@euanbarker
Do you mean the bandwidth requirement for the communication between the HA1 and HA2 links of the HA ?
If yes, I dont readily see any document stating this information (it is tricky for the vendor to publish any specfic values).
The following DOC explains what is synchronized via which link (HA1/HA2). You will find that this is very less bandwidth intensive info (more than bandwidth, stablility and availability is important here)
https://live.paloaltonetworks.com/twzvq79624/attachments/twzvq79624/documentation_tkb/568/1/HA_Synchronization_RevD.pdf
Also, since this is not a Active/Active setup, there is not transfer of actual traffic between the nodes (via HA3) which might have required high bandwidth (or aggregated )HA3 link.
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