If you deploy Exchange 2013 servers in your Exchange 2010 organization and you have external mailflow configured to pass emails through exchange 2013 Edge Transport servers, you should configure subscription for Exchange 2013 edge servers to your existing Exchange 2010 hub servers.
You can subscribe an edge server in a site to multiple HUB servers if it is(Exchange 2007/2010) and CAS & Mailbox Combined together if it is Exchange 2013 servers.
You can subscribe a 2007/2010 edge to 2013 Exchange CAS & HUB combined servers. This can be done vice versa as well.
You can import the Edge Subscription file and run EdgeSync on a standalone Exchange 2013 Mailbox server, or on a server where the Mailbox server and the Client Access server are installed on the same computer.
You can’t import the Edge Subscription file or run EdgeSync only on a standalone Exchange 2013 Client Access server.
You cannot subscribe an edge servers to multiple site since edge servers are bounded to site specific and can be subscribed to multiple mailbox & CAS servers in a single site
Make sure you open the below ports on the firewall
SMTP – TCP port 25 (from Internet)
SMTP – TCP port 25 (from Edge server to Hub server on internal network)
SMTP – TCP/UDP port 25 (from Edge to Internet)
SMTP – TCP/UDP port 25 (from Hub to Edge server)
Very IMP : Do not open the below mentioned ports on perimeter firewall. These ports should be open only on intranet firewall.
LDAP for Edge Sync – TCP port 50389 (from Mailbox to Edge server) Secure LDAP for Edge Sync – TCP port 50636 (from Mailbox to Edge server).
MVP – Exchange Server