To install blockdiag:

yum install python-reportlab python-imaging
easy_install "blockdiag[pdf]"

Generate a pdf

nwdiag -Tpdf routing.diag -f /usr/share/fonts/dejavu/DejaVuSans.ttf

Generate a SVG

nwdiag -Tsvg routing.diag

Generate a png.  (Not as pretty, actually.)

nwdiag routing.diag