Concept
Replication Transport And Routing
How DR replication data moves between sites: the hub-routed path via SHA, the direct site-to-site path, and how routes are chosen.
Control Plane Versus Data Plane
SHA is the control plane on every route. It schedules the sync, selects the appliances, applies the route policy, prepares the recovery-side ingress, and receives health and progress reporting. Changing the route never moves control traffic off SHA.
The route policy changes only the replication data path between the two sites. Both data paths end identically at the target SNA, which passes the stream to the Sendense Controller inside the recovery site.
Roles On The Replication Data Path
Four roles participate in every controller-backed replication sync. SHA is the control plane: it schedules the sync, selects healthy appliances at each site, chooses the data route, and tracks progress and errors. The source SNA reads protected workload data at the protected site. The target SNA is the replication ingress at the recovery site. The Sendense Controller is the per-VM destination appliance that holds the replica disks.
The final hop is the same on every route: the target SNA hands the replication stream to the Sendense Controller inside the recovery site. Controller traffic stays within the recovery site network.
DR replication topology
Both replication routes end the same way: the target SNA receives the replication stream and passes it to the Sendense Controller for that VM inside the recovery site.
Text equivalent
- Protected VMs at the protected site are read by the source SNA.
- Route A, hub-routed: source SNA to SHA to target SNA. Solid lines.
- Route B, direct: source SNA straight to target SNA. Dashed line.
- Both routes end at the target SNA, which passes the stream to the Sendense Controller inside the recovery site.
- The Sendense Controller reports its health to SHA. Dotted line.
Route A: Via SHA
Source SNA to SHA to target SNA to Sendense Controller. Replication data travels inside the always-on encrypted appliance connections, so each site only needs its normal outbound connection to SHA.
Route B: Direct P2P
Source SNA directly to target SNA to Sendense Controller. Replication data crosses the site-to-site network after a live reachability probe. In Automatic mode, a failed probe falls back to Route A.
The Two Data Routes
Route Policy
The route is controlled per replication pattern with the replication route policy, and it is evaluated for each sync.
Choosing A Route Policy
- Use Automatic unless there is a specific reason not to. It prefers the direct path when it is reachable and falls back safely to the hub route.
- Use Via SHA when there is no network path between the sites, when firewall policy only allows outbound connections to SHA, or when replication data must stay inside the Sendense encrypted appliance connections end to end.
- Use Direct P2P when the sites are connected by a private, high-bandwidth link, replication traffic should stay off the SHA data path, and a hard failure is preferred over silent rerouting.
Why The Target SNA Is On Both Routes
The target SNA is the recovery site's replication ingress on every route, not just the direct one. On the hub route it receives the stream from SHA; on the direct route it receives the stream from the source SNA. In both cases it is the component that writes into the Sendense Controller for that VM.
That is why the recovery-site hop from the target SNA to the Controller has to be planned even in deployments that only ever use Via SHA.
One Destination Listener Per Replica Disk
On the target SNA, Sendense prepares one replication listener for each replica disk of the protected VM and maps each listener to the matching replica-disk export on the Sendense Controller. A three-disk protected VM therefore uses three listeners during its sync.
Listeners are leased from the target appliance's assigned range for the duration of a sync and released afterwards. The size of that range therefore also bounds how many replica disks that appliance can carry concurrently.
Route-Specific Requirements
Requirements differ by route. The first item applies to every deployment; the second applies only when the direct route can be selected; the third applies on every controller-backed route.
Transport security
The hub route carries replication data inside the encrypted appliance connections end to end. The direct route inherits the security characteristics of the site-to-site network it crosses, so use it over private or trusted links such as a VPN, MPLS circuit or private interconnect. Control and orchestration traffic always flows through SHA on either route.
Multi-SNA Implications
Because SHA resolves a healthy appliance per sync, a site with several SNAs does not have one fixed replication pairing. SHA may select any healthy eligible SNA at the source site and any healthy eligible SNA at the recovery site, and skips appliances that are unhealthy, stale, or in maintenance mode.
For the direct route this means network policy has to cover every permitted source-SNA-to-target-SNA pairing, and each target SNA's own assigned range. Covering only one pairing makes the direct route succeed or fall back depending on which appliances were selected for that sync.
Planning The Network
Replication Network And Firewall Planning is the detailed companion to this page. It has the full connectivity matrix, per-appliance assigned ranges and how to read yours, worked firewall examples for single and multi-SNA sites, security characteristics, a validation checklist and troubleshooting.
What SHA Does For Every Sync
SHA orchestrates every sync regardless of which data route is used.
- Resolves a healthy SNA at the source site and at the recovery site. Sites can run multiple SNAs; SHA balances work across the healthy ones and skips appliances that are unhealthy, stale, or in maintenance mode.
- Applies the pattern's replication route policy and, where relevant, the reachability probe result to select the route.
- Prepares the target-side ingress on the target SNA and confirms the Sendense Controller for the VM is ready to receive data.
- Starts and monitors the sync, records sync and checkpoint recovery state, and reports progress, RPO status, and errors through the GUI and API.
Scope Of The Routing Choice
- The route policy controls how replication data travels during sync. Failover behavior is defined by the failover operation, not the route.
- The direct route is not a separate feature to license or install. It is a per-pattern routing choice.
- The hub route relays the replication stream through SHA; SHA does not keep a copy.
- Operators set the policy on the pattern; Sendense applies it to each sync. There is no manual per-sync route selection.
Related Docs