  In the cops and robber games played on a simple graph $G$, Aigner and
Fromme's lemma states that one cop can guard a shortest path in the sense that
the robber cannot enter this path without getting caught after finitely many
steps. In this paper, we extend Aigner and Fromme's lemma to cover a larger
family of graphs and give metric characterizations of these graphs. In
particular, we show that a generalization of block graphs, namely vertebrate
graphs, are 1-guardable. We use this result to give the cop number of some
special class of multi-layer generalized Peterson graphs.
