/mcp<#root> ip22-75a# configure terminal Enter configuration commands, one per line. End with CNTL/Z. ip22-75a(config)# access-list 101 permit udp host 10.1.1.1 host 224.1.1.1
ip22-75a(config)# end
ip22-75a# debug ip mpacket 101
IP multicast packets debugging is on for access list 101 ip22-75a# *Jan 17 09:04:08.714: IP: s=10.1.1.1 (Ethernet0/0) d=224.1.1.1 len 60, threshold denied *Jan 17 09:04:08.762: IP: s=10.1.1.1 (Ethernet0/0) d=224.1.1.1 len 60, threshold denied *Jan 17 09:04:08.814: IP: s=10.1.1.1 (Ethernet0/0) d=224.1.1.1 len 60, threshold denied
The debug messages indicate that Router 75a does not forward the multicast packets because the TTL threshold has been reached. Look at the router configuration in order to see whether you can find the reason. This output shows the culprit:
interface Ethernet0/1 ip address 10.2.1.1 255.255.255.0 ip pim sparse-dense-mode ip multicast ttl-threshold 15
The router has a TTL threshold of 15, but this does not mean that anything greater than a TTL of 15 is not sent. Actually, the opposite is true. The application is sent with a TTL of 15. By the time it gets to Router 75a, the multicast packets have a TTL less than 15. The ip multicast ttl-threshold <value> command means that any packets with a TTL lower than the specified threshold, in this case, 15, are not forwarded. This command is usually used in order to provide a border to keep internal multicast traffic from drifting out of the intranet. Possible Fixes Either remove the ip multicast ttl-threshold <value> command with the no form of this command, which reverts to the default TTL threshold value of 0, or lower the TTL threshold so that the traffic can pass. Multiple Equal Cost Paths Result in Unwanted RPF Behavior This section shows how equal cost paths to a multicast source can cause unwanted RPF behavior. It also describes how to configure IP multicast in order to avoid this behavior. This network diagram is used as an