Expand all | Collapse all

Multicast with TTL of 1 - how to route across subnets - PIM not working

Jump to Best Answer
  • 1.  Multicast with TTL of 1 - how to route across subnets - PIM not working

    Posted 03-06-2016 09:53

    Hello all...


    I'm pulling my hair out here trying to figure a way to send multicast packets with a TTL of 1 generated by our PBX paging system, over to our other voice subnets. I've opened a case w JTAC..,,and they claim setting up PIM between routers SHOULD overcome the TTL, and route the traffic. My unicast routing is fine. Have spare mode setup w a reachable RP on our campus core. All works fine if the packet ttl is 4 or so. I'm using mcast.exe for simulation rather than sending pages from our voice system all you can set the ttl w mcast.exe (windows 2003 resource kit btw)


    This will also affect our ability to bridge over discovery protocols like SSDP and Bonjour which fall into the same category....TTL=1. 


    I've also tried sparse-dense mode, adding the multicast group IP that the pager send to as a dense group - no difference. 


    Is it possible to make this work using PIM on Juniper devices. (4300 and 3300 EX switches)? And if so, what needs to be done in the config to allow passing of multicast packet with a TTL of 1? I'm at a loss now ,,,,have searched google high and low....  my next step is lab it up on Cisco gear...there's a TTL threshold level you can set at the interface level I believe...and setting to 0 is an option. Can not find out of if Juniper has the same thing, and if it's even a viable solution anyway....realizing that every hop decrements the TTL.


    Any help is appreciated .....I know somebody out there had to deal with the same thing at some point!


    Thanks in advance....Dennis

  • 2.  RE: Multicast with TTL of 1 - how to route across subnets - PIM not working
    Best Answer

    Posted 03-07-2016 00:15

    Hi Dennis,


    due to RFC791, every router has to decrement the TTL value. Nothing you can do about this. Having said this, there are two solutions: (a) sent packets with TTL > 1 or (b) setup a layer-2 connection (e.g. VPLS) to avoid layer-3 processing.




  • 3.  RE: Multicast with TTL of 1 - how to route across subnets - PIM not working

    Posted 03-07-2016 21:49

    Hi !

    I cannot offer any help on Junos but as you spoke about cisco equipment

    Cisco IOS has a feature called "Multicast Service Reflection"  this allows to reinstate a mutlicast stream with normal TTL


    To implement multicast service reflection, you would complete the following
    1. Identify the multicast groups to be translated.
    2. On the Vif1 interface, statically configure an IGMP join to the service
    provider multicast group(s).
    3. Configure the IP reflect service application to translate the destination
    addresses in the content provider’s multicast group into a pool of
    multicast addresses that can be used in your PIM domain, and define a
    new source for those addresses.
    4. Advertise the newly created source address to your interior gateway
    protocol (OSPF, IS-IS, EIGRP,and so on).