DVMRP exploits IPIP tunnels to route multicasts via Internet.
mrouted creates
IPIP tunnels listed in its configuration file automatically.
From kernel and user viewpoints there are no differences between
tunnels, created in this way, and tunnels created by ip tunnel.
I.e. if mrouted created some tunnel, it may be used to
route unicast packets, provided appropriate routes are added.
And vice versa, if administrator has already created a tunnel,
it will be reused by mrouted, if it requests DVMRP
tunnel with the same local and remote addresses.
Do not wonder, if your manually configured tunnel is destroyed, when mrouted exits.