Multicast questions

So I am starting my studies on multicast. This is a technology that I have never configured so I am learning it for the first time. I have a few questions about the basics of multicast. I also have listed a few things I believe to be true about multicast based on my studies so far. 

 

1. The 224.0.0.0/24 is said to be used locally only and not to be forwarded by multicast routers. Can someone explain this more. 

2. The way I understand it, multicast is used for one to many and if clients join a multicast stream, doesn't that imply that the stream is live?

3. Is this a good example of multicast-- my cable box would be a client configured with a multicast group address and when I go to change the channel on my cable box, that channel that I change to is configured with a different multicast group. When I change the channel I join that group based on the channels multicast group. Is the cable box in this example a Client or receiver? And what would the name be for the cable modem? Also what multicast addresses would be used in this example-- something public or some private range? 

 

So I am really trying to get a good feel for a real would example of multicast. Once I have that I can build on it.

Comments

  • The 224.0.0.0/24 is kept local by setting the TTL to 1. Think of the routing protocols like OSPF sends to 224.0.0.5 or 224.0.0.6, EIGRP to 224.0.0.10 and RIP to 224.0.0.9. If you look at it in Wireshark most of them should set the TTL to 1.

    Most often multicast will be used to do one to many but it can alse be used to do many to many like in a stock exchange or so.

    If you watch TV at home via multicast your cable box would be a receiver, a client or receiver is the same thing from my point of view but receiver is a better name for it. When you look at some channel your box will send IGMP join towards your default gateway. This router will convert that to a PIM join and send that upstream. Of course if someone is already watching that channel the router may already have joined the multicast tree. The addressing used isn't that relevant since most providers don't exchange multicast with each other but the 239.0.0.0/8 range is set aside for private use. For IPTV many will use SSM and that is in the 232.0.0.0/8 range.

    Hope that cleared some things out.

  • Hi.

    1. 224.0.0.0/24 is a Link-Local multicast address scope. Routers must not forward these messages outside the subnet in which they originate. The idea is to use multicast, since there might be multiple receivers for the traffic, but all of them are on the local segment only. Mostly used for control-plane traffic, for example:

    Routing Protocols (RIPv2 uses 224.0.0.9, OSPF uses 224.0.0.5 & 224.0.0.6 etc.)
    First-Hop Redundancy Protocols (VRRP uses 224.0.0.18, HSRP uses 224.0.0.102 etc.)
    All Routers on this Subnet (224.0.0.2)
    All Systems on this Subnet (224.0.0.1)

    2. Multicast can also be used for many-to-many applications (e.g. Bidirectional PIM).
    Didn't really understand your question about the stream being "live".

    3. Yes, IPTV is a good example of IP Multicast.
    IGMP and PIM are both used. The cable box would be a client/receiver. The modem/router is the "last-hop" router, receiving an IGMP Report message, and generating a PIM Join up the Reverse Path Tree towards the RP (if PIM Sparse Mode is used).

    More info on IPTV Multicast:
    www.banglajol.info/index.php/JSR/article/download/2633/2632

    P.S.
    Daniel - you beat me to it while I was writing my post. :)

  • Well technically HSRPv2 uses 224.0.0.102, HSRPv1 (the default) still runs over 224.0.0.2

    Not to nit pick or anything :P


    David Rothera




    On Thu, Aug 23, 2012 at 7:06 AM, ArtiomL <[email protected]> wrote:

    Hi.

    1. 224.0.0.0/24 is a Link-Local multicast address scope. Routers must not forward these messages outside the subnet in which they originate. The idea is to use multicast, since there might be multiple recievers for the traffic, but all of them are on the local segment only. Mostly used for control-plane traffic, for example:



    Routing Protocols (RIPv2 uses 224.0.0.9, OSPF uses 224.0.0.5 & 224.0.0.6 etc.)
    First-Hop Redundancy Protocols (VRRP uses 224.0.0.18, HSRP uses 224.0.0.102 etc.)
    All Routers on this Subnet (224.0.0.2)
    All Systems on this Subnet (224.0.0.1)

    2. Multicast can also be used for many-to-many applications (e.g. Bidirectional PIM).
    Didn't really understand your question about the stream being "live".

    3. Yes, IPTV is a good example of IP Multicast. IGMP and PIM are both used. The cable box would be a client/reciever. The modem/router is the "last-hop" router, receiving an IGMP Report message, and generating a PIM Join up the reverse path tree towards the RP (if PIM Sparse Mode is used).



    More info on IPTV Multicast:
    www.banglajol.info/index.php/JSR/article/download/2633/2632






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  • Hi David.

    Yes, you are right.
    I didn't want to go into it though, as the question wasn't about HSRP.

    Anyway, thanks for your comment.

    Cheers.

  • Scope  is the keyword.

    224.0.0.0/24 range has local scope.

    The TTL is set to 1 so that the packet is not forwarded beyond one router.

    To undestand this just think about this 

    OSPF routers send  LSA's as multicast DR and DR sends it other routers using another multicast address.

    They  use both the multicast address which are in the 224.0.0.0/24 range. 

    HSRP also; If you think HSRP messages are also link local or local scope. Is there any need to forward the HSRP message outside onto antoher link- No right. But I have to reach many routers within my link then use Link local multicast

  • 224.0.0.2 which HSRP v1 used is also used by all routers multicast.

    Say I have 4 routers in my link. R1 and R2 HSRP  v1 enabled . R3 and R4 do not have HSRP enabled .

    In case of HSRP v1  even R3 and R4 will get HSRP messages. They have to process it as all routers are members of 224.0.0.2.

    HSRP v2 removed this inefficency by using 224.0.0.102 address. Now only R1 and R2 will get HSRP messages if v2 is enabled on both of them

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