> For the complete documentation index, see [llms.txt](https://docs.parallels.com/landing/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://docs.parallels.com/landing/ras-admin-guide/parallels-ras-21-administrators-guide/load-balancing-and-halb/high-availability-load-balancing-halb/prerequisites.md).

# Prerequisites

The below highlights the prerequisites required to use HALB:

* Firewall or router in front of a HALB configured to preserve the source IPs of client devices

### Sizing recommendations

<table data-header-hidden="false" data-header-sticky><thead><tr><th width="128">Traffic Profile</th><th>Concurrent TCP Sessions</th><th>Recommended vCPUs</th><th>Recommended RAM</th><th>Notes</th></tr></thead><tbody><tr><td><strong>Low-intensity payloads</strong></td><td>Up to <strong>12,500</strong></td><td><strong>2 vCPUs</strong></td><td><strong>2 GB</strong></td><td>Safe and stable for small payloads and low send frequency</td></tr><tr><td><strong>Moderate-intensity payloads</strong></td><td>Up to <strong>12,500</strong></td><td><strong>4 vCPUs</strong></td><td><strong>2–4 GB</strong></td><td>Provides headroom and improved CPU efficiency under sustained load</td></tr><tr><td><strong>Moderate-intensity payloads</strong></td><td>Up to <strong>20,000</strong></td><td><strong>8 vCPUs</strong></td><td><strong>4 GB</strong></td><td>Required to avoid CPU saturation at high concurrency</td></tr><tr><td><strong>All profiles</strong> (general guidance)</td><td>Any</td><td>—</td><td><strong>4 GB (preferred)</strong></td><td>4 GB RAM provides additional safety margin and resilience during prolonged high-load operation</td></tr></tbody></table>


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