Sunday, November 14, 2010


We learned that Multicast allows one to send one data stream to multiple hosts. However, how a router handles such traffic is a mystery to us all. Recall that in IPv4, there are three ways to send data:-Unicast - One to One
-Broadcast - One to All
-Multicast - One to Selected Group

An example of a Multicast application is Norton Ghost. In a normal Fast Ethernet switched network comprising of 20 computers, imaging each computer with Unicast would result in a gross bandwidth of 100Mbps/20=5Mbps for each system. For each computer, a unique traffic stream is required, so 20 identical streams would be sent out if one used Unicast.

Broadcast is much better when it comes to this. When imaging with Broadcast, a single stream is sent out which is received by all computers in the network. All computers would have a gross bandwidth of 100Mbps. However, even computers that didn't need the imaging would have their links clogged.

The good thing about Multicast is that it's the best of both worlds. It allows a single stream to be split into multiple streams destined for only selected hosts. In IPv6, there no longer is Broadcast. Multicast has completely replaced it.

All Multicast communication uses UDP. Multicast addresses lie in the Class D range ( to Systems would need to subscribe to a Multicast server before it can receive the streaming data. This is analogously similar to a user tuning their radio to receive sound from a particular frequency.

The entire range is reserved by IANA. A list of current Multicast addresses can be viewed here. Here are a few well-known Multicast addresses:
- All Multicast-capable hosts in subnet (Replacement for Broadcast)
- All Multicast-capable routers in subnet
- RIPv2 Routers

Multicast Global range is from to It is originally designed for the MBone (Multicast Backbone). MBone, however, is a dead concept. Global addresses are always assigned dynamically.

The Private Multicast range goes from to It is designed for private use within an organization and can be statically assigned.

We'll first talk about IGMP. There are three major versions of IGMP and IGMPv2 is the most popular. When a client wants to look for a Multicast server, it sends an IGMP message to the router. It's then the router's job to search for the best path to the server, then subscribe to it.

In IGMPv1, servers would occasionally send Query messages to clients to check if they are still listening to the stream. Once the clients stop responding to Query messages, it would send a Stop message to the upstream routers and so on. In IGMPv2, the clients have the ability to stop the stream if they no longer need it without waiting for the Query message to time the stream out. IGMPv3 allows the clients to choose the source of the stream with the Unicast address (e.g. I want from

By default, switches treat Multicast messages the same as Broadcast. Cisco uses the proprietary CGMP (Cisco Group Management Protocol) on their routers and switches. As the stream returns from the internet, it is accompanied with a CGMP message. The CGMP message tells the switch of the Multicast MAC address of the client. This is Cisco's way of making a switch Multicast-aware without being a full Layer 3 device.

The second method is IGMP snooping which allows the switch to interpret passing IGMP messages. This adds a processing load on the switch. IGMP snooping can only be enabled on a Layer 3 Switch.

Normal routing protocols do not work for Multicast. Multicast routing, however, is heavily reliant on the Unicast table. Multicast is routed with two types of protocols: Source Trees and Shared Trees. For Multicast, we're more concerned about the source than the destination.

There is a Loop prevention mechanism known as RPF (Reverse Path Forwarding). The router simply chooses the best interface to reach the server. Any multicast packets coming in from interfaces other than the RPF-tagged interface would be dropped.

In the source tree system, every router tracks every Multicast source. Every Multicast source is mapped with a Unicast address. Every single router along the path to the clients would map it like this. Every client would also be mapped. The routers would then be able to route the packets around according to the Unicast table.

Source tree configuration is very easy to set up, but becomes extremely large if it's used in the Internet.

In the Shared tree world, one router would be elected as the RP (Rendezvous Point). The servers would all stream towards the RP, and clients would all request the stream from the RP. The Shared tree design is efficient because not all routers need to keep track of every single Multicast source, but because of the RP location some hosts may not get the best path.

The two main Multicast routing protocols in use are: PIM Dense Mode, PIM Sparse Mode. Dense Mode uses a Source Tree algorithm, while Sparse Mode uses the Shared Tree algorithm. Dense Mode is like RIP: You simply turn it on and it works.

In Dense Mode, whenever a server starts streaming, the router would flood the entire stream through the network. The routers would then send back Prune messages towards the source if there are no clients in need of the stream. Typically only a few packets get flooded, so it's not as inefficient as one might think.

PIM Sparse Mode requires setting of a Rendezvous Point. There is a mechanism known as Auto-RP to automatically propagate the RP address.

To enable Multicasting on a router, we'll need to type:
ip multicast-routing

By default, Multicast packets are dropped. You'll need to turn it on before the packets get through. We'll now need to turn on Multicast routing under specific interfaces. Here I'll use Dense Mode:
in f0/0
ip pim dense

At the "ip pim" menu, you'll be able to also run Sparse-Dense-Mode which runs both modes (e.g. Bush out AND set RP).

If you're running Sparse Mode, we'll need to go back to Global Configuration and type:
ip pim rp-address ADDRESS

You can also automatically announce that the router is RP by using:
ip pim send-rp-announce

On other routers, you'll need to type:
ip pim send-rp-discovery scope 10

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