traffic is normally treated like an unknown MAC address or broadcast frame that causes the frame to be flooded out every port within a VLAN. This treatment is acceptable for unknowns and broadcasts, but, as noted earlier, IP multicast hosts may join and be interested in only specific multicast groups. On most Layer 2 switches, this traffic is forwarded out all ports, resulting in wasted bandwidth on both of the segments and on the end stations. One method that Cisco Catalyst switches used to circumvent this is to allow the administrator to configure the switch manually to associate a multicast MAC address with various ports. For example, the administrator configures ports 5, 6, and 7 so that they are the only ones that receive the multicast traffic destined for the multicast group. This method works but is not scalable. IP multicast hosts dynamically join and leave groups using IGMP to signal to the multicast router. Dynamic configuration of the forwarding tables of the switches is more effective and reduces user administration.
Content 7.2 IGMP and Layer 2 Issues 7.2.6 Multicast in Layer 2 Solutions Many multicast switching solutions have been developed to improve the behavior of the switches when they receive multicast frames, such as the following: CGMP is a Cisco Systems proprietary protocol that runs between a multicast router and a switch. This protocol enables the Cisco multicast router to inform the switch about the information contained in the IGMP packet after it receives IGMP messages sent by hosts. With IGMP snooping, a switch must examine every multicast data packet to determine if it contains any pertinent IGMP control information and then updates the MAC table accordingly. IGMP snooping implemented on a low-end switch with a slow CPU could have a severe performance impact when data is sent at high rates. The solution is to implement IGMP snooping on high-end switches with special application-specific integrated circuits (ASICs) that can perform the IGMP checks in hardware. CGMP is a better option for low-end switches without special hardware.
Content 7.2 IGMP and Layer 2 Issues 7.2.7 Cisco Group Management Protocol (CGMP) CGMP is the most common multicast switching solution, and it was first implemented by Cisco. CGMP is based on a client/server model, where the router is considered a CGMP server and the switch assumes the client role. There are software components running on both devices, with the router translating IGMP messages into CGMP commands, which are then processed in the switches and used to populate the Layer 2 forwarding tables with the correct multicast entries. The basis of CGMP is that the IP multicast router sees all IGMP packets and informs the switch when specific hosts join or leave multicast groups. Routers use well-known CGMP multicast MAC addresses to send CGMP control packets to the switch. The switch then uses this information to program the forwarding table. When the router sees an IGMP control packet, it creates a CGMP packet that contains the request type (join or leave), the Layer 2 multicast MAC address, and the actual MAC address of the client. This packet is sent to the well-known CGMP multicast MAC address of 0x0100.0cdd.dddd that all CGMP switches listen to. The CGMP control message is then interpreted, and the proper entries are created in the switch content-addressable memory (CAM) table to constrain the forwarding of multicast traffic for this group.
Content 7.2 IGMP and Layer 2 Issues 7.2.8 IGMP Snooping The second multicast switching solution is IGMP snooping. As its name implies, switches become IGMP-aware and listen in on the IGMP conversations between hosts and routers. This activity requires the processor in each switch to identify and intercept a copy of all IGMP packets flowing between routers and hosts and vice versa. It includes these IGMP packets: If care is not taken as to how IGMP snooping is implemented, a switch may have to intercept all Layer 2 multicast packets to identify IGMP packets. This action can have a significant impact on switch performance. Proper designs require special hardware (Layer 3 ASICs) to avoid this problem, which may directly affect the overall cost of the switch. Switches must be Layer 3-aware to avoid serious performance problems because of IGMP snooping.
Content 7.3 Multicast Routing Protocols 7.3.1 Protocols Used in Multicast Multicast distribution trees define the path from the source to the receivers over which the multicast traffic flows.There are two types of multicast distribution trees: With a source tree, a separate tree is built for each source to all members of its group. Because the source tree takes the shortest path from the source to its receivers, it is also called a shortest path tree (SPT). Each source/group pair requires its own state information. For groups that have a very large number of sources, or networks that have a very large number of groups with a large number of sources in each group, source trees can stress the storage capability of routers. Note
Source trees are also referred to as source-based trees or source-root trees. Shared tree protocols create multicast forwarding paths that rely on a central core router that serves as a rendezvous point (RP) between multicast sources and destinations. Sources initially send their multicast packets to the RP, which in turn forwards data through a shared tree to the members of the group. A shared tree is less efficient than an SPT (paths between the source and receivers are not necessarily the shortest), but it is less demanding on routers (memory, CPU). There are basically two types of multicast routing protocols: dense mode and sparse mode:
Content 7.3 Multicast Routing Protocols 7.3.2 Multicast Distribution Trees Figure shows an SPT between source 1 and receivers 1 and 2. It is appropriately assumed that the path between the source and receivers over routers A, C, and E is the path with the lowest cost. Packets are forwarded according to source and group address pair. The forwarding state associated with the SPT is referred to by the notation (S, G), pronounced “S comma G”, where S is the IP address of the source, and G is the multicast group address. Figure shows another example of SPT, where source 2 is active and is sending multicast packets to receivers 1 and 2. A separate SPT is built for this purpose, this time with source 2 at the root of the SPT. The main point is that a separate SPT is built for every source S sending to group G. Figure shows a shared distribution tree. Router D is the root of this shared tree, which is built from router D to routers C and E toward receivers 1 and 2. In Protocol-Independent Multicast (PIM), the root of the shared tree is an RP. Packets are forwarded down the shared distribution tree to the receivers. The default forwarding state for the shared tree is identified by the notation (*, G), pronounced “star comma G”, where the asterisk (*) is a wildcard entry, meaning any source, and G is the multicast group address. In Figure , source 1 and source 2 are sending multicast packets toward a RP via SPTs, and from the RP, the multicast packets are flowing via a shared distribution tree toward receivers 1 and 2.
Content 7.3 Multicast Routing Protocols 7.3.3 Multicast Distribution Trees Identification The multicast forwarding entries that appear in multicast forwarding tables are read in the following