Posters at soekris-tech have reported problems with sis(4) bridged to
ral(4) where after some amount of traffic has passed, sis(4) stops
passing traffic until 'ifconfig sisX up' is issued.
Some posts linked below, but basically manualling
re-enabling the receiver at the end of interrupt processing
as done by the included diff (adapted from one for freebsd
posted to soekris-tech by peter at alastria dot net) is
reported to avoid the problem.