The encapsulation of IP packets in most Link Layer protocols (except PPP) allows a receiver to distinguish broadcasts and multicasts from unicasts simply by examining the Link Layer protocol headers (most commonly, the Link Layer destination address). The rules in this section that refer to Link Layer broadcasts apply only to Link Layer protocols that allow broadcasts to be distinguished; likewise, the rules that refer to Link Layer multicasts apply only to Link Layer protocols that allow multicasts to be distinguished.
A router MUST NOT forward any packet that the router received as a Link Layer broadcast, unless it is directed to an IP Multicast address. In this latter case, one would presume that link layer broadcast was used due to the lack of an effective multicast service.
A router MUST NOT forward any packet which the router received as a Link Layer multicast unless the packet's destination address is an IP multicast address.
A router SHOULD silently discard a packet that is received via a Link Layer broadcast but does not specify an IP multicast or IP broadcast destination address.
When a router sends a packet as a Link Layer broadcast, the IP destination address MUST be a legal IP broadcast or IP multicast address.