Planning Routing for IP Multicast Traffic


Plan a routing strategy.

  • Plan routing for IP multicast traffic.

This section covers IP multicast routing. We spend some time examining what multicast means and why knowing this information is important. To do that, we have to compare and contrast other forms of traffic traversing your LAN or WAN, which includes unicast, broadcast, and multicast.

  • Unicast ” When a single node on the network sends data to its destination node under one single packet, it knows where the destination node is and gets the data to it. Remember uni as being a single transmission.

  • Broadcast ” Broadcasting (think of the word broad ) occurs when the destination is not known or cannot be found, and that node (which needs to find the destination node) sends out packets to all nodes on the network segment to see whether it can find its destination node. This process is conducive to increased traffic, collisions on hubbed networks, and so on. Broadcasts are at times a necessary evil, but most of the time you should prevent them from happening often or control their number. Basically, broadcast traffic adds overhead to your network (bandwidth utilization) and its devices (processing packets they don't need to look at).

  • Multicast ” Multicast is the happy medium but still can cause problems. A multicast transmission is based on a group. In simple terms, think of 20 nodes on a network, 5 of which need to communicate at all times and 15 of which do not need to ever know what the other 5 are doing. Say these nodes are OSPF routers. OSPF routers send out updates to each other via a multicast, so you might ask, "What about devices that do not need to know this information?" If you use a multicast address, such as 224.0.0.5 (the all-OSPF routers multicast address) for OSPF-based networks, only those devices listen for transmissions from other nodes using this service. This reduces the problems caused by broadcast traffic and enables you to cause one sender to get information to multiple ( group ) nodes without that message going to every node on the network. Consequently, multicasting is good, but as with any other technology, it can cause problems as well or require an advanced level of administration to make it work properly. This section focuses on these issues and how they revolve around the Windows Server 2003 platform.

So now that you are familiar with the concept of multicast networks, let's consider the problems with routing such networks. Routing of multicast information is important, but first you should know why you need to route it.

EXAM TIP

Get more information For more information on this technology, you can visit the following RFCs:

  • RFC 1075 defines the Distance Vector Multicast Routing Protocol (DVMRP).

  • RFC 1584 defines the Multicast Open Shortest Path First (MOSPF) protocol.

Neither protocol is tested on the 70-293 exam; this information is provided just so you know how to plan for such protocols in your environment.


Considering multicast routing is very important because your network must be able to build packet distribution trees that allow sources to send packets to all receivers. These trees ensure that each packet on the network exists one time only and is found only on a specific network. If this limitation did not exist, you would have problems with IP multicast routing. Also, multicast routing, which is the propagation of multicast listening information, is provided by multicast routing protocols such as Distance Vector Multicast Routing Protocol (DVMRP) and Multicast Open Shortest Path First (MOSPF), which is an extension to OSPF that allows it to support IP multicasting. Such protocols ease manual configuration of VPN- and OSPF-based networks and are becoming the standard for these types of networks.

As a Windows Server 2003 administrator, you need to know this information. For example, if you set up a remote access solution with Windows Server 2003 (such as a router), this router would need to know what to do with multicast traffic that approaches it. In other words, if you were to set up a Windows Server 2003 router on a multicast network, it could participate if configured properly.

The Windows Server 2003 family does not provide any multicast routing protocols like DVMRP. Other vendors , such as Cisco, are leaders in this arena. The purpose of Microsoft's product line is to conform with industry trends, vendor-neutral functionality, and so on; therefore, it is important for the server to at least be able to "participate." How can it participate then? It can use technologies to make sure that IP multicast routing does happen, even though it may not be directly responsible for routing of such information. Windows Server 2003 handles multicast traffic and its routing by forwarding this traffic. To do this, you can use the Internet Group Management Protocol (IGMP).

IGMP, which is described in RFC 2236, is an Internet protocol that allows nodes which are configured to use it to communicate as a group, instead of individually. IGMP allows a node to report its multicast group membership (the group to which it is assigned) to adjacent routers. Multicasting allows a node to send data to multiple other nodes that have also identified themselves as being part of that group. This cuts down on broadcast traffic and bandwidth consumption.

EXAM TIP

What are multicast packets ? Multicast packets are IP-based packets. They must contain the right multicast address as the destination IP address. Remember the previous OSPF example in which the multicast address used to inform neighboring routers was 224.0.0.5. Only nodes listening for this destination address in the IP packet can receive multicast traffic.


Now that you know what IGMP does, let's look at the features you must plan and design into your infrastructure to work with Windows Server 2003, especially because it can't natively route multicast traffic based on the multicast routing protocol. If you run Windows Server 2003 in your environment to help with multicast traffic, you can set it up to use the IGMP routing protocol, IGMP router mode, and IGMP proxy mode to provide multicast forwarding in a single router-based network or when connecting a single router-based LAN to the Internet. This means that you can use a Windows Server 2003 system on your network to participate in multicast routing as a forwarding service.

With a service such as multicast forwarding, a router is able to forward multicast traffic to any network where nodes also configured to use the multicast service are listening. Only multicast-capable nodes can forward multicast traffic across an internetwork. A multicast-capable node must be able to send and receive multicast network traffic as well as be able to register the multicast address that it and other routers are listening to. Again, Windows Server 2003 is capable of doing this.

Windows Server 2003 Multicast Specifics

Windows Server 2003 enables you to use TCP/IP (refer to Chapter 2); therefore, it can work with multicast traffic. Windows Server 2003 uses a multicast forwarding table to make decisions about where to forward its incoming multicast traffic as well as to listen for IGMP report messages thus provided by the IGMP routing protocol on a single interface operating in IGMP router mode. You can plan to use the multicast routing protocol to propagate to other multicast-capable routers any information, such as multicast group listening information. Remember, though, RRAS does not provide a routing protocol for multicasting; it provides forwarding only.

The Windows Server 2003 IGMP routing protocol is used for the maintenance of entries found in the multicast forwarding table on the router. When you add this component in RRAS, plan to configure them in one of two possible ways: IGMP router mode or IGMP proxy mode.

EXAM TIP

The IGMP routing protocol Do not be confused by the IGMP routing protocol provided by Windows Server 2003. This protocol is NOT a multicast routing protocol.

The MBone The multicast-capable portion of the Internet is known as the Internet multicast backbone, or MBone . This is the place where all the multicast forwarding messages and reports go.


IGMP router mode allows you to set Windows Server 2003 to listen for IGMP Membership Report packets as well as to track group membership.

IGMP proxy mode allows you to configure your router (you must have two or more interfaces or NICs) with different settings on different interfaces ”one acting as a proxy multicast host that sends IGMP membership reports on one of its interfaces.



MCSE Windows Server 2003 Network Infrastructure (Exam 70-293)
MCSE 70-293 Exam Prep: Planning and Maintaining a Microsoft Windows Server 2003 Network Infrastructure (2nd Edition)
ISBN: 0789736500
EAN: 2147483647
Year: 2003
Pages: 151
Authors: Will Schmied

flylib.com © 2008-2017.
If you may any questions please contact us: flylib@qtcs.net