Illustrated TCP/IP Illustrated TCP/IP
by Matthew G. Naugle
Wiley Computer Publishing, John Wiley & Sons, Inc.
ISBN: 0471196568   Pub Date: 11/01/98  

Previous Table of Contents Next


Chapter 249
Multicast Caveats

Multicast Caveats

•  Multicast is not simply a “turn it on and it works” protocol.
•  Much thought must go into implementing a multicast network
•  There are many protocols that run with multicast.
•  Multicast requires as much thought to implement as OSPF.
•  Many different types of protocols can be implemented and some are still experimental
•  WAN networks as well as LANs must be considered.
•  Frame relay is still only partially multicast ready
•  The Internet is not multicast ready
•  There is a learning curve with multicast.

Multicast transmission and reception can take place anywhere it is enabled. There are, however, a few obstructions in the path to widespread implementation of multicast: routers and switches must be multicast ready; the backbone of your internet must be multicast ready; the WAN must be multicast ready; and so on and so on. Add to this the general lack of knowledge and ample studies on the effects of multicast and you should be able to see why implementation is slow. The Unicast forwarding Internet currently plays a part in multicast, but only as a transport between multicast-enabled networks. Tunnels can be built across the Internet using the loose source routing feature of IP. In this way, two “islands” of multicast can be connected together to provide connectivity. Otherwise, the Internet (at the time of this writing) is not multicast-enabled and probably won’t be for some years to come.

Many corporate networks have moved to frame relay as their WAN protocol, which consists of the customer device (called the CPE for customer premise equipment, usually a router) and the frame relay provider’s equipment (frame relay switches). Making the frame relay cloud multicast ready, however, is not that easy. There are many studies and tests that have to be performed to see how multicast reacts to an existing network that has many customers already enabled who expect 99.999 percent uptime. The frame relay providers will multicast-enable their WAN networks, but it may not happen for a while.

Corporate networks are not multicast-enabled either. These environments have just redone their topologies to allow for ATM and LAN switches to be employed and are now looking at multicast. Most corporate environments do not even realize that their routers are multicast-ready and believe that new equipment must be purchased before multicast is enabled. All routers support the multicast protocol of IGMP. Most routers support the multicast routing practice of DMVRP, PIM, and a few support MOSPF. These will be discussed in detail later but suffice it to say, these are software features that can be turned on, but very carefully. Multicast may be an extension of the IP protocol, but routing multicast packets is a different story.


Previous Table of Contents Next