MD5 was never broken, but it was discovered there could be generated collisions hence weakening it.
I would suggest OSPFv3, it allows SHA or MD5.
OSPF is not as simple as RIP, it is intended for more complex networkings, but IMHO would be a very good choice complexity/authentication security.
This is a sample cisco config using SHA-512 (source)
interface GigabitEthernet 0/0
ospfv3 1 ipv4 authentication key-chain ospf-1
router ospfv3 1
address-family ipv6 unicast vrf vrf1
area 1 authentication key-chain ospf-1
area 1 virtual-link 220.127.116.11 authentication key-chain ospf-1
area 1 sham-link 18.104.22.168 authentication key-chain ospf-1
authentication mode deployment
key chain ospf-1