no ip pim dm-fallback !!!

Plz Plz...
univercd show this command over sparse-mode but is realy necessary over a ONLY sparse-mode domain ???
Some times cisco says that if you configure only sparse-mode this is not required but univercd show diferent.


  • Hello,

    This command is important in all setubs, if you don't need to fallback to Dense mode

    actually it is tricky here , as we have the interface status and the group status

    1- if the interface configured ONLY sparse mode , so it will not FWD the dense groups BUT

    2- the Group is dense or sparse based on if there is RP for that group or not which means you can find a situation where Interface is sparse mode but the Group ( when use show ip mroute ) is Dense mode : Flag D

    In this case traffic willnot be FWD because of the interface but still the groups became dense , so to avoid such situation you should use the command : no ip pim dm-fallback

    That how I understand

  • If you're using only sparse-mode, then you don't need the "no ip pim dm-fallback" command. This command is useful in sparse-dense domains.
  • "A single RP can be configured for multiple groups that are defined by an access list. If no RP is configured for a group, the router treats the group as dense using the PIM dense mode techniques. (You can prevent this occurrence by configuring the no ip pim dm-fallback command.)"

    I also do not understand why the Cisco docs say to configure this command on all sparse-mode routers. it's only relevant to sparse-dense mode.
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