| 8.4 |  | Routing and Multicast Configuration |
| 8.4.3 |  | Multicast routing |

| IP multicasting is a bandwidth-conserving technology that reduces traffic by simultaneously delivering a single stream of information to multiple recipients. The following applications can utilize multicast:

- Video conferencing
- Corporate communications
- Distance learning
- Distribution of software
- Stock quotes
- News

IP multicasting is actually the transmission of an IP datagram to a host group, which is a set of hosts identified by a single IP destination address. In order for this to work, hosts that wish to receive multicasts must tune in to the multicast by joining a multicast host group. Routers that forward multicast datagrams must know which hosts belong to which group. Routers discover this information by sending Internet Group Management Protocol (IGMP) query messages through their attached local networks. Host members of a multicast group respond to the query by sending IGMP reports noting the multicast groups to which they belong. If a host is removed from a multicast group, it sends a ‘leave’ message to the multicast router.

In software versions 6.2 and higher, the PIX Security Appliance supports Stub Multicast Routing (SMR), which enables it to pass multicast traffic. This feature is necessary when hosts that need to receive multicast transmissions are separated from the multicast router by a PIX Security Appliance. With SMR, the PIX Security Appliance acts as an IGMP proxy agent. It forwards IGMP messages from hosts to the upstream multicast router. This router is responsible for forwarding multicast datagrams from one multicast group to all other networks that have members in the group. When SMR is used, it is not necessary to construct Generic Route Encapsulation (GRE) tunnels to allow multicast traffic to bypass the PIX Security Appliance. Figure illustrates how the PIX Security Appliance supports multicasting.

| Web Links Digital Convergence http://www.networkcomputing.com/1204/ 1204f1c1.html?ls=NCJS_1204rt Protocol Directory
http://www.protocols.com/pbook/tcpip2.htm

IGMP
http://www.networksorcery.com/
enp/protocol/igmp.htm