Multicast Architectures
Pages: 1, 2

Routing Multicast

The problem IP multicast routing resolves is the sending of a single IP packet to multiple hosts. The operative mechanism, in multicast, is to send the packet to an IP multicast group. An IP multicast group is uniquely identified by a class D IP address. Routers typically use Internet Group Management Protocol (IGMP) to handle multicast traffic. Other multicast routing protocols exist; IGMP is the most prevalent.

IGMP

Routers and hosts use IGMP messaging to start, manage, and stop multicast services. A host will send an IGMP join-group message to request multicast services. If the group is active, the router will forward the desired multicast group packets to the interface the join request was detected on. The complementary message to join is the IGMP leave message. A host sends a message to leave a group when it no longer desires to receive the multicast services of that specific group.

A multicast service may not be localized to the router. The router maintains multicast route tables of groups it has learned about and learns about groups from routing protocols.

PIM

The router and the hosts use IGMP to send messages on group states. A router uses its routing protocol to learn routes and build routing tables. Protocol Independent Multicast (PIM) uses the routing table information to manage the multicast packet forwarding. PIM does not maintain an explicit multicast routing table. It relies on the router's routing table for forwarding multicast packets.

Configuring the Sprockets' Multicast Network

Server Load Balancing By Tony Bourke August 2001 0-596-00050-2, Order Number: 0502 192 pages, $34.95

Let’s take a look at Nanna Spacely’s multicast solution for the Sprockets’ network. For the moment, Nanna only intends to route information for multicast group 239.6.1.11 in the manufacturing network. Nanna later plans to add voice-over-ip and multimedia data streams which will require a complex multicast network. At first, Nanna is just going to address the needs of the manufacturing floor and enable IP multicast for the CNC robots.

Using Cisco routers, Nanna must enable IP multicast routing in the router’s configuration. She enters the following commands to accomplish this.

manfloor1# config t 

Enter configuration commands, one per line. End with CNTL/Z. 

manfloor1(config)# ip multicast-routing 

manfloor1(config)#end 

manfloor1# 

The router now has IP multicast routing enabled. PIM must now be configured on an interface basis to manage multicast flow out of the desired interfaces. Figure 3 shows the fast Ethernet interfaces on the router we desire to enable PIM on.

Nanna configures PIM on the manfloor1 router fast Ethernet interface, id 0/0, with the following command IOS command syntax.

manfloor1# config t 

Enter configuration commands, one per line. End with CNTL/Z. 

manfloor1(config)# interface fast eth 0/0 

manfloor1(config-if)# ip pim sparse-dense-mode 

manfloor1(config-if)#end 

manfloor1# 

The other two interfaces on the router, fe 0/1 and fe 0/2, will be configured in a likewise manner. Upon completion of the configuration modifications, IP multicast routing will be enabled. Remember, PIM is required for extrapolating group addresses and routes from the routing unicast-based tables. If you’re curious about the sparse-dense-mode syntax, you should read up on the multicast PIM density modes.

Figure 3. Fast Ethernet interfaces which require PIM configurations.

Cheap and Superficial

It’s a modest multicast network, but Nanna managed to get the services up and running for the manufacturing CNC robots. There are many complexities involved with IP multicast networks. The current multicast bible is the book Developing IP Multicast Networks Volume I, by Beau Williamson. Word on the streets is that volume II is in the works. For a quick and dirty primer that is free, checkout Multicast Quick Start Configuration Guide on Cisco System’s online documentation site.

This multicast network was a simple confidence builder for Nanna. Now she’s considering adding voice-over-ip multicast and videoconference streams. But that's another day. Right now, Nanna wants to treat herself to a nice afternoon at the movies. A new action-martial arts flick from Hong Kong has been downloaded to her local high tech multiplex cinema.

Michael J. Norton is a software engineer at Cisco Systems.

Read more Networking as a 2nd Language columns.

---

Return to the O'Reilly Network.