Use cases for Layer 2 forwarding Virtual servers?

Have you looked at Lori MacVittie's (F5) article, Back to Basics: The Many Modes of Proxies? It provides some information on L2 Forwarding Virtual Servers. Basically, it is not uncommon for a BIG-IP system to be placed in a network location where it sits in the path between two existing networks. As a default-deny system, any traffic that is not explicitly allowed by the BIG-IP system's configuration is denied. Therefore, if access is needed between networks where the BIG-IP system is in the path, a forwarding virtual server provides an ideal solution. A forwarding virtual server essentially changes your BIG-IP system from full-proxy mode to packet-by-packet proxy mode, at least for the traffic the forwarding virtual server processes:

 

The BIG-IP system provides forwarding services in two ways:

 

• For simple packet forwarding, where the destination is not based on a pooled resource but simply on a routing table, a forwarding IP virtual server turns the BIG-IP full proxy into a packet layer forwarder.
• For situations where a proxy should be used to bridge two different collision domains, a layer 2 forwarding virtual server can be used, and configured to be an opaque, semi-opaque, or transparent bridge. (From Lori's article.) A forwarding layer 2 virtual server shares the same IP address as a node in an associate VLAN, and is used in conjunction with a VLAN group.

Does this help?