Hi,
We were testing multicast routing in the network and we observed odd results. Apparently, the network core is a Meraki that supports multicast. It's a very small network but the results are different from a lab environment that is using pure Cisco.
The following were used.
Desktop PC
Meraki L3 switch
Cisco 2900 router that will act as multicast receiver
From the Meraki switch, we enabled PIM-SM on the desktop VLAN and the Cisco router VLAN. We configured the router with "ip igmp join-group 239.1.1.1" command. We can see on the Meraki mroute table that it can see the Cisco router VLAN as outgoing interface for the multicast traffic that's being sent from the desktop. We were also able to verify that the Meraki switch is sending the packets on the port facing the Cisco router. Source=Desktop IP, Destination=239.1.1.1.
The PC is Windows 10, when we try to do "ping 239.1.1.1", we get no response from the router. If we increase the TTL above 2, the router will not respond. However, when we changed the TTL to just 2, the router responds with its LAN IP. It appeared that the TTL must just be 1 when it reaches the Cisco router so it will respond to mcast ICMP. We checked Wireshark and Windows 10 is sending ICMP with a value of 128 by default if no options configured.
This is a bit weird because we have tested this on a lab environment first that consists of IOU devices. Regardless of the size of the network, we can route the multicast traffic end to end (as I cannot change the TTL value of the ICMP in IOS using extended ping).
Also one thing that we have tested, we tried to use PIM-SM on the Cisco router so it would establish PIM neighbor with the Meraki switch. When we did that, we can successfully ping the router from the desktop using multicast IP.
Any idea what's wrong with the TTL?
JL
