Explanation/Reference:
Explanation:
By default, all site links are transitive. This means that if Site A has a link to Site B, and Site B has a link to Site C, Site A is transitively linked to Site C The transitive link between Site A and Site C is also known as a site-link bridge. A message sent from Site A to Site C would be sent directly to Site C.
In your Exchange organization, you may want to force all message delivery through a specific Active Directory site. You can use the Shell to designate an Active Directory site as a hub site. When you do this, you cause additional overall overhead because more servers are involved in message delivery. For example, consider a message that's sent from Site A to Site E If the least-cost routing path is Site A- Site B-Site C-Site D-Site E, and you designate Site C as a hub site, the message is relayed from Site A to Site C and then relayed from Site C to Site E.
Whenever a hub site exists along the least-cost routing path for message delivery, the messages are queued and are processed by the Transport service on Mailbox servers in the hub site before they're relayed to their ultimate destination.
After the least-cost routing path is chosen, routing determines whether there's a hub site along that routing path. If a hub site is configured, messages stop at a Mailbox server in the hub site before they're relayed to the target destination.
References:
https://technet.microsoft.com/en-us/library/jj916681(v=exchg.160).aspx