Multicast Administrative Scoping

Routing loops must be avoided in IP multicast networks. Because multicast routers must replicate packets for each downstream branch, not only do looping packets not arrive at a destination, but each pass around the loop multiplies the number of looping packets, eventually overwhelming the network.

Scoping limits the routers and interfaces that can be used to forward a multicast packet. Scoping can use the time-to-live (TTL) field in the IP packet header, but TTL scoping depends on intimate knowledge of the network topology by the network administrator. This topology can change as links fail and are restored, making TTL scoping a poor solution for multicast.

Multicast scoping is administrative in the sense that a range of multicast addresses is reserved for scoping purposes, as described in RFC 2365. Routers at the boundary must be able to filter multicast packets and make sure the packets do not stray beyond the established limit.

Administrative scoping is much better than TTL scoping, but in many cases the dropping of administratively scoped packets is still determined by the network administrator. For example, the multicast address range 239/8 is defined in RFC 2365 as administratively scoped, and packets using this range are not to be forwarded beyond a network “boundary,” usually a routing domain. But only the network administrator knows where the border routers are and can implement the scoping correctly.

Multicast groups used by unicast routing protocols, such as 224.0.0.5 for all Open Shortest Path First (OSPF) routers, are administratively scoped for that LAN only. This scoping allows the same multicast address to be used without conflict on every LAN running OSPF.