When a call reaches Expressway, Expressway classifies it based on source and destination address and port.Based on classification, the call is sent to a specific «zone».Except for the Local Zone , the other zones connect to remote systems as in the case of a SIP Trunk on CUCM

Different policies can be applied per zone, such as:
• signaling and media encryption
• protocol usage (i.e. SIP and/or H.323)
• message authentication (PAI header for SIP)
• use of TLS with Mutual Authentication
• Others

Most common used zones on Expressway

• Neighbor Zone: This zone will be a neighbor of the local Expressway and most similar to a SIP Trunk.SIP/H.323 Permanent Trunk to a remote peer.SIP/H.323 Permanent Trunk to a remote peer.
• Traversal Zone: it’s a special neighbor zone with firewall traversal capabilities.There is a firewall between the zones. Permanent Trunk between Expressway-C and -E with firewall traversal capabilities and media multiplexing
• DNS Zone: it’s a special neighbor zone used for outbound B2B calls supporting DNS SRV.This zone contains endpoints discoverable by DNS lookup.
• Default Zone: it’s a special neighbor zone used for inbound B2B calls.Ad-hoc inbound Trunk for B2B calls from any unknown IP/host
• Unified Communications traversal: This zone traverses a firewall at the edge of a Unified Communications deployment. The Expressway and its counterpart across the firewall must both use this zone type.

Sending the Call to the Destination Zone

Inbound vs Outbound Zone
• Neighbour and Traversal both inbound and outbound
• Default Zone outbound only
• DNS Zone inbound only, unless SAN is configured and matched, and mapping enabled

Cisco Expressway Connectivity Overview

Local Zone and Subzones

The collection of all devices registered with the Expressway makes up its Local Zone.

The Local Zone is divided into subzones. These include an automatically created Default Subzone and up to 1000 manually configurable subzones.When an endpoint registers with the Expressway, it’s allocated to an appropriate subzone based on subzone membership rules. These rules specify the range of IP addresses or alias pattern matches for each subzone. If an endpoint’s IP address or alias does not match any of the membership rules, it is assigned to the Default Subzone. The Local Zone may be independent of network topology, and may comprise multiple network segments. The

Expressway also has two special types of subzones:
■ Traversal Subzone, which is always present
■ Cluster Subzone, which is always present but only used when the Expressway is part of a cluster