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36 C1640M-B (3/06)
TROUBLESHOOTING
If your multicast over GRE tunnel is not working, one of the following could be the cause:
• Tunnel not UP/UP: The tunnel source and destination do not match on each end of the tunnel. For example, if the tunnel destination on
R102 was changed to the IP address 10.2.2.2 instead of 2.2.2.2 while the configuration on R104 remained the same, the tunnel would not
come up. To verify the status of the tunnel, use the show interface tunnel 0 command.
• RPF Failure: Use the show ip mroute count command to verify that the multicast packets are dropped because of RPF failure. A sample
output of the show ip mroute count command and its increasing counters for RPF failure is shown in bold below:
r104# show ip mroute count
IP Multicast Statistics
3 routes using 1642 bytes of memory
2 groups, 0.50 average sources per group
• Forwarding Counts: Pkt Count/Pkts per second/Avg Pkt Size/Kilobits per second
Other counts: Total/RPF failed/Other drops(OIF-null, rate-limit etc)
Group: 224.0.1.40, Source count: 0, Packets forwarded: 0, Packets received: 0
Group: 239.1.1.20, Source count: 1, Packets forwarded: 11, Packets received: 45
Source: 10.1.1.1/32, Forwarding: 11/0/100/0, Other: 25/14/0
!--- After some time, the show ip mroute count command is issued again.
!--- You can see the RPF failed counter increasing:
r104# show ip mroute count
IP Multicast Statistics
3 routes using 1642 bytes of memory
2 groups, 0.50 average sources per group
Forwarding Counts: Pkt Count/Pkts per second/Avg Pkt Size/Kilobits per second
Other counts: Total/RPF failed/Other drops(OIF-null, rate-limit etc)
Group: 224.0.1.40, Source count: 0, Packets forwarded: 0, Packets received: 0
Group: 239.1.1.20, Source count: 1, Packets forwarded: 11, Packets received: 50
Source: 10.1.1.1/32, Forwarding: 11/0/100/0, Other: 30/19/0
r104#
Use the show ip rpf source command to ensure that the RPF interface is the same as that on which the source multicast packets are
received—Tunnel 0 in this example. For more information on RPF failures, go to or click http://www.cisco.com/warp/public/105/
mcastguide0.html to access the IP Multicast Troubleshooting Guide.
• PIM Neighbors: Router R102 is not forwarding over the Tunnel0 interface because it is not seeing a PM neighbor R104. You can use the
show ip pim neighbor command on R102 to show the neighbor R104 over the tunnel. You can also use the show ip pim int command to
show that there is a neighbor. Verify that the interface level ip pim sparse-dense-mode command is configured on both ends of the tunnel
and that IP multicast-routing is enabled.