diff --git a/en/chap02.tex b/en/chap02.tex
index 8d9744d..db8b2bd 100644
--- a/en/chap02.tex
+++ b/en/chap02.tex
@@ -213,7 +213,9 @@ area 0.0.0.1
 		router 201.0.113.2
 \end{lstlisting}
 
-The tree as output by BIRD has three levels, we call them top-level, level-2
+The tree as output by BIRD\footnote{The format was determined by
+experimentation and inspecting of \texttt{proto/ospf/ospf.c} in BIRD's source
+code\cite{bird-src}.} has three levels, we call them top-level, level-2
 and level-3. The top level only contains directives of form \texttt{area
 AreaID}, with the AreaID being written in the quad-dotted notation.