize}
    \item RSTP {\scriptsize Rapid Spanning Tree Protocol}
    \begin{itemize}
        \item Ieee 802.1w
        \item A future development of the original 802.1D standard meant to provide faster convergance. As the original stp standard wasn't actually that fast.
    \end{itemize}
    \item RPVST+ {\scriptsize Rapid Per Vlan Spanning Tree Plus}
    \begin{itemize}
        \item A cisco implementation of rstp based upon pvst+.
    \end{itemize}
    \item MST {\scriptsize Multiple Spanning Tree}
    \begin{itemize}
        \item Originally a cisco developed protocol. Mst has since been developed as an ieee standard.
        \item Mst can as cst map multiple vlans to a single stp instance.
        \item Mst \textit{differently} than cst supports multiple stp instances.
        \item Fx. Instance 1: Vlan 1-99, Instane 2: Vlan 100-199.
    \end{itemize}
\end{itemize}

\subsection{Features}

\subsubsection{BPDU}
\textbf{B}ridge \textbf{P}rotocol \textbf{D}ata \textbf{U}nits is on cisco equipment sent out every 2 seconds and generally catogorizes into 2 categories:
\begin{itemize}
    \item \textit{Configuration} bpdu used for stp calculations and
    \item \textit{Topology change notifications} bpdus used to notify other network nodes of a change in the network.
\end{itemize}

Any network node with switchports and stp + bpdu enabled sends out bpdu packets with the ports mac as the src address. The destination mac is is designated stp multicast addr 01:80:C2:00:00:00.

\subsubsection{Root bridge}
Using a \textbf{R}oot \textbf{B}rigde as the reference point for the stp instance and calculation of root/designated/non-designated ports.\\This election process uses a pre-configured bridge priority (ranges from $0$ to $2^{16}$) (defaults to $2^{15}$). If a tie in priority is found the switch in possession of the lowest mac address wins the root bridge election.

\subsubsection{Port}

\begin{itemize}
	\item PortFart
    \begin{itemize}
        \item
    \end{itemize}
	\item UplinkFast
    \begin{itemize}
        \item
    \end{itemize}
	\item BackboneFast
    \begin{itemize}
        \item
    \end{itemize}
\end{itemize}

\subsubsection{Loop prevention}

\begin{itemize}
	\item BPDU Guard
	\item BPDU Filter
	\item Root Guard
	\item Loop Guard
\end{itemize}

\subsubsection{Link}

\begin{itemize}
	\item Unidirectional Link Detection (UDLD)
	\item FlexLinks
\end{itemize}