Hi neeeewbie ,
If spanning option is not enable environments that require spanning do not process traffic.
Starting from BIG-IP 12.1.0 , when you upgrade your system from BIG-IP 10.x to BIG-IP 12.1.0 or later when you have a virtual address with address resolution protocol (ARP) disabled and spanning feature got enable.
Prior to upgrading, the virtual address is configured with ARP disabled.
After you upgrade to 15.1.6.1, the virtual server address is still configured with ARP disabled;
However the virtual server address now have the required Spanning option enabled.
You can manually modify the virtual server address to enable/disable spanning.
To manually enable/disable spanning, perform the following procedure:
1. Log in to the Configuration utility.
2. Navigate to Local Traffic > Virtual Servers > Virtual Address List.
3. Click the affected virtual server address.
4. Under Configuration, select/unselect the Spanning check box to enable/disable spanning.
5. Click Update.
Spanning feature enables all BIG-IP systems in a device group to listen for and process traffic on the same virtual address.
Spanning for a virtual address occurs when you enable the spanning option on a device, and then sync the virtual address to the other members of the device group.
Spanning also relies on the upstream router to distribute application flows to the BIG-IP systems using ECMP routes. ECMP defines a route to the virtual address using distinct Floating self-IP addresses configured on each BIG-IP system.
You must also configure MAC masquerade addresses and disable arp on the virtual address when Spanning is enabled.
When creating a new virtual address, if this parameter is not specified, the default valus is no.
HTH