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RE: Multicast over a GRE Tunnel posted 01/10/2007
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Configure the ip igmp join-group command at the remote ethernet segments.
Then try to ping the multicast from the hub. Use sho ip mroute and  debug
ip mpacket commands for troubleshooting.  If you are using GRE tunnels be
aware that the MTU size will need to account for the 24 byte header
adjusting MTU to 1476.  You may want to set the MTU to 1300-1400 on the
tunnels. If the DF bit is set the router will not fragment but drop the
packet.  You can use debug ip icmp to determine if this is the case and
you can clear the DF bit by using a route-map.

rr 

  --------------------------------------------------------------------

  From:  Cisco study <groupstudy@xxxxxxxxxxxxxx>
  Reply-To:  Cisco study <groupstudy@xxxxxxxxxxxxxx>
  To:  ccielab@xxxxxxxxxxxxxx, cisco@xxxxxxxxxxxxxx
  Subject:  Multicast over a GRE Tunnel
  Date:  Wed, 10 Jan 2007 10:41:37 +0000 (GMT)
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  >Hello All,
  >
  >   Hope you are well.
  >
  >   I have a multicast question.
  >
  >   We have setup ip pim sparse-dense-mode Multicast GRE tunnels at
  each of the 5 regional hub sites .  The purpose of the GRE tunnels is
  to stream the multicast traffic from 4 of the hub sites to the main
  central hub site.  A Windows server at the main hub site collects the
  multicast data and is suppose to display phone calling statistics for
  external user calls.
  >
  >   On the server (at the main hub site) we are seeing intermittent
  multicast traffic coming through.  After checking the multicast
  traffic on the routers locally, the traffic is working fine for each
  of the local regional sites.  However, we are seeing packet loss when
  the Multicast traffic is going over the GRE tunnels.
  >
  >   The initial configuration was setup as a fully meshed
  environment, i.e. each site had two tunnels connected to the other
  sites.  But this was causing Multicast packets to drop because of RPF
  failure.
  >
  >   We overcame this problem by removing the fully mesh setup to only
  having a single tunnel from each of the hub sites to the central hub
  site.  Now we are not seeing any RPF failures.  Good.
  >
  >   The troubleshooting on the following site has been covered and
  exhausted:
  >  
  http://www.cisco.com/en/US/tech/tk828/technologies_configuration_example09186a00801a5aa2.shtml
  >
  >   We still are not seeing 100% multicast packets at the central
  site, only around 50% multicast traffic?
  >
  >   Could anyone shed some light if there is a way to check if the
  multicast packets are going over the GRE tunnels?  Can you recommend
  a Windows based software or tool, router commands, etc so that
  multicast source to destination packets can be traced.
  >
  >   Any questions or suggestions welcome.
  >
  >   Regards!
  >John
  >
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