Multicast is slow



  • Hello everyone, i’m new here.
    I`ll go to the point. As the title suggest i have some issues with multicast speeds.
    Server : Old hp pc, one core AMD, 2GB ddr ram, 2xwd 300GB hdd raid 1.
    FOG version: 1.4.4.
    OS: Ubuntu Server 14.04 LTS x64
    FOG is deployed on the network with existing DHCP server i used this tutorial. Unicast speeds reaches up to 1.6 GBpm which is fine. If i deploy two pc’s in unicast one gets ~900MBpm and the other ~600MBpm which again is fine HOWEVER if i were to try to run the same machines with the same cables without changing anything in multicast i get 65MBpm. I looked over the internet for a possible solution without much luck… I thought it might have something to do with DNS so in /etc/resolv.conf i changed 127.0.0.1 to my network DNS. That fixed the issue where FOG webui would show errors on information like version. I tried plugin in FOG server to a separate switch and connecting two laptops to that switch so that nothing else could interfere. Multicast speeds now were 1.5 GBpm. How can i test what is slowing down multicast so much when its connected to a network with DHCP ?


  • Developer

    @xpalis said in Multicast is slow:

    multicast speeds reach up to 1.5-1.6 GB/min ONLY if i connected fog server separately from my network.

    So we see that your FOG server and the clients are able to do full speed. So it’s probably your network equipment or some kind of configuration of those. Would you mind drawing up a quick schema of your network structure including subnet information so we know better what your setup looks like?



  • @sebastian-roth multicast speeds reach up to 1.5-1.6 GB/min ONLY if i connected fog server separately from my network. As for the results being the same in case of DHCP server > CISCO > FOG server and two laptops two the same CISCO. The speeds are still at 60MB/min.


  • Developer

    @xpalis said in Multicast is slow:

    the results were the same

    Can you please be more precise. Same as what?! I am asking because in your initial post you said: “I tried plugin in FOG server to a separate switch and connecting two laptops to that switch so that nothing else could interfere. Multicast speeds now were 1.5 GBpm”. So we have no idea if same here means same slow or same fast or what.



  • @sebastian-roth yes ALL computers are in the same subnet and i already tried that… because the school network is old so i thought maybe the cable is wrong somewhere, maybe the switch is dying so i connected two laptops with new cat5e cables to cisco (fog is already connected to cisco) and tried multicast with only two laptops… the results were the same


  • Developer

    @xPalis Slow multicast speed is usually due to one client being slow (disk dying or such) or some kind of misconfiguration in your network. You can’t really compare unicast speed to multicast so let’s focus on the later.

    Are the client’s and the FOG server all in one single subnet? Please multicast again with FOG server and two client all connected to the Cisco switch. Next try the same but using one of the TP-Links. See any difference there?



  • The DHCP server is a simple old PC with gigatybe network card running on Debian. Server is directly connected to Cisco SG100-24 swtiches (i read about settings that should be set on CISCO however these switches do not have any webui interface). From CISCO it goes to multiple TP-LINK SF1024 switches (100Mbps). This is a school with rooms that have different pc`s. I remember playing around with DNS and Fog PXE options (on fog server) i cant remember what i did but then
    i had to reinstall 10 machines (Desktop pc’s same models, same hardware) in a library. They were all connected to TP-LINK SF1024 i created the task for multitask and the speed was 1.6GB/min. (which is great). We dont keep our computers on 24/7 on a weekends they are off. Ever since i turned off FOG server i cant replicate what i did. I thought it was DNS that i typed in /etc/resolv.conf that changed on next boot however that didnt work.


  • Developer

    @xPalis Please explain your network structure in more detail. How are clients and FOG server connected normally? How many switches or routers are between clients and FOG server? Different subnets?


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