spanningtree.tex: Added codeblock showing STP Bridge ID Frame

chapter/section/spanningtree.tex

@@ -377,3 +377,9 @@ It is the network admins job to propagate an even configuration to all switches
 
 \fig{spanningtree/mstregions2}{mstregions2}{MST Regions vlans mapped to different instances.}
 
+\begin{lstlisting}[language=TeX,numbers=none,frame=trBL,frameround=fttt,backgroundcolor=\color{white}]
++----------+-----------+---------+
+| Bridge   | Extended  | MAC     |
+| priority | system ID | Address |
++----------+-----------+---------+
+\end{lstlisting}