Home > Multicast Questions

Multicast Questions

January 27th, 2021 Go to comments

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.

RP_join_source.jpg

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

Comments
  1. Anonymous
    February 16th, 2021

    Question 4

    What is the purpose of an RP in PIM?

    RPs do not receive IGMP joins, but PIM joins from the DRs.

  2. Ing_MX
    March 12th, 2021

    I agree, IGMP Membership is only received by the fist hop Router connected to Receiver. The RP sends Join messages toward the source so letter D is the correct answer.

    https://www.cisco.com/c/en/us/td/docs/switches/lan/catalyst3850/software/release/16-6/configuration_guide/ip_mcast_rtng/b_166_ip_mcast_rtng_3850_cg/configuring_pim.html

  3. contoso
    March 29th, 2021

    Agree Q4 answer should be D. send join messages toward a multicast source SPT (Source Path Tree).
    https://www.ciscolive.com/c/dam/r/ciscolive/apjc/docs/2018/pdf/BRKIPM-1261.pdf
    Page 43.

    https://www.juniper.net/documentation/us/en/software/junos/multicast/topics/topic-map/mcast-pim-sparse-mode.html
    If the RP router has interested receivers in the PIM sparse-mode domain, it sends a PIM join message toward the source to build a shortest-path tree (SPT) back to the source.

  4. Dens
    March 30th, 2021

    @digitaltut please confirm Question 4. If D is the correct answer. thanks.

  5. contoso
    April 12th, 2021

    @digitut
    Please review for Q4.

  6. bob
    April 15th, 2021

    @digitaltut, please fix the following:

    Which router is elected the IGMP Querier when more than one router is in the same LAN segment?

    A. The router with the lowest IP address
    B. The router with the highest IP address
    C. The router with the shortest uptime
    D. The router with the longest uptime

    The correct answer is B

    Source:

    https://www.ciscopress.com/articles/article.asp?p=2738463&seqNum=6

    IGMPv1 also does not have the capability of electing a querier. If there are multiple queriers (routers) on the subnet, a designated router (DR) is elected using PIM to avoid sending duplicate multicast packets. The elected querier is the router with the highest IP address

    And:

    What is the purpose of an RP in PIM?

    A. secure the communication channel between the multicast sender and receiver.
    B. receive IGMP joins from multicast receivers.
    C. ensure the shortest path from the multicast source to the receiver.
    D. send join messages toward a multicast source SPT

    The correct answer is D not B.

    (B) is not correct. The LHR (last hop router) receives the IGMP from the receiver and sends it upstream towards the RP as a PIM join/prune msg (*/G). So the IGMP msg is only between the receiver and the LHR.

    B is correct. Once the multicast source registers with the RP, the RP then has to join the SPT source (S.G) to start receiving the multicast traffic

    Great video here:
    https://www.youtube.com/watch?v=QaNFiRJiTNM&ab_channel=DecodingPackets

  7. bob
    April 15th, 2021

    sorry, last sentence should read:

    D is correct. Once the multicast source registers with the RP, the RP then has to join the SPT source (S.G) to start receiving the multicast traffic

  8. Anonymous
    April 15th, 2021

    Disregard the – Which router is elected the IGMP Querier when more than one router is in the same LAN segment? question

    After further reading I’ve noticed:

    IGMPv1 – also does not have the capability of electing a querier
    ICMPv2 – If there are multiple routers on a subnet, the DR is the device with the highest IP address and the querier is the device with the lowest IP address.

    So yes the querier is the lowest IP (A)

    However “What is the purpose of an RP in PIM”? is definitely:
    D. send join messages toward a multicast source SPT

  9. digitaltut
    April 15th, 2021

    Thanks for your information. We have just updated Q4.

  1. No trackbacks yet.