Least cost path
Site A to Site D has two paths.
Site A+Site B+Site D (cost=5+15)
Site A+Site C+Site D (cost=5+5)
The path A+C+D is chosen.
How do you understand the direct delivery?After the least cost path is selected, Hub transport server in Site A will directly deliver messages to the Hub Transport server in Site D. The Hub Transport server in Site C does not process the messages at all.
If you want the HUB Transport Server in SITE C to process the message, configure it as a hub site.
Set-AdSite -identity "Site C' -HubSiteEnabled:$true
Why would you do that?
One example will be that a company implements the centralized journaling.
Most of the time, you should not define a HUB Site. For this scenario, if Hub transport server in SITE C does not work, because Site C is a hub site, the messages from SITE A cannot be delivered to Site D.
If Site C is not defined as a HUB SITE, mail is directly delivered from SITE A to SITE D. Direct relay is not affected by lack of network connectivity between SITE A and Site C.
