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DVMRP Tunnels and the Internet Multicast Backbone

DVMRP Tunnels and the Internet Multicast Backbone

Tunnels are used to transport one protocol within another. For example, in Figure 5-32, we have a network that is running IP and IPX applications, but only IP is enabled between routers A and B. For the IPX traffic from router A to get to the client attached to router B, the IPX datagram is sent through an IP tunnel connecting the two routers.

Figure 5-32: Tunneling IPX in IP

Assume that the Netware server in Figure 5-32 is sending an IPX packet to the Netware client. The data from the server is encapsulated in an IPX packet at layer 3 and sent to the ethernet module at layer 2. The ethernet module then encapsulates the IPX packet in an ethernet frame with destination and source ethernet addresses. The IPX packet is treated as data inside the ethernet frame. When router A receives the frame, the data is removed and router A determines that this is an IPX packet destined for the IPX network attached to router B. Because IP is the only protocol enabled between the routers, a tunnel needs to be configured to carry the IPX packet in an IP packet. Assuming the tunnel has been configured, router A encapsulates the IPX packet in an IP packet. Notice that we are encapsulating one layer 3 protocol, IPX, in another layer 3 protocol, IP. This is typically the characteristic of tunneling. When the IP packet reaches the other end of the tunnel, router B removes the IPX packet from the IP packet and forwards the IPX packet onto the network on which the Netware client is attached.

The Internet Multicast Backbone (MBONE) is a logical multicast network overlaid onto the physical unicast Internet (see Figure 5-33).

Figure 5-33: The Internet and the MBONE

Multicast traffic that travels between DVMRP sections of the Internet needs to be sent over an IP tunnel that encapsulates the multicast packet into a unicast packet (see Figure 5-34).

Figure 5-34: A DVMRP tunnel

The two DVMRP routers and the tunnel form the logical or virtual multicast network that is a subset of the physical Internet. Tunnels are needed because not all routers on the Internet support multicast routing. Even if they did, the maximum hop count for DVMRP is 32, which is not sufficient to span the entire Internet. DVMRP tunnels are IP in IP tunnels, as shown in Figure 5-35.

Figure 5-35: Multicast traffic encapsulated in an IP in IP tunnel

Cisco routers do not implement DVMRP but can interact with DVMRP, as we shall see in later chapters. CGMP can act as a proxy for a non-Cisco DVMRP router using the interface command

ip cgmp proxy

Consider the network in Figure 5-36. Here we have a non-Cisco DVMRP router connected to a Cisco switch that has CGMP enabled, and with CGMP enabled on the interface connected to the switch. With CGMP proxy enabled on the router, the router listens to the DVMRP messages and determines the groups for which DVMRP will be forwarding traffic. The proxy router then informs the switch using CGMP about any DVMRP hosts attached to the switch that wish to receive the traffic.