excerpt from man ip(8):
protocol RTPROTO
the routing protocol identifier of this route.
RTPROTO may be a number or a string from the
file /etc/iproute2/rt_protos. If the routing
protocol ID is not given, ip assumes protocol
boot (i.e. it assumes the route was added by
someone who doesn't understand what they are doing).
Several protocol values have a fixed interpretation.
Namely:
redirect - the route was installed due to an ICMP redirect.
kernel - the route was installed by the kernel during
autoconfiguration.
boot - the route was installed during the bootup sequence.
If a routing daemon starts, it will purge all of them.
static - the route was installed by the administrator to
override dynamic routing. Routing daemon will
respect them and, probably, even advertise them
to its peers.
ra - the route was installed by Router Discovery protocol.
The rest of the values are not reserved and the administrator is free
to assign (or not to assign) protocol tags.