Use Cases for Service Routes for a Virtual System
One use case for configuring service routes at the virtual system level is when a large customer (such as an ISP) needs to support multiple individual tenants on a single Palo Alto Networks firewall. The ISP has configured virtual systems on the firewall, and wants to have separate service routes for each virtual system, rather than services routes configured at the global level. Each tenant requires service route capabilities so that it can customize service route parameters for DNS, email, Kerberos, LDAP, NetFlow, RADIUS, SNMP trap, syslog, TACACS+, User-ID Agent, and VM Monitor.
Another use case is an IT organization that wants to provide full autonomy to groups that set servers for services. Each group can have a virtual system and define its own service routes.
If Multi Virtual System Capability is enabled, any virtual system that does not have specific service routes configured inherits the global service and service route settings for the firewall.
An organization can have multiple virtual systems, but use a global service route for a service rather than different service routes for each virtual system. For example, the firewall can use a shared email server to originate email alerts to its virtual systems.
A firewall with multiple virtual systems must have interfaces and subinterfaces with non-overlapping IP addresses.
A per-virtual system service route for SNMP traps or for Kerberos is for IPv4 only.
You can select a virtual router for a service route in a virtual system; you cannot select the egress interface. After you select the virtual router and the firewall sends the packet from the virtual router, the firewall selects the egress interface based on the destination IP address. Therefore:
If a virtual system has multiple virtual routers, packets to all of the servers for a service must egress out of only one virtual router. A packet with an interface source address may egress a different interface, but the return traffic would be on the interface that has the source IP address, creating asymmetric traffic.

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