Rob,
I have already answer for you.
Yes, my Pi is running Raspbian 3.6.11+ #538
PREEMPT Fri Aug 30 20:42:08 BST 2013 armv6l
updated&upgraded as often as possible.
Line from config.gz file you're asking about is as follow:
#CONFIG_IP_MROUTE is not set
But here is my AMPRNet tunnel interface:
tunl0 Link encap:UNSPEC
inet addr: 44.165.2.2 Mask: 255.255.255.248
UP RUNNING NOARP MULTICAST MTU:1480 Metric:1
I've also 3 ax ifces, 3 nr ifces, 1 rose ifce and tun0 ifce from linux to
my JNOS.
Traffic flows in both directions IN&OUT just fine.
Best regards.
Tom - sp2lob