Multicast Questions

Multicast Quick Summary Multicast routing is a method of network communication that allows data to be efficiently sent from one source to multiple recipients. Multicast is only sent to devices that are part of a specific multicast group. IGMP (Internet Group Management Protocol) is a protocol used by hosts and routers to manage multicast group memberships. It allows routers to know which multicast groups have members on a particular local network segment. + IGMPv1 is the simplest form, where devices can join a group but cannot leave it explicitly; they simply stop responding to the router’s queries if they’re no longer interested. + IGMPv2 introduces the “Leave” message to IGMPv1 which gives the ability for devices to leave groups explicitly, making the process more efficient. + IGMPv3 adds support for source-specific multicast (SSM), allowing devices to specify not only the multicast group they want to join but also from which sources they wish to receive data. PIM (Protocol Independent Multicast) is used for routing multicast traffic across a network. It does not depend on a specific unicast routing protocol, hence the name “protocol independent”. When PIM is enabled on a Layer 3 interface (SVI or routed interface) then IGMP is automatically enabled at the same time in most modern Cisco IOS versions. IGMP is then used to listen for IGMP Membership Reports (requests to join a particular Multicast Group so to say) from endpoints on that LAN. PIM is sub-divided into two well known modes: Dense-Mode and PIM Sparse-Mode: + PIM Dense Mode (PIM-DM) is ideal for networks where group members are densely packed, this mode initially floods the data to all points, then prunes back the paths where there are no interested receivers. It’s like casting a wide net and then pulling it back in areas without fish. + In PIM Sparse-Mode there has to be a Layer 3 device designated as the Rendezvous Point (RP) role. The RP acts as a place where multicast sources and multicast receivers can “meet”.

Question 1

Explanation

A rendezvous point (RP) is required only in networks running Protocol Independent Multicast sparse mode (PIM-SM).

By default, the RP is needed only to start new sessions with sources and receivers.

Reference: https://www.cisco.com/c/en/us/td/docs/ios/solutions_docs/ip_multicast/White_papers/rps.html

For your information, in PIM-SM, only network segments with active receivers that have explicitly requested multicast data will be forwarded the traffic. This method of delivering multicast data is in contrast to the PIM dense mode (PIM-DM) model. In PIM-DM, multicast traffic is initially flooded to all segments of the network. Routers that have no downstream neighbors or directly connected receivers prune back the unwanted traffic.

Question 2

Explanation

PIM dense mode (PIM-DM) uses a push model to flood multicast traffic to every corner of the network. This push model is a brute-force method of delivering data to the receivers. This method would be efficient in certain deployments in which there are active receivers on every subnet in the network. PIM-DM initially floods multicast traffic throughout the network. Routers that have no downstream neighbors prune the unwanted traffic. This process repeats every 3 minutes.

PIM Sparse Mode (PIM-SM) uses a pull model to deliver multicast traffic. Only network segments with active receivers that have explicitly requested the data receive the traffic. PIM-SM distributes information about active sources by forwarding data packets on the shared tree. Because PIM-SM uses shared trees (at least initially), it requires the use of an RP. The RP must be administratively configured in the network.

Answer C seems to be correct but it is not, PIM spare mode uses sources (not receivers) to register with the RP. Sources register with the RP, and then data is forwarded down the shared tree to the receivers.

Reference: Selecting MPLS VPN Services Book, page 193

Question 3

Explanation

The concept of joining the rendezvous point (RP) is called the RPT (Root Path Tree) or shared distribution tree. The RP is the root of our tree which decides where to forward multicast traffic to. Each multicast group might have different sources and receivers so we might have different RPTs in our network.

Question 4

Explanation

In the figure below, we can see RP sent “join 234.1.1.1” message toward Source.

Reference: https://www.ciscolive.com/c/dam/r/ciscolive/apjc/docs/2018/pdf/BRKIPM-1261.pdf

Question 5

Explanation

Query messages are used to elect the IGMP querier as follows:
1. When IGMPv2 devices start, they each multicast a general query message to the all-systems group address of 224.0.0.1 with their interface address in the source IP address field of the message.
2. When an IGMPv2 device receives a general query message, the device compares the source IP address in the message with its own interface address. The device with the lowest IP address on the subnet is elected the IGMP querier.
3. All devices (excluding the querier) start the query timer, which is reset whenever a general query message is received from the IGMP querier. If the query timer expires, it is assumed that the IGMP querier has gone down, and the election process is performed again to elect a new IGMP querier.

Reference: https://www.cisco.com/c/en/us/td/docs/switches/lan/catalyst3750x_3560x/software/release/15-2_2_e/multicast/configuration_guide/b_mc_1522e_3750x_3560x_cg/b_ipmc_3750x_3560x_chapter_01000.html