# Cisco Expressway clustering

Canonical: https://warmtransfer.net/knowledge/expressway-clustering

Last verified: 2026-09-25

An Expressway cluster can contain up to 6 Expressway peers[^27]. Within the cluster, 1 cluster peer is nominated as the primary, and its configuration is replicated to the other peers[^40].

## Capacity and sizing

The X15.4 cluster deployment guide states clustering can increase Expressway capacity by a maximum factor of 4 compared with a single Expressway, and that there is no capacity gain after 4 peers[^5]. From X14.2, in the 4+1 redundancy model up to 5 Expressway systems can be clustered for a total capacity of 4 times a single system, with 1 node counted as redundancy[^1]. From X14.2, in the 5+1 redundancy model up to 6 Expressway systems can be clustered for a total capacity of 5 times a single system, with 1 node counted as redundancy; this is disputed, because another section of the guide says clustering adds no capacity beyond 4 peers[^2][^5].

Clustering Small VM Expressways provides redundancy only and gives no additional capacity[^51]. Cisco cautions that Expressway capacity figures assume single-service scenarios and that capacity limits cannot be given for individual, specific deployments[^6]. Regarding host sizing, 2 Large Expressway VMs can co-reside on a UCS server with two eight-core 3.2 GHz processors dedicated to Expressway when hyperthreading is enabled[^22].

Expressway capacity licensing is per cluster: capacity licenses installed on any peer, including Rich Media Session (RMS) and room/desktop system registration licenses, are available to every peer[^24]. A call between endpoints registered to different peers uses no RMS licence if its media does not traverse the cluster peers[^20].

## Node and platform requirements

An Expressway cluster must contain only Expressway-C nodes or only Expressway-E nodes; the 2 cannot be mixed in 1 cluster[^29]. Clusters mixing Expressway and Expressway Select peers are also not supported[^32]. All peers in an Expressway cluster must run the same software version, except for the short period while the cluster is being upgraded[^48].

Peers on standard appliances can be clustered with peers on 2-core Medium VMs, but not with peers on 8-core Large VMs[^3]. All cluster peers must have the same set of option keys installed, except RMS licenses and room system and desktop system registration licenses[^35]. Every peer in an Expressway cluster must have the same routing capabilities, because the cluster assumes any peer can route to any destination another peer can reach[^47].

## Network and inter-peer communication

Expressway clustering supports a round-trip delay of up to 80 ms between peers[^46]. There must be no NAT between Expressway cluster peers, and any firewall between them must open the required ports[^30]. WarmTransfer's reading of the sources is that because peers must be within 80 ms round trip with no NAT between them, a single Expressway cluster can only span sites with low-latency routed connectivity, and widely separated regions would need separate clusters[^17].

From X8.8, cluster peers use TCP 4371 for cluster recovery and TLS on TCP 4372 for cluster communication and database synchronization[^37]. Before X8.8, Expressway clustering used IPsec (AH and ISAKMP on UDP 500) between peers rather than TLS[^38]. H.323 mode must be on for each Expressway cluster peer because clustering uses H.323 communication between peers; if it is off, an Invalid clustering configuration alarm is raised[^19].

## DNS and endpoint interaction

Cisco recommends a DNS SRV record for the Expressway cluster containing A or AAAA records for each peer[^14]. For Mobile and Remote Access, a collab-edge SRV record should be created for each peer in the Expressway-E cluster[^28].

An H.323 endpoint registering to 1 peer receives a list of alternate gatekeepers containing the IP addresses of all other cluster peers in random order[^18]. SIP endpoints that support RFC 5626 can register simultaneously to multiple Expressway cluster peers[^50]. In addition, cluster peers share bandwidth usage information, so bandwidth consumed in a subzone or pipe by 1 peer is unavailable to the other peers[^4].

If Cisco TMS is used with an Expressway cluster it must run version 13.2 or later, or 12.6 or later if TMS is not used for provisioning or FindMe[^55].

## Cluster formation and configuration replication

To form a cluster, an administrator first creates a cluster of 1 (the primary) and restarts it before adding other peers[^10]. The Expressway cluster name should be the routable FQDN used in the SRV records that address the cluster, and the same cluster name must be configured on every peer[^9].

Cluster TLS verification mode defaults to Permissive; Enforce requires valid peer certificates[^54]. All peers in an Expressway cluster must use the same TLS verification mode[^53]. Cisco recommends forming the cluster with IP addresses in Permissive mode, then changing peer addresses to FQDNs, and then switching TLS verification mode to Enforce[^16].

Configuration changes made on a peer other than the primary are not replicated and will be overwritten at the next replication from the primary[^52]. DNS server settings are peer-specific and are not replicated; each peer can use a different set of DNS servers[^13]. NTP servers are peer-specific, and each peer may use 1 or more different NTP servers[^33]. Trusted CA certificates, the server certificate, and CRLs must be uploaded to each Expressway peer individually; they are not replicated[^7]. Conference Factory template configuration is not replicated across Expressway cluster peers; Cisco documents this as expected behaviour[^11].

## Maintenance, cluster changes, and troubleshooting

To remove a live peer, an administrator clears all Peer N address fields on it, saves, and restarts, which factory-resets it; the primary and remaining peers then need identical, gap-free peer lists[^41]. If a removed dead peer is later recovered, its configuration must be cleared before it is reconnected to the network[^12]. Changing a clustered peer's IP address, hostname, or FQDN requires removing it from the cluster, making the change, and adding it back[^8]. Furthermore, a backup cannot be restored to an Expressway while it is part of a cluster; it must first be removed from the cluster[^45].

When upgrading an Expressway cluster, upgrade the primary peer first and then the subordinate peers 1 at a time[^58]. Cisco recommends upgrading the Expressway-E cluster before the Expressway-C cluster, starting with the primary peer in each[^56]. While a cluster is mid-upgrade, configuration changes are not replicated to subordinate peers running a different version from the primary[^31]. The X15.4 cluster deployment guide defers cluster upgrade instructions to the release notes for the target version[^57]. Cisco TAC's X14.x upgrade note advises confirming no cluster alarms exist and taking a backup of each server before upgrading[^39].

In a cluster, a peer in maintenance mode has its workload handled by the other peers, and Cisco advises enabling maintenance mode on only 1 peer at a time in large deployments[^26].

Cluster replication health is checked at System > Clustering on each peer, where the cluster database status should read Active[^43]. For the alarm requiring manual synchronization of configuration, Cisco directs running xcommand ForceConfigUpdate as admin on the subordinate, which deletes its configuration and forces an update from the primary; back up first[^15]. The documented cluster restart sequence is to restart the primary, wait until its web interface is reachable and replication is healthy, then restart the other peers 1 at a time[^44]. A cluster replication error can also be raised because the NTP server is unreachable; the fix is to configure an accessible NTP server on System > Time[^34].

## See also

See also [Expressway and Mobile and Remote Access](https://warmtransfer.net/knowledge/cucm-expressway-mra).
See also [Setting up Mobile and Remote Access on Cisco Expressway](https://warmtransfer.net/knowledge/expressway-mra-setup).

## Applicability

Applies to: Cisco Expressway. Deployments: on-premises. Sources checked 2026-09-25. As of 2026-09-25, Cisco's Expressway release-notes listing includes an X15.5.x release note above X15.4.x, whose release note is dated April 13, 2026[^23].

## What remains uncertain

The following items are not covered by the sources below:
- X15.5 release note clustering and upgrade content is not covered by the sources below.
- How to reconcile the 4x capacity gain limit with the 5+1 redundancy model scaling statement is not covered by the sources below.
- Client Auth EKU sunset in public CA certs and its interaction with cluster TLS Enforce mode is not covered by the sources below.
- MRA deployment guide guidance on cluster DNS and configuring traversal zones to multiple Expressway-E peers is not covered by the sources below.
- Cisco Smart Licensing behaviour for pooled RMS licences across cluster peers is not covered by the sources below.
- Numeric alarm IDs for cluster alarms are not covered by the sources below.
- The CE1300 hardware platform MRA registration capacity figure is not covered by the sources below.
- Certificate Subject Alternative Name (SAN) requirements for cluster TLS Enforce mode are not covered by the sources below.

## Sources

[^1]: From X14.2, in the 4+1 redundancy model up to five Expressway systems can be clustered for a total capacity of four times a single system, with one node counted as redundancy. Source: [Cisco Expressway Administrator Guide (X15.4) - Expressway Capacity and Sizing](https://www.cisco.com/c/en/us/td/docs/voice_ip_comm/expressway/admin_guide/X15-4/exwy_b_cisco-expressway-administrator-guide-x154/exwy_m_expressway-capacity-and-sizing.html), Expressway Capacity and Sizing chapter, Important Caveats, 4+1 redundancy model sentence. Checked 2026-09-25.
[^2]: From X14.2, in the 5+1 redundancy model up to six Expressway systems can be clustered for a total capacity of five times a single system, with one node counted as redundancy. Source: [Cisco Expressway Administrator Guide (X15.4) - Expressway Capacity and Sizing](https://www.cisco.com/c/en/us/td/docs/voice_ip_comm/expressway/admin_guide/X15-4/exwy_b_cisco-expressway-administrator-guide-x154/exwy_m_expressway-capacity-and-sizing.html), Expressway Capacity and Sizing chapter, Important Caveats, 5+1 redundancy model sentence. Checked 2026-09-25.
[^3]: Peers on standard appliances can be clustered with peers on 2-core Medium VMs, but not with peers on 8-core Large VMs. Source: [Cisco Expressway Cluster Creation and Maintenance Deployment Guide (X15.4) - Clustering Requirements](https://www.cisco.com/c/en/us/td/docs/voice_ip_comm/expressway/config_guide/X15-4/cluster_creation_maint/exwy_b_cisco-expressway-cluster-creation-and-maintenance-deployment-guide-x154/exwy_m_clustering-requirements.html), Clustering Requirements chapter, Platform and Software Versions Match. Checked 2026-09-25.
[^4]: Cluster peers share bandwidth usage information, so bandwidth consumed in a subzone or pipe by one peer is unavailable to the other peers. Source: [Cisco Expressway Administrator Guide (X15.4) - Clustering and Peers](https://www.cisco.com/c/en/us/td/docs/voice_ip_comm/expressway/admin_guide/X15-4/exwy_b_cisco-expressway-administrator-guide-x154/exwy_m_clustering-and-peers.html), Clustering and Peers chapter, bandwidth sharing section. Checked 2026-09-25.
[^5]: The X15.4 cluster deployment guide states clustering can increase Expressway capacity by a maximum factor of four compared with a single Expressway and that there is no capacity gain after four peers. Source: [Cisco Expressway Cluster Creation and Maintenance Deployment Guide (X15.4) - Clustering Basics](https://www.cisco.com/c/en/us/td/docs/voice_ip_comm/expressway/config_guide/X15-4/cluster_creation_maint/exwy_b_cisco-expressway-cluster-creation-and-maintenance-deployment-guide-x154/exwy_m_clustering-basics.html), Clustering Basics chapter, overview of reasons to cluster (capacity). Checked 2026-09-25.
[^6]: Cisco cautions that Expressway capacity figures assume single-service scenarios and that capacity limits cannot be given for individual, specific deployments. Source: [Cisco Expressway Administrator Guide (X15.4) - Expressway Capacity and Sizing](https://www.cisco.com/c/en/us/td/docs/voice_ip_comm/expressway/admin_guide/X15-4/exwy_b_cisco-expressway-administrator-guide-x154/exwy_m_expressway-capacity-and-sizing.html), Expressway Capacity and Sizing chapter, Important Caveats. Checked 2026-09-25.
[^7]: Trusted CA certificates, the server certificate and CRLs must be uploaded to each Expressway peer individually; they are not replicated. Source: [Cisco Expressway Administrator Guide (X15.4) - Clustering and Peers](https://www.cisco.com/c/en/us/td/docs/voice_ip_comm/expressway/admin_guide/X15-4/exwy_b_cisco-expressway-administrator-guide-x154/exwy_m_clustering-and-peers.html), Clustering and Peers chapter, Peer-Specific Items in Clustered Systems (security certificates). Checked 2026-09-25.
[^8]: Changing a clustered peer's IP address, hostname or FQDN requires removing it from the cluster, making the change, and adding it back. Source: [Cisco Expressway Cluster Creation and Maintenance Deployment Guide (X15.4) - How to Change a Cluster](https://www.cisco.com/c/en/us/td/docs/voice_ip_comm/expressway/config_guide/X15-4/cluster_creation_maint/exwy_b_cisco-expressway-cluster-creation-and-maintenance-deployment-guide-x154/exwy_m_how-to-change-a-cluster.html), How to Change a Cluster chapter, change peer IP address or identity. Checked 2026-09-25.
[^9]: The Expressway cluster name should be the routable FQDN used in the SRV records that address the cluster, and the same cluster name must be configured on every peer. Source: [Cisco Expressway Cluster Creation and Maintenance Deployment Guide (X15.4) - How to Form a Cluster](https://www.cisco.com/c/en/us/td/docs/voice_ip_comm/expressway/config_guide/X15-4/cluster_creation_maint/exwy_b_cisco-expressway-cluster-creation-and-maintenance-deployment-guide-x154/exwy_m_how-to-form-a-cluster.html), How to Form a Cluster chapter, cluster name field. Checked 2026-09-25.
[^10]: To form a cluster you first create a cluster of one (the primary) and restart it before adding other peers. Source: [Cisco Expressway Cluster Creation and Maintenance Deployment Guide (X15.4) - How to Form a Cluster](https://www.cisco.com/c/en/us/td/docs/voice_ip_comm/expressway/config_guide/X15-4/cluster_creation_maint/exwy_b_cisco-expressway-cluster-creation-and-maintenance-deployment-guide-x154/exwy_m_how-to-form-a-cluster.html), How to Form a Cluster chapter, primary peer set-up. Checked 2026-09-25.
[^11]: Conference Factory template configuration is not replicated across Expressway cluster peers; Cisco documents this as expected behaviour. Source: [Cisco Expressway Cluster Creation and Maintenance Deployment Guide (X15.4) - Troubleshooting](https://www.cisco.com/c/en/us/td/docs/voice_ip_comm/expressway/config_guide/X15-4/cluster_creation_maint/exwy_b_cisco-expressway-cluster-creation-and-maintenance-deployment-guide-x154/exwy_m_troubleshooting.html), Troubleshooting chapter, Conference Factory Template Does Not Replicate. Checked 2026-09-25.
[^12]: If a removed dead peer is later recovered, its configuration must be cleared before it is reconnected to the network. Source: [Cisco Expressway Cluster Creation and Maintenance Deployment Guide (X15.4) - How to Change a Cluster](https://www.cisco.com/c/en/us/td/docs/voice_ip_comm/expressway/config_guide/X15-4/cluster_creation_maint/exwy_b_cisco-expressway-cluster-creation-and-maintenance-deployment-guide-x154/exwy_m_how-to-change-a-cluster.html), How to Change a Cluster chapter, Remove a Dead Peer. Checked 2026-09-25.
[^13]: DNS server settings are peer-specific and are not replicated; each peer can use a different set of DNS servers. Source: [Cisco Expressway Administrator Guide (X15.4) - Clustering and Peers](https://www.cisco.com/c/en/us/td/docs/voice_ip_comm/expressway/admin_guide/X15-4/exwy_b_cisco-expressway-administrator-guide-x154/exwy_m_clustering-and-peers.html), Clustering and Peers chapter, Peer-Specific Items in Clustered Systems (DNS servers). Checked 2026-09-25.
[^14]: Cisco recommends a DNS SRV record for the Expressway cluster containing A or AAAA records for each peer. Source: [Cisco Expressway Cluster Creation and Maintenance Deployment Guide (X15.4) - Clustering Requirements](https://www.cisco.com/c/en/us/td/docs/voice_ip_comm/expressway/config_guide/X15-4/cluster_creation_maint/exwy_b_cisco-expressway-cluster-creation-and-maintenance-deployment-guide-x154/exwy_m_clustering-requirements.html), Clustering Requirements chapter, DNS Configuration Is Done. Checked 2026-09-25.
[^15]: For the alarm requiring manual synchronization of configuration, Cisco directs running xcommand ForceConfigUpdate as admin on the subordinate, which deletes its configuration and forces an update from the primary; back up first. Source: [Cisco Expressway Cluster Creation and Maintenance Deployment Guide (X15.4) - Troubleshooting](https://www.cisco.com/c/en/us/td/docs/voice_ip_comm/expressway/config_guide/X15-4/cluster_creation_maint/exwy_b_cisco-expressway-cluster-creation-and-maintenance-deployment-guide-x154/exwy_m_troubleshooting.html), Troubleshooting chapter, Cluster replication error: manual synchronization of configuration is required. Checked 2026-09-25.
[^16]: Cisco recommends forming the cluster with IP addresses in Permissive mode, then changing peer addresses to FQDNs, and then switching TLS verification mode to Enforce. Source: [Cisco Expressway Cluster Creation and Maintenance Deployment Guide (X15.4) - How to Form a Cluster](https://www.cisco.com/c/en/us/td/docs/voice_ip_comm/expressway/config_guide/X15-4/cluster_creation_maint/exwy_b_cisco-expressway-cluster-creation-and-maintenance-deployment-guide-x154/exwy_m_how-to-form-a-cluster.html), How to Form a Cluster chapter, TLS verification mode guidance. Checked 2026-09-25.
[^17]: Because peers must be within 80 ms round trip with no NAT between them, a single Expressway cluster can only span sites with low-latency routed connectivity; widely separated regions would need separate clusters (inferred). Source: [Cisco Expressway Cluster Creation and Maintenance Deployment Guide (X15.4) - Clustering Requirements](https://www.cisco.com/c/en/us/td/docs/voice_ip_comm/expressway/config_guide/X15-4/cluster_creation_maint/exwy_b_cisco-expressway-cluster-creation-and-maintenance-deployment-guide-x154/exwy_m_clustering-requirements.html), Clustering Requirements chapter, Network Conditions Are Met. Checked 2026-09-25.
[^18]: An H.323 endpoint registering to one peer receives a list of alternate gatekeepers containing the IP addresses of all other cluster peers in random order. Source: [Cisco Expressway Administrator Guide (X15.4) - Clustering and Peers](https://www.cisco.com/c/en/us/td/docs/voice_ip_comm/expressway/admin_guide/X15-4/exwy_b_cisco-expressway-administrator-guide-x154/exwy_m_clustering-and-peers.html), Clustering and Peers chapter, H.323 registrations. Checked 2026-09-25.
[^19]: H.323 mode must be on for each Expressway cluster peer because clustering uses H.323 communication between peers; if it is off, an Invalid clustering configuration alarm is raised. Source: [Cisco Expressway Cluster Creation and Maintenance Deployment Guide (X15.4) - Troubleshooting](https://www.cisco.com/c/en/us/td/docs/voice_ip_comm/expressway/config_guide/X15-4/cluster_creation_maint/exwy_b_cisco-expressway-cluster-creation-and-maintenance-deployment-guide-x154/exwy_m_troubleshooting.html), Troubleshooting chapter, Invalid clustering configuration: H.323 mode. Checked 2026-09-25.
[^20]: A call between endpoints registered to different peers uses no RMS licence if its media does not traverse the cluster peers. Source: [Cisco Expressway Administrator Guide (X15.4) - Clustering and Peers](https://www.cisco.com/c/en/us/td/docs/voice_ip_comm/expressway/admin_guide/X15-4/exwy_b_cisco-expressway-administrator-guide-x154/exwy_m_clustering-and-peers.html), Clustering and Peers chapter, Intracluster Calls. Checked 2026-09-25.
[^21]: In X15.x a standalone Large VM Expressway is rated for 5,000 native registrations, 500 video or 1,000 audio calls, and 3,500 MRA registrations. Source: [Cisco Expressway Administrator Guide (X15.4) - Expressway Capacity and Sizing](https://www.cisco.com/c/en/us/td/docs/voice_ip_comm/expressway/admin_guide/X15-4/exwy_b_cisco-expressway-administrator-guide-x154/exwy_m_expressway-capacity-and-sizing.html), Expressway Capacity and Sizing chapter, standalone capacity table, Large VM row. Checked 2026-09-25.
[^22]: Two Large Expressway VMs can co-reside on a UCS server with two eight-core 3.2 GHz processors dedicated to Expressway when hyperthreading is enabled. Source: [Cisco Expressway Install and Upgrade Guide (X15.4) - Requirements and Limitations](https://www.cisco.com/c/en/us/td/docs/voice_ip_comm/expressway/install_guide/X15-4/install-upgrade/exwy_b_cisco-expressway-install-and-upgrade-guide-x154/exwy_m_requirements-and-limitations.html), Requirements and Limitations chapter, ESXi Requirements, co-residency note. Checked 2026-09-25.
[^23]: As of 2026-09-25 Cisco's Expressway release-notes listing includes an X15.5.x release note above X15.4.x, whose release note is dated April 13, 2026. Source: [Cisco Expressway Series - Release Notes](https://www.cisco.com/c/en/us/support/unified-communications/expressway-series/products-release-notes-list.html), Release Notes listing, X15.5.x and X15.4.x entries. Checked 2026-09-25.
[^24]: Expressway capacity licensing is per cluster: capacity licenses installed on any peer, including Rich Media Session and room/desktop system registration licenses, are available to every peer. Source: [Cisco Expressway Cluster Creation and Maintenance Deployment Guide (X15.4) - Clustering Basics](https://www.cisco.com/c/en/us/td/docs/voice_ip_comm/expressway/config_guide/X15-4/cluster_creation_maint/exwy_b_cisco-expressway-cluster-creation-and-maintenance-deployment-guide-x154/exwy_m_clustering-basics.html), Clustering Basics chapter, licensing statement. Checked 2026-09-25.
[^25]: In maintenance mode, existing calls on the peer continue until they end, while new calls and registrations go to other peers. Source: [Cisco Expressway Administrator Guide (X15.4) - Maintenance](https://www.cisco.com/c/en/us/td/docs/voice_ip_comm/expressway/admin_guide/X15-4/exwy_b_cisco-expressway-administrator-guide-x154/exwy_m_maintenance.html), Maintenance chapter, Enable Maintenance Mode. Checked 2026-09-25.
[^26]: In a cluster, a peer in maintenance mode has its workload handled by the other peers, and Cisco advises enabling maintenance mode on only one peer at a time in large deployments. Source: [Cisco Expressway Administrator Guide (X15.4) - Maintenance](https://www.cisco.com/c/en/us/td/docs/voice_ip_comm/expressway/admin_guide/X15-4/exwy_b_cisco-expressway-administrator-guide-x154/exwy_m_maintenance.html), Maintenance chapter, Enable Maintenance Mode. Checked 2026-09-25.
[^27]: An Expressway cluster can contain up to six Expressway peers. Source: [Cisco Expressway Cluster Creation and Maintenance Deployment Guide (X15.4) - Clustering Basics](https://www.cisco.com/c/en/us/td/docs/voice_ip_comm/expressway/config_guide/X15-4/cluster_creation_maint/exwy_b_cisco-expressway-cluster-creation-and-maintenance-deployment-guide-x154/exwy_m_clustering-basics.html), Clustering Basics chapter, opening overview section. Checked 2026-09-25.
[^28]: For Mobile and Remote Access, a collab-edge SRV record should be created for each peer in the Expressway-E cluster. Source: [Cisco Expressway Cluster Creation and Maintenance Deployment Guide (X15.4) - Clustering Requirements](https://www.cisco.com/c/en/us/td/docs/voice_ip_comm/expressway/config_guide/X15-4/cluster_creation_maint/exwy_b_cisco-expressway-cluster-creation-and-maintenance-deployment-guide-x154/exwy_m_clustering-requirements.html), Clustering Requirements chapter, DNS Configuration Is Done, MRA item. Checked 2026-09-25.
[^29]: An Expressway cluster must contain only Expressway-C nodes or only Expressway-E nodes; the two cannot be mixed in one cluster. Source: [Cisco Expressway Cluster Creation and Maintenance Deployment Guide (X15.4) - Clustering Requirements](https://www.cisco.com/c/en/us/td/docs/voice_ip_comm/expressway/config_guide/X15-4/cluster_creation_maint/exwy_b_cisco-expressway-cluster-creation-and-maintenance-deployment-guide-x154/exwy_m_clustering-requirements.html), Clustering Requirements chapter, deployment restrictions. Checked 2026-09-25.
[^30]: There must be no NAT between Expressway cluster peers, and any firewall between them must open the required ports. Source: [Cisco Expressway Cluster Creation and Maintenance Deployment Guide (X15.4) - Clustering Requirements](https://www.cisco.com/c/en/us/td/docs/voice_ip_comm/expressway/config_guide/X15-4/cluster_creation_maint/exwy_b_cisco-expressway-cluster-creation-and-maintenance-deployment-guide-x154/exwy_m_clustering-requirements.html), Clustering Requirements chapter, Network Conditions Are Met. Checked 2026-09-25.
[^31]: While a cluster is mid-upgrade, configuration changes are not replicated to subordinate peers running a different version from the primary. Source: [Cisco Expressway Administrator Guide (X15.4) - Maintenance](https://www.cisco.com/c/en/us/td/docs/voice_ip_comm/expressway/admin_guide/X15-4/exwy_b_cisco-expressway-administrator-guide-x154/exwy_m_maintenance.html), Maintenance chapter, Process to Upgrade a Clustered System. Checked 2026-09-25.
[^32]: Clusters mixing Expressway and Expressway Select peers are not supported. Source: [Cisco Expressway Cluster Creation and Maintenance Deployment Guide (X15.4) - Clustering Requirements](https://www.cisco.com/c/en/us/td/docs/voice_ip_comm/expressway/config_guide/X15-4/cluster_creation_maint/exwy_b_cisco-expressway-cluster-creation-and-maintenance-deployment-guide-x154/exwy_m_clustering-requirements.html), Clustering Requirements chapter, deployment restrictions. Checked 2026-09-25.
[^33]: NTP servers are peer-specific; each peer may use one or more different NTP servers. Source: [Cisco Expressway Administrator Guide (X15.4) - Clustering and Peers](https://www.cisco.com/c/en/us/td/docs/voice_ip_comm/expressway/admin_guide/X15-4/exwy_b_cisco-expressway-administrator-guide-x154/exwy_m_clustering-and-peers.html), Clustering and Peers chapter, Peer-Specific Items in Clustered Systems (NTP servers). Checked 2026-09-25.
[^34]: A cluster replication error can be raised because the NTP server is unreachable; the fix is to configure an accessible NTP server on System > Time. Source: [Cisco Expressway Cluster Creation and Maintenance Deployment Guide (X15.4) - Troubleshooting](https://www.cisco.com/c/en/us/td/docs/voice_ip_comm/expressway/config_guide/X15-4/cluster_creation_maint/exwy_b_cisco-expressway-cluster-creation-and-maintenance-deployment-guide-x154/exwy_m_troubleshooting.html), Troubleshooting chapter, Cluster replication error: NTP server unreachable. Checked 2026-09-25.
[^35]: All cluster peers must have the same set of option keys installed, except RMS licenses and room system and desktop system registration licenses. Source: [Cisco Expressway Cluster Creation and Maintenance Deployment Guide (X15.4) - Clustering Requirements](https://www.cisco.com/c/en/us/td/docs/voice_ip_comm/expressway/config_guide/X15-4/cluster_creation_maint/exwy_b_cisco-expressway-cluster-creation-and-maintenance-deployment-guide-x154/exwy_m_clustering-requirements.html), Clustering Requirements chapter, Basic Configuration Is Done. Checked 2026-09-25.
[^36]: If a replication error persists after ForceConfigUpdate, Cisco advises upgrading the primary again with the original upgrade package; the alarm typically clears 10 to 20 minutes after reboot. Source: [Cisco Expressway Cluster Creation and Maintenance Deployment Guide (X15.4) - Troubleshooting](https://www.cisco.com/c/en/us/td/docs/voice_ip_comm/expressway/config_guide/X15-4/cluster_creation_maint/exwy_b_cisco-expressway-cluster-creation-and-maintenance-deployment-guide-x154/exwy_m_troubleshooting.html), Troubleshooting chapter, replication error persisting after ForceConfigUpdate. Checked 2026-09-25.
[^37]: From X8.8, cluster peers use TCP 4371 for cluster recovery and TLS on TCP 4372 for cluster communication and database synchronization. Source: [Cisco Expressway IP Port Usage Configuration Guide (Includes X14.3 and X15.0 releases) - Clustering Connections](https://www.cisco.com/c/en/us/td/docs/voice_ip_comm/expressway/config_guide/X15-0/ip-port/exwy_b_cisco-expressway-ip-port-usage-configuration-guide-x15/exwy_m_clustering-connections.html), Clustering Connections chapter, Table 2 (cluster database synchronization). Checked 2026-09-25.
[^38]: Before X8.8, Expressway clustering used IPsec (AH and ISAKMP on UDP 500) between peers rather than TLS. Source: [Cisco Expressway IP Port Usage Configuration Guide (Includes X14.3 and X15.0 releases) - Clustering Connections](https://www.cisco.com/c/en/us/td/docs/voice_ip_comm/expressway/config_guide/X15-0/ip-port/exwy_b_cisco-expressway-ip-port-usage-configuration-guide-x15/exwy_m_clustering-connections.html), Clustering Connections chapter, Cluster Connections Before X8.8. Checked 2026-09-25.
[^39]: Cisco TAC's X14.x upgrade note advises confirming no cluster alarms exist and taking a backup of each server before upgrading. Source: [Upgrade Video Communication Server (VCS) / Expressway X14.x](https://www.cisco.com/c/en/us/support/docs/unified-communications/expressway/217743-upgrade-of-video-communication-server-v.html), Pre-upgrade requirements section (cluster stability check; backup). Checked 2026-09-25.
[^40]: One cluster peer is nominated as the primary, and its configuration is replicated to the other peers. Source: [Cisco Expressway Cluster Creation and Maintenance Deployment Guide (X15.4) - Clustering Basics](https://www.cisco.com/c/en/us/td/docs/voice_ip_comm/expressway/config_guide/X15-4/cluster_creation_maint/exwy_b_cisco-expressway-cluster-creation-and-maintenance-deployment-guide-x154/exwy_m_clustering-basics.html), Clustering Basics chapter, overview. Checked 2026-09-25.
[^41]: To remove a live peer you clear all Peer N address fields on it, save and restart, which factory-resets it; the primary and remaining peers then need identical, gap-free peer lists. Source: [Cisco Expressway Cluster Creation and Maintenance Deployment Guide (X15.4) - How to Change a Cluster](https://www.cisco.com/c/en/us/td/docs/voice_ip_comm/expressway/config_guide/X15-4/cluster_creation_maint/exwy_b_cisco-expressway-cluster-creation-and-maintenance-deployment-guide-x154/exwy_m_how-to-change-a-cluster.html), How to Change a Cluster chapter, Remove a Live Peer. Checked 2026-09-25.
[^42]: Configuration is copied from the primary to the other peers about every two minutes, and changes can take up to about a minute to appear on all peers. Source: [Cisco Expressway Cluster Creation and Maintenance Deployment Guide (X15.4) - How to Form a Cluster](https://www.cisco.com/c/en/us/td/docs/voice_ip_comm/expressway/config_guide/X15-4/cluster_creation_maint/exwy_b_cisco-expressway-cluster-creation-and-maintenance-deployment-guide-x154/exwy_m_how-to-form-a-cluster.html), How to Form a Cluster chapter, post-restart verification steps. Checked 2026-09-25.
[^43]: Cluster replication health is checked at System > Clustering on each peer, where the cluster database status should read Active. Source: [Cisco Expressway Cluster Creation and Maintenance Deployment Guide (X15.4) - Troubleshooting](https://www.cisco.com/c/en/us/td/docs/voice_ip_comm/expressway/config_guide/X15-4/cluster_creation_maint/exwy_b_cisco-expressway-cluster-creation-and-maintenance-deployment-guide-x154/exwy_m_troubleshooting.html), Troubleshooting chapter, Check Replication Status. Checked 2026-09-25.
[^44]: The documented cluster restart sequence is to restart the primary, wait until its web interface is reachable and replication is healthy, then restart the other peers one at a time. Source: [Cisco Expressway Cluster Creation and Maintenance Deployment Guide (X15.4) - Troubleshooting](https://www.cisco.com/c/en/us/td/docs/voice_ip_comm/expressway/config_guide/X15-4/cluster_creation_maint/exwy_b_cisco-expressway-cluster-creation-and-maintenance-deployment-guide-x154/exwy_m_troubleshooting.html), Troubleshooting chapter, Restarting Sequence. Checked 2026-09-25.
[^45]: A backup cannot be restored to an Expressway while it is part of a cluster; it must first be removed from the cluster. Source: [Cisco Expressway Administrator Guide (X15.4) - Maintenance](https://www.cisco.com/c/en/us/td/docs/voice_ip_comm/expressway/admin_guide/X15-4/exwy_b_cisco-expressway-administrator-guide-x154/exwy_m_maintenance.html), Maintenance chapter, Restoring a Previous Backup. Checked 2026-09-25.
[^46]: Expressway clustering supports a round-trip delay of up to 80 ms between peers. Source: [Cisco Expressway Cluster Creation and Maintenance Deployment Guide (X15.4) - Clustering Requirements](https://www.cisco.com/c/en/us/td/docs/voice_ip_comm/expressway/config_guide/X15-4/cluster_creation_maint/exwy_b_cisco-expressway-cluster-creation-and-maintenance-deployment-guide-x154/exwy_m_clustering-requirements.html), Clustering Requirements chapter, Network Conditions Are Met. Checked 2026-09-25.
[^47]: Every peer in an Expressway cluster must have the same routing capabilities, because the cluster assumes any peer can route to any destination another peer can reach. Source: [Cisco Expressway Cluster Creation and Maintenance Deployment Guide (X15.4) - Clustering Basics](https://www.cisco.com/c/en/us/td/docs/voice_ip_comm/expressway/config_guide/X15-4/cluster_creation_maint/exwy_b_cisco-expressway-cluster-creation-and-maintenance-deployment-guide-x154/exwy_m_clustering-basics.html), Clustering Basics chapter, routing statement. Checked 2026-09-25.
[^48]: All peers in an Expressway cluster must run the same software version, except for the short period while the cluster is being upgraded. Source: [Cisco Expressway Cluster Creation and Maintenance Deployment Guide (X15.4) - Clustering Requirements](https://www.cisco.com/c/en/us/td/docs/voice_ip_comm/expressway/config_guide/X15-4/cluster_creation_maint/exwy_b_cisco-expressway-cluster-creation-and-maintenance-deployment-guide-x154/exwy_m_clustering-requirements.html), Clustering Requirements chapter, Platform and Software Versions Match. Checked 2026-09-25.
[^49]: Cisco's X15.4 documentation is internally inconsistent on the maximum capacity multiplier of an Expressway cluster: the cluster deployment guide says four times with no gain after four peers, while the Administrator Guide capacity chapter describes a 5+1 model yielding five times (disputed). Source: [Cisco Expressway Administrator Guide (X15.4) - Expressway Capacity and Sizing](https://www.cisco.com/c/en/us/td/docs/voice_ip_comm/expressway/admin_guide/X15-4/exwy_b_cisco-expressway-administrator-guide-x154/exwy_m_expressway-capacity-and-sizing.html), Expressway Capacity and Sizing chapter, Important Caveats; compare Cluster Creation and Maintenance Deployment Guide X15.4, Clustering Basics. Checked 2026-09-25.
[^50]: SIP endpoints that support RFC 5626 can register simultaneously to multiple Expressway cluster peers. Source: [Cisco Expressway Administrator Guide (X15.4) - Clustering and Peers](https://www.cisco.com/c/en/us/td/docs/voice_ip_comm/expressway/admin_guide/X15-4/exwy_b_cisco-expressway-administrator-guide-x154/exwy_m_clustering-and-peers.html), Clustering and Peers chapter, SIP registrations. Checked 2026-09-25.
[^51]: Clustering Small VM Expressways provides redundancy only and gives no additional capacity. Source: [Cisco Expressway Cluster Creation and Maintenance Deployment Guide (X15.4) - Clustering Basics](https://www.cisco.com/c/en/us/td/docs/voice_ip_comm/expressway/config_guide/X15-4/cluster_creation_maint/exwy_b_cisco-expressway-cluster-creation-and-maintenance-deployment-guide-x154/exwy_m_clustering-basics.html), Clustering Basics chapter, Small VM note. Checked 2026-09-25.
[^52]: Configuration changes made on a peer other than the primary are not replicated and will be overwritten at the next replication from the primary. Source: [Cisco Expressway Administrator Guide (X15.4) - Clustering and Peers](https://www.cisco.com/c/en/us/td/docs/voice_ip_comm/expressway/admin_guide/X15-4/exwy_b_cisco-expressway-administrator-guide-x154/exwy_m_clustering-and-peers.html), Clustering and Peers chapter, Other Configuration for the Cluster. Checked 2026-09-25.
[^53]: All peers in an Expressway cluster must use the same TLS verification mode. Source: [Cisco Expressway Administrator Guide (X15.4) - Clustering and Peers](https://www.cisco.com/c/en/us/td/docs/voice_ip_comm/expressway/admin_guide/X15-4/exwy_b_cisco-expressway-administrator-guide-x154/exwy_m_clustering-and-peers.html), Clustering and Peers chapter, Basics of Cluster Configuration. Checked 2026-09-25.
[^54]: Cluster TLS verification mode defaults to Permissive; Enforce requires valid peer certificates. Source: [Cisco Expressway Cluster Creation and Maintenance Deployment Guide (X15.4) - How to Form a Cluster](https://www.cisco.com/c/en/us/td/docs/voice_ip_comm/expressway/config_guide/X15-4/cluster_creation_maint/exwy_b_cisco-expressway-cluster-creation-and-maintenance-deployment-guide-x154/exwy_m_how-to-form-a-cluster.html), How to Form a Cluster chapter, TLS verification mode field. Checked 2026-09-25.
[^55]: If Cisco TMS is used with an Expressway cluster it must run version 13.2 or later, or 12.6 or later if TMS is not used for provisioning or FindMe. Source: [Cisco Expressway Cluster Creation and Maintenance Deployment Guide (X15.4) - Clustering Requirements](https://www.cisco.com/c/en/us/td/docs/voice_ip_comm/expressway/config_guide/X15-4/cluster_creation_maint/exwy_b_cisco-expressway-cluster-creation-and-maintenance-deployment-guide-x154/exwy_m_clustering-requirements.html), Clustering Requirements chapter, TMS Is Configured (When Essential). Checked 2026-09-25.
[^56]: Cisco recommends upgrading the Expressway-E cluster before the Expressway-C cluster, starting with the primary peer in each. Source: [Cisco Expressway Administrator Guide (X15.4) - Maintenance](https://www.cisco.com/c/en/us/td/docs/voice_ip_comm/expressway/admin_guide/X15-4/exwy_b_cisco-expressway-administrator-guide-x154/exwy_m_maintenance.html), Maintenance chapter, Process to Upgrade a Clustered System. Checked 2026-09-25.
[^57]: The X15.4 cluster deployment guide defers cluster upgrade instructions to the release notes for the target version. Source: [Cisco Expressway Cluster Creation and Maintenance Deployment Guide (X15.4) - How to Change a Cluster](https://www.cisco.com/c/en/us/td/docs/voice_ip_comm/expressway/config_guide/X15-4/cluster_creation_maint/exwy_b_cisco-expressway-cluster-creation-and-maintenance-deployment-guide-x154/exwy_m_how-to-change-a-cluster.html), How to Change a Cluster chapter, upgrading a cluster note. Checked 2026-09-25.
[^58]: When upgrading an Expressway cluster, upgrade the primary peer first and then the subordinate peers one at a time. Source: [Cisco Expressway Administrator Guide (X15.4) - Maintenance](https://www.cisco.com/c/en/us/td/docs/voice_ip_comm/expressway/admin_guide/X15-4/exwy_b_cisco-expressway-administrator-guide-x154/exwy_m_maintenance.html), Maintenance chapter, Process to Upgrade a Clustered System. Checked 2026-09-25.
[^59]: Expressway Small and Medium VMs use 2 vCPU and the Large VM uses 8 vCPU, which is why Large VMs are the 8-core peers that cannot be clustered with standard appliances (inferred). Source: [Virtualization for Cisco Expressway](https://www.cisco.com/c/dam/en/us/td/docs/voice_ip_comm/uc_system/virtualization/virtualization-cisco-expressway.html), VM configuration table, Small/Medium/Large rows. Checked 2026-09-25.