Unlike the preceding multi-tenancy scenario, virtual systems on a firewall can be under the control of a single organization. The organization wants to both isolate traffic between virtual systems and allow communications between virtual systems. This common use case arises when the organization wants to provide departmental separation and still have the departments be able to communicate with each other or connect to the same network(s). In this scenario, the inter-vsys traffic remains within the firewall, as described in the following topics:
External Zone
The communication desired in the use case above is achieved by configuring security policies that point to or from an external zone. An external zone is a security object that is associated with a specific virtual system that it can reach; the zone is external to the virtual system. A virtual system can have only one external zone, regardless of how many security zones the virtual system has within it. External zones are required to allow traffic between zones in different virtual systems, without the traffic leaving the firewall.
The virtual system administrator configures the security policies needed to allow traffic between two virtual systems. Unlike security zones, an external zone is not associated with an interface; it is associated with a virtual system. The security policy allows or denies traffic between the security (internal) zone and the external zone.
Because external zones do not have interfaces or IP addresses associated with them, some zone protection profiles are not supported on external zones.
Remember that each virtual system is a separate instance of a firewall, which means that each packet moving between virtual systems is inspected for security policy and App-ID evaluation.
External Zones and Security Policies For Traffic Within a Firewall
In the following example, an enterprise has two separate administrative groups: the departmentA and departmentB virtual systems. The following figure shows the external zone associated with each virtual system, and traffic flowing from one trust zone, out an external zone, into an external zone of another virtual system, and into its trust zone.
To create external zones, the firewall administrator must configure the virtual systems so that they are visible to each other. External zones do not have security policies between them because their virtual systems are visible to each other.
To communicate between virtual systems, the ingress and egress interfaces on the firewall are either assigned to a single virtual router or else they are connected using inter-virtual router static routes. The simpler of these two approaches is to assign all virtual systems that must communicate with each other to a single virtual router.
There might be a reason that the virtual systems need to have their own virtual router, for example, if the virtual systems use overlapping IP address ranges. Traffic can be routed between the virtual systems, but each virtual router must have static routes that point to the other virtual router(s) as the next hop.
Referring to the scenario in the figure above, we have an enterprise with two administrative groups: departmentA and departmentB. The departmentA group manages the local network and the DMZ resources. The departmentB group manages traffic in and out of the sales segment of the network. All traffic is on a local network, so a single virtual router is used. There are two external zones configured for communication between the two virtual systems. The departmentA virtual system has three zones used in security policies: deptA-DMZ, deptA-trust, and deptA-External. The departmentB virtual system also has three zones: deptB-DMZ, deptB-trust, and deptB-External. Both groups can control the traffic passing through their virtual systems.
In order to allow traffic from deptA-trust to deptB-trust, two security policies are required. In the following figure, the two vertical arrows indicate where the security policies (described below the figure) are controlling traffic.
Security Policy 1: In the preceding figure, traffic is destined for the deptB-trust zone. Traffic leaves the deptA-trust zone and goes to the deptA-External zone. A security policy must allow traffic from the source zone (deptA-trust) to the destination zone (deptA-External). A virtual system allows any policy type to be used for this traffic, including NAT.
No policy is needed between external zones because traffic sent to an external zone appears in and has automatic access to the other external zones that are visible to the original external zone.
Security Policy 2: In the preceding figure, the traffic from deptB-External is still destined to the deptB-trust zone, and a security policy must be configured to allow it. The policy must allow traffic from the source zone (deptB-External) to the destination zone (deptB-trust).
The departmentB virtual system could be configured to block traffic from the departmentA virtual system, and vice versa. Like traffic from any other zone, traffic from external zones must be explicitly allowed by policy to reach other zones in a virtual system.
In addition to external zones being required for inter-virtual system traffic that does not leave the firewall, external zones are also required if you configure a Shared Gateway, in which case the traffic is intended to leave the firewall.

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