understand multicasting from the client/host perspective .

Discussion in 'Cisco' started by April, Jun 25, 2006.

  1. April

    Sam Wilson Guest

    Consider the alternatives - unicast or broadcast just wouldn't work.
    Switches and NICs are built to make make multicast packets go to the
    right places and (mostly) only the right places.

    Sam Wilson, Jun 26, 2006
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  2. April

    J Guest

    To respond to your inquiry from further down and to put it into a
    decent analogy, you've asked a forum of race car-driving pros to teach
    you how to corner and power brake. All racing pros of course consider
    this rudimentary racing knowledge. No offense but I would recommend
    that you learn the same way we did. Get yourself a copy of TCP/IP
    Illustrated Volume 1 & 2. If ever there was a "User's Guide" for
    TCP/IP, those books are it. Once you learn the basics of how it's
    supposed to work then we can better help you with the specifics of the
    implementation on Cisco hardware.

    J, Jun 26, 2006
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    Brad Reese
    Cisco Repair
    www.BradReese.Com, Jun 26, 2006
  4. April

    April Guest

    If you don't know or cannot understand, you don't have to say anything
    ...there are people can relate and try to help.

    I'm a pro in my field, and I know how to help when I got questions like
    this when I got time. ;-)
    April, Jun 26, 2006
  5. April

    April Guest

    Thanks Brad ...
    April, Jun 26, 2006
  6. April

    April Guest

    Oh by the way, try this specifi one ...

    The multicast MAC part is where I have difficulty to understand well ..

    like why this is needed? As the sender, it seems to me it only needs
    to send to an IP ... I can think this signifies the frame is multicast,

    and somehow the switch can relate the multicast MAC to the MAC of a
    host that joined the group, but not sure this is all it about?

    Thanks in advance .
    April, Jun 26, 2006
  7. April

    April Guest

    I could take this route .. however it would just leave a grey area in
    the whole picture though.
    April, Jun 26, 2006
  8. April

    Sam Wilson Guest

    Well then I'm afraid I'm going to have to side with the folks who say
    you need to know a lot more background before we can answer your

    Sam Wilson, Jun 26, 2006
  9. April

    Rod Dorman Guest

    Because there could be dozens or hundreds of hosts that want to
    receive the data.

    Its much more efficient to send a packet to *one* multicast address
    than to sent it to *multiple* IP addresses.
    Rod Dorman, Jun 26, 2006
  10. April

    April Guest

    k let's talk just the last leg here .. based on what Mike said, the
    multicast IP is converted to the multicast MAC at the last router ...

    In non multicast case, a switch will just just based on the IP and find
    the MAC of a host, and forward a frame to the card that has the MAC.

    However, in the multicast case, once the router converted the IP to
    MAC, what this MAC will be used for? How does a switch know a
    multicast frame should be forwarded to a specific host, which is a
    member of a multicast group? Did the IP and MAC, and their affiliation
    to the multicast group is registered somewhere on the switch, say the
    CAM table?

    Is the purpose of the multicast MAC is only for diferenciating possible
    multi multicast traffics?
    April, Jun 26, 2006
  11. April

    BernieM Guest

    All multicast IP addresses map to corresponding mac address but there's not
    a one-to-one mapping. Rather than me retype it please read the info at this
    link, it explains how 32 different multicast IP's all map to the same
    Ethernet address.


    BernieM, Jun 26, 2006
  12. April

    April Guest

    Thanks Bernie .. I understand this part - 2^5 to 1

    My prob is I don't understand what this MAC is for, even it is 1-1.
    April, Jun 26, 2006
  13. Just think logically. You have a hundred workstations, connected to the
    switch. Router knows, that somebody "subscribed" to certain "multicast
    stream", and did everything to bring the stream. Then it has to deliver this
    stream to the client. But switch is not aware, which port "subscribed" to
    the stream. So, it sends frames to all ports. however, if switch is smart
    enough, it will watch for an initial conversation between the client(s) and
    the router, and will remember, that port 3/15 has subscribed to the group In this case it will send the frame directly to that port,
    and everybody else will not hear anything. It's like having a headphones
    with your radio... But if your radio has no headphones jack, you can not use
    this feature... :)

    Good luck,

    HeadsetAdapter.com, Jun 26, 2006
  14. OK. Get back to the "4th grade language" :)

    What's a difference between IP address and MAC address. Let's say, the IP
    address is your "Public Mailing Address", for example, 1234 Main Street,
    Room 234, Best Town, CA, 95001. People send you a correspondence to that
    address. But they don't care what color is your mailbox, what row is it
    located, etc. But mailmen does. He knows that your mailbox is third one on
    the fourth row, right over the corner.

    Now, let's say some Men's magazine sends a preview issue to all male, who
    live in this house. They just sent a bunch of magazines to your house, and
    mailmen will place them into the boxes, where male tenants live. That will
    be "multicast".

    But if mailmen is not aware, who lives where, it will place the magazine in
    each mailbox. He "converts" a "multicast" (Men's magazine) to "broadcast"
    (what if there is a men in the house...). Publisher's intent was to deliver
    the issue to male population only, but since mailmen was not aware, he
    delivered them to every mailbox.

    And another situation - your town wants every person in the town to receive
    an invitation. They send a big box of postcards to a local post office. And
    mailmen instructed to put the postcard into each and every mailbox. So, he
    does not care, who lives there - he just delivers a mail.

    And now back to "technical language"... Each network device has it's own MAC
    address. Your switch knows, which MAC address available thriugh which port.
    So, in case on unicast, it knows, that specific freame should be delivered
    to the specific port. Also there is a "Layer 2 broadcast" MAC address (often
    named Ethernet broadcast). Any frame with the bradcast MAC address as the
    destination, will be delivered to each and every port on the switch. And
    third "group" of MAC addresses is defined to be a Multicast MAC addresses.
    You can look at this MAC address as on "conditional broadcast". It will
    reach the port only if certain condition met (port subscribed to specific
    Multicast group). Switch does not care what was the IP address of the group,
    nor what was the IP address of the sender. It knows, that it has to deliver
    the frame based on the destination MAC address.

    Good luck,

    HeadsetAdapter.com, Jun 26, 2006
  15. April

    April Guest

    Mike, you are the best!

    I think IGMP has to bee used for sending to hosts in a specific
    multicast group, my teacher told me that ...

    I think my remaining question is on the conversion of the last leg
    multicast IP to MAC, why that is required, and how it is being used (by
    the switch?)?

    I think we are getting there ... :)
    April, Jun 26, 2006
  16. It's the same reason why a unicast IP address has to be translated to a
    MAC address when sending to the final destination. Everything on a LAN
    has to be sent to a MAC address, that's how LANs work.

    In the case of unicast we we ARP to find out the MAC address that
    corresponds to a particular IP address. But for multicast you can't use
    ARP because each member of the group has a different MAC address. So
    instead there's a direct translation algorithm, where a piece of the
    multicast IP address is appended to a standard MAC prefix.
    Barry Margolin, Jun 27, 2006
  17. April

    April Guest

    So, at the last leg, the router translate the multicast IP to the
    multicast MAC (according to Mike), and forward to the switch. The
    switch then delivers the multicast frames to the multicast MAC.
    Although this multicast MAC does not exist on the subnet - not relate
    to any physical NIC, the NIC belongs to the host that joined the
    multicast group knows it, and picks up the frames that sent to the
    multicast MAC, is this the picture I should get?

    It seems to me the switch should play a more active role here?
    April, Jun 27, 2006
  18. April

    SAto Guest

    April skrev:
    Hasn't this just been answered?
    A switch has no concept of IP addresses it does not know what to do
    with them or even care about them. The only thing a switch cares about
    are MAC addresses.

    Thats why the router which cares about both IP addresses and MAC
    addresses has to convert the IP address to a address that the switch
    can understand.

    SAto, Jun 27, 2006
  19. April

    Sam Wilson Guest

    Let me nitpick - the word "conversion" seems slightly inappropriate
    here. The IP addresses remain the same but the IP packet is
    encapsulated in a frame with a multicast destination MAC address derived
    from the multicast destination IP address.

    For the avoidance of doubt, YMMV etc etc.

    Sam Wilson, Jun 27, 2006
  20. April

    rdymek Guest

    The switch doesn't really do a whole lot with Multicast other than deal
    with the multicast address and identify which ports belong to that
    multicast group. Nothing more. Its reality a simple concept - I
    believe you're probably over thinking this just a bit.

    Think of the layers of your OSI model. If the multicast mac is
    assigned to your PC (and possibly hundreds of others and the network
    propagates) then data destined for your mac will be sent to you via the
    switch. The router encapsulates the MAC frame into an IP Packet with
    IP addressing and the process goes on and on. Just like a Unicast. In
    fact, most of the network looks at Multicast as a Unicast, even though
    its in all reality a type of broadcast. Its a hybrid so to speak.

    But since Layer 2 requires a MAC to communicate you couldn't possibly
    map the multicast IP to your real MAC, because there'd have to be a
    separate ARP mapping for each host involved. And you can't have an ARP
    entry that is 1 to many mapping. Otherwise you'd be saying that the
    real MAC of your PC translates to multiple IP's and vise versa (one IP
    mapping to multiple MAC). Since that's not the way ARP works, you need
    sort of a broadcast method involved. Its similar to your network
    broadcast (i.e. which goes to all systems on the
    network and all systems choose to listen to that broadcast. With
    Multicast, its the same concept but only the systems that needs to
    receive the packet/frame get it.

    So end result it you're talking last leg, layer 2. Layer 2 needs a
    mapping, normally done through ARP. Since ARP doesn't work with 1 to
    many, you have to do it slightly differently and assign a MAC to IP
    mapping. This is an RFC standard and is the same from vendor to

    Ultimately, multicast doesn't work without a mechanism to use it though
    - your original post mentioned how to use it from the end user
    perspective. You need an application or something to say what
    multicast addresses to use to communicate with (whether it be the
    client [receive] or the server [sending]). Without the application in
    place, Multicast won't function.

    There are entire courses set aside just for Multicast - it can get very
    detailed, but the basic concept isn't much different than from your
    basic IP knowledge, just with a little twist on it.

    rdymek, Jun 27, 2006
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