Recently I worked on a two nodes cluster issue.
The software was : CentOS 5 + heartbeat + DRBD + Pacemaker
Due to some issue I rebooted one node and tried to speak with the other via heartbeat/crm manual commands.
After this I saw these strange messages ( never faced before ):
heartbeat[5855]: 2013/08/13_20:58:15 ERROR: Cannot rexmit pkt 133 for system04: seqno too low
Looking on the documentation/web I did not found any solution or tip related to this message.
I verified that network connection was active and not low. No collisions , right speed and duplex , everything was ok. I tried first to stop heartbeat on one node and the messages did not stop. I halted the node then and nothing changed!!!!
After other tests I resolved rebooting both nodes.
This is bad solution, but I usually see heartbeat/DRBD/Pacemaker are not stable. I can understand with DRBD there is no need to buy any storage but it’s better have even a little DAS and use CentOS native Cluster Suite!
Alessio dini's Blog 