A lot of dynamic routing protocols uses ip multicast for neighbor discovery, hellos and other things.
Here are some well known multicast adresses:
224.0.0.1 all hosts on a subnet
224.0.0.2 all routers on a subnet
224.0.0.4 Distance Vector Multicast Routing Protocols (DVMRP)
224.0.0.5 OSPF routers
224.0.0.6 OSPF designated routers
224.0.0.9 RIP Version 2 routers
224.0.0.10 EIGRP routers
224.0.0.13 Protocol independent Multicast (PIM) routers