According to this link: http://www.cisco.com/c/dam/en/us/td/docs/solutions/SBA/August2012/Cisco_SBA_BN_EmailSecurityUsingESADeploymentGuide-Aug2012.pdf
High Availability
The Cisco IronPort ESA functions as part of the mail transfer chain, and there is a reasonable amount of resiliency built into the system because a mail server in the chain stores a message for some period of time if the destination server is unresponsive. You can achieve additional resiliency by adding a second IronPort ESA.
You should configure the second IronPort ESA the same as the first IronPort ESA, and then add an additional MX record to the Domain Name System (DNS). For any additional devices, you need to add access lists and static Network Address Translation (NAT) rules to the appliance.
Sceraio: There is no concept of redundancy in the Ironport ESA appliances, what I mean is that the 2 ESAs do not work as one Cluster , we should rely on the network to do the failover . For the Outbound mail traffic , in case the ESA at the main site fails down , the Exchange will be configured to point to the ESA at the DR Site which was in Standby State. And for the Inbound traffic, we will rely on the MX-records to route the Inbound traffic to the ESA at the DR Site in case the ESA at the HQ fails.
Question: In such above scenario where the 2 ESAs work in Active/Standby mode but we are relying on the Exchange and MX-records to perform the failover , we still need 1 set of licenses which will be activate on both appliances?
