There is a version called Crab that is used for router simulation by running Quagga in multiple UMLs.
http://www.isk.kth.se/proj/crab/
Quickstart:
Download and boot the LiveCD, start a terminal and run:
xterm -title Router1 -exec /crab/.uml/kernel ubd0=/crab/.uml/Router1.cow,/crab/.uml/rootfs hostname=Router1 eth0=daemon,00:00:00:00:00:01 e0=192.168.0.1,255.255.255.0 mem=16M &
I am currently involved in creating an new router simulator that uses Xen instead of UML.