Hi Team, Issue : We are performing a rolling upgrade from Pacemaker v2.1.6 to v3.0.1 in a two-node cluster. According to the [Pacemaker 3.0 Changes documentation]( ⚡ Pacemaker 3.0 Changes<https://projects.clusterlabs.org/w/projects/pacemaker/pacemaker_3.0_changes/> ), rolling upgrades from Pacemaker 2.0.0 and later should be supported with minimal changes.
However, after upgrading the first node (Node 1) to Pacemaker 3.0.1, the second node (Node 2) running Pacemaker 2.1.6 is unable to connect to the cluster after reboot of node2. Pacemaker fails to start on Node 2 with the error: Error: error running crm_mon, is pacemaker running? crm_mon: Connection to cluster failed: Connection refused Expected Behaviour: During a rolling upgrade, both nodes should remain operational and communicate with each other until the second node is upgraded. The non-upgraded node (2.1.6) should continue to function while the upgraded node (3.0.1) acts as DC. Questions: 1. Is this a known limitation with the rolling upgrade path from 2.1.6 to 3.0.1? 2. Are there specific compatibility issues between Pacemaker 3.0.1 and 2.1.6 that prevent cluster communication? 3. Are there any additional logs or diagnostic information needed to troubleshoot this issue? Environment: - Cluster: 2-node RHEL 8 cluster with Corosync - Node 1 (node1): Upgraded to Pacemaker 3.0.1-1.el8 - Node 2 (node2): Running Pacemaker 2.1.6-1.el8 (not yet upgraded) Node 1 (node1) - Upgraded: Pacemaker: 3.0.1-1.el8 Corosync: 3.1.10-1.el8 PCS: 0.12.2-1.el8 libknet1: 1.33-1.el8 resource-agents: 4.17.0-1.el8 Node 2 (node2) - Not Upgraded: Pacemaker: 2.1.6-1.el8 Corosync: 3.1.7-1.el8 PCS: 0.10.19-2.el8 libknet1: 1.25-1.el8 resource-agents: 4.12.0-1.el8 Node1: [root@node1 testadmin]# pcs status pcsd node1: Online node2: Online [root@node1 testadmin]# pcs status Cluster Summary: * Stack: corosync (Pacemaker is running) * Current DC: node1 (version 3.0.1-1.el8-3.0.1) - partition with quorum * Last updated: Sat Feb 7 11:39:00 2026 on node1 * Last change: Sat Feb 7 11:22:01 2026 by hacluster via hacluster on node1 * 2 nodes configured * 23 resource instances configured (2 DISABLED) Node List: * Online: [ node1 ] * OFFLINE: [ node2 ] Full List of Resources: * SNMP_node2 (ocf:pacemaker:ClusterMon): Stopped * SNMP_node1 (ocf:pacemaker:ClusterMon): Started node1 Daemon Status: corosync: active/enabled pacemaker: active/enabled pcsd: active/enabled [root@node1 testadmin]# rpm -qa | egrep "pcs|pacemaker|corosy|pacemak|resource-a|libknet" libknet1-1.33-1.el8.x86_64 libknet1-compress-lz4-plugin-1.33-1.el8.x86_64 libknet1-compress-zstd-plugin-1.33-1.el8.x86_64 libknet1-crypto-plugins-all-1.33-1.el8.x86_64 pacemaker-libs-3.0.1-1.el8.x86_64 pcs-0.12.2-1.el8.x86_64 corosynclib-3.1.10-1.el8.x86_64 libknet1-compress-bzip2-plugin-1.33-1.el8.x86_64 libknet1-compress-lzma-plugin-1.33-1.el8.x86_64 libknet1-compress-zlib-plugin-1.33-1.el8.x86_64 libknet1-compress-plugins-all-1.33-1.el8.x86_64 libknet1-crypto-openssl-plugin-1.33-1.el8.x86_64 libknet1-plugins-all-1.33-1.el8.x86_64 pacemaker-schemas-3.0.1-1.el8.noarch pacemaker-cluster-libs-3.0.1-1.el8.x86_64 pacemaker-3.0.1-1.el8.x86_64 corosync-3.1.10-1.el8.x86_64 libknet1-compress-lzo2-plugin-1.33-1.el8.x86_64 libknet1-crypto-nss-plugin-1.33-1.el8.x86_64 resource-agents-4.17.0-1.el8.x86_64 pacemaker-cli-3.0.1-1.el8.x86_64 [root@node1 testadmin]# Node2: [root@node2 testadmin]# pcs status Error: error running crm_mon, is pacemaker running? crm_mon: Connection to cluster failed: Connection refused [root@node2 testadmin]# pcs status pcsd node2: Online node1: Online [root@node2 testadmin]# [root@node2 testadmin]# rpm -qa | egrep "pcs|pacemaker|corosy|pacemak|resource-a|libknet" libknet1-compress-bzip2-plugin-1.25-1.el8.x86_64 libknet1-compress-zlib-plugin-1.25-1.el8.x86_64 libknet1-crypto-openssl-plugin-1.25-1.el8.x86_64 pacemaker-schemas-2.1.6-1.el8.noarch pacemaker-2.1.6-1.el8.x86_64 libknet1-1.25-1.el8.x86_64 corosync-3.1.7-1.el8.x86_64 libknet1-compress-lz4-plugin-1.25-1.el8.x86_64 libknet1-compress-lzo2-plugin-1.25-1.el8.x86_64 libknet1-compress-zstd-plugin-1.25-1.el8.x86_64 libknet1-crypto-nss-plugin-1.25-1.el8.x86_64 libknet1-crypto-plugins-all-1.25-1.el8.x86_64 resource-agents-4.12.0-1.el8.x86_64 pacemaker-libs-2.1.6-1.el8.x86_64 pacemaker-cli-2.1.6-1.el8.x86_64 pcs-0.10.19-2.el8.x86_64 corosynclib-3.1.7-1.el8.x86_64 libknet1-compress-lzma-plugin-1.25-1.el8.x86_64 libknet1-compress-plugins-all-1.25-1.el8.x86_64 libknet1-plugins-all-1.25-1.el8.x86_64 pacemaker-cluster-libs-2.1.6-1.el8.x86_64 Thanks and Regards, S Sathish S
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