I believe it is random based on the proxies you have available but if you modify the job you can force it.

Q: How is the backup proxy selected in cases when there are multiple proxies available?

A: For each VM in the job, backup server classifies all backup proxies available to the job into 3 groups by access type they have to the VM's disks: SAN (1), hot add (2) and network (3). Proxy with least tasks currently assigned is picked from group 1, and if responding, VM processing task is assigned to this proxy. Otherwise, the next least busy proxy is selected from group 1. If all available proxies group 1 are already running max number of concurrent tasks, selection process switches to group 2 and repeats. Group 3 is only used if groups 1 and 2 have no responding proxies. For group 3, proxy subnet is also considered when picking the best proxy.

https://forums.veeam.com/post39946.html#p39946

You can actually look at the job logs, and see the selection process/decisions being made by the Veeam server. 

Thanks @Cragdoo  for your answers, yes that’s confirm my post, unfortunately in my case I don’t know why Veeam doesn’t used the remote proxy…

Unfortunately I dont have access anymore to the logs, I ll check this next time I’m encounter this problem

when you say remote proxy, do you mean you have VBR server in one location, controlling backups in another location? If so ,can the VBR server route to/from the proxy subnet? Can the proxy route to/from the host network? Does the proxy have access to the storage the VMs being backed up reside on?

Yes I have a VBR on my main site, I manage a remote proxy to backup a remote ESX with a local repository.

Everything works well when I force the proxy in the job, just in automatic VBR choose a proxy in my main site ...

Do both proxies have the same transport mode configured and the same number of task slots? 

Also remember that Direct SAN won’t work remotely.  So if your proxies are Direct SAN, and 1 stops working, it will fail over to Hot-Add locally as a second choice.  I find many people have their Veeam BR Server as a standby proxy which gets used in this situation. 

It’s often not a terrible idea to create a few VM’s in your virtual infrastructure as Veeam Proxies I find on the rare occurrence they are needed.  Personally though, I’m in the Direct SAN for life crew because it works so well. 

Yes the both proxy are in hotadd but not the same number of task slots.

 

Direct San is a good option but sometime the infrastructure is not compatible or if you have vm in thin provisioning, the proxy will not be used so it’s necessary to create a vm to have a hotadd proxy.