Local Zone: This is a collection of all devices that are registered with Cisco Expressway. The local zone is divided to subzones. These subzones include the automatically crated default subzone and up to 1000 manually configured subzones. In addition, there are two special types of subzones in Cisco Expressway.
-Traversal Subzone: This subzone is a conceptual subzone that does not have any registered endpoints. Its sole purpose is to control the bandwidth that is used by traversal calls.
-Cluster Subzone: This subzone is created when two or more Cisco Expressways are clustered. Any Calls between two peers in the cluster will briefly traverse this subzone during call setup. This subzone cannot include any registered endpoints.
Local Zone is a permanent zone. The local zone refers to the Cisco Expressway itself. Devices that are registered to Cisco Expressway are defined as registered to the local zone (in other words, this Cisco Expressway). The local zone contains subzones with other zone around the edge. The Local Zone represents the Cisco Expressway, the other zones are routes to other devices on cisco Unified Communications network.
Neighbor Zone: This zone is a connection to a neighbor system of the local Cisco Expressway. Neighbors can be part of your own enterprise network (such as a Cisco Unified Communication Manager or another Cisco Expressway), part of a separate network, or even a standalone system (such as third party SIP devices).
Traversal Client: This zone is used when another Cisco Expressway wants to connect to this Cisco Expressway through a firewall.
Traversal Server: This zone is used when another Cisco Expressway wants to connect to this Cisco Expressway through a firewall.
ENUM: This zone contains endpoints that discoverable by E.164 lookup.
DNS: This zone contains endpoints that discoverable by DNS lookup.
Unified Communications traversal: This zone is either a traversal client or traversal zone that is used for Cisco Unified Communication features such as mobile and remote access MRA.
With the zones in this scenario, you can see that the initial call setup message, in this case the SIP INVITE message, passes from the neighbor zone in Cisco Expressway A to the neighbor zone on Cisco Expressway B and then through the second neighbor zones on Expressway B to the neighbor zone on Cisco Expressway C.
However, the call route is now defined, and because optimal call routing is enabled, Cisco Expressway B drops out of the call path so the signaling passes directly from Cisco Expressway A to Cisco Expressway C. All messages leaving or entering a Cisco Expressway must pass through a zone for the calls to be managed. In this instance, there is no neighbor zone between the two Cisco Expressways, so the signaling passes through the default zone on both servers because because neither server knows about the other. Therefore, the call is classed as coming from and going to an unknown place. This behavior is a typical use of the default zone of the Cisco Expressway-C servers inside the corporate firewall. Cisco Expressway-E will use the default zone to send and receive call to other organizations or locations.
In this instance, there is no neighbor zone between the two Cisco Expressways, so the signaling passes through the default zone on both servers because neither server knows about the other.