Today I had a strange problem on a network with a stack of Cisco 3750X running IOS 15.0(2)SE4.
All unicast traffic was working but for some reason I noticed that multicast traffic (more specifically OSPF Hello packets) between routers did not pass. One of the routers was configured as a trunk ports with many VLANs and LACP.
What I saw was that the firewall could se one router, one router could see both the firewall and the other router and the second firewall could only see the first router.
This happened after a reload of the entire switch stack, something that I have done many times before. So I can not tell for sure exactly what I did different to cause this problem this time. The switch is configured for src-dst-ip load balancing.
This seems to be a bug within the Cisco switch as the quick fix that worked for me was to take down one of the ports in the port-channel and later bring it back up.
Troubleshooting commands that you can use. I did not so I am not sure what its result would be.
show platform forward gigabitEthernet 1/0/2 vlan 333 1111.2222.3333 3333.2222.1111
test etherchannel load-balance interface po 2 mac c000.1111.1111 2222.2222.2222
test etherchannel load-balance interface po 2 ip 172.30.126.1 188.8.131.52