640-861 Designing for Cisco Internetwork Solutions (DESGN) Practice Exams

Demo Question 2.

Answer: B, C, E

Explanation: Layer 2 Multicasting The concept of forming a multicast group is the basis for IP multicast. This means that any group or collection of receivers can indicate interest in receiving a particular stream of data. The group itself isn't limited by physical or departmental boundaries, and the hosts can be located anywhere on the network as well. Hosts join the group by using the Internet Group Management Protocol (IGMP) in order to receive data going to that group. Multicast routing protocols such as Protocol Independent Multicast (PIM) guide the delivery of traffic through multicast-enabled routers. The router to the switch port forwards the incoming multicast stream. One problem is that the default for an L2 switch is to forward all multicast traffic to every port that belongs to the same VLAN on the switch. Talk about a monkey wrench in the works. This activity defeats the very purpose of the switch, which is to send traffic only to the ports that need to receive the data. Bummer! The good news is that there are several ways Cisco switches can circumvent this little problem. Ones commonly used include: Cisco Group Management Protocol (CGMP) A Cisco proprietary solution found on all Cisco LAN switches. The multicast receiver registration (using the IGMP) is accepted by the router and communicated by CGMP to the switch; the switch updates its forwarding table with that information. IGMP snoopingThe switch intercepts multicast receiver registrations and updates the forwarding table with that information. The IGMP snooping means that the switch is aware of L3 because IGMP is a Network layer protocol. Typically, the IGPM packet recognition is hardware assisted.