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.
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{Alternative port} is an active port in network with an alternative path to the root bridge. A port in alternative mode will remain active but \textit{discards} all traffic until the the current designated path fails.
\item\textbf{Backup port} is running in active mode and \textit{discards} all traffic it recieves until the current designated port on the segment the backup port is connected to, fails.
\end{itemize}
Election of ports goes in order of the following values (low is best): 1) root bridge id, 2) lowest path cost to root bridge, 3) sender bridge id, 4) sender port bridge id
\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.
\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:
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.
\textit{\textbf{NB:} beware that when working with bundled links (aka. ether-/port-channel). Then the link cost will be calculated based upon the summarized bandwidth accross all links.}
Cisco did on their part early on enhance the original spanning tree standard with some proprietary portroles that can (on cisco switch equipment) skip steps in the port role election process. And configure a STP switchport to a specific behavior as described below:
\item\cliline{switch(config)# spanning-tree backbonefast} enables the feature.
\item\textit{Scenario:} If switch needs searching new path root bridge. BackboneFast shortens process.
\begin{enumerate}
\item Switch will search for alternative path to root.
\item If BPDU recieved on blocked port. Port considered alternative path path to root.
\item If alternate path identified. RQL\footnote{\textbf{R}equest \textbf{L}ink \textbf{B}locking} packets are out for identify either A) an alternative path to the root bridge \textit{or} B) an up-/downstream switch with a path to the root bridge.
\item Beware to \underline{only} enable BPDU filter on ports connected to end hosts. Consequence if not followed \underline{can} result in creating bridging loops.
