Layer 2 forwarding is the means by which frames are exchanged directly between hosts, with no
IP routing required. This is accomplished using a simple forwarding table for each VLAN. The L2
forwarding table is a list that shows, for each host in the VLAN, the MAC address of the host,
along with the interface that the BIG-IP® system needs for sending frames to
that host. The intent of the L2 forwarding table is to help the BIG-IP system determine the
correct interface for sending frames, when the system determines that no routing is required.
The format of an entry in the L2 forwarding table is:
<MAC address> -> <if>
For example, an entry for a host in the VLAN might look like this:
00:a0:c9:9e:1e:2f -> 2.1
The BIG-IP system learns the interfaces that correspond to various MAC entries as frames pass
through the system, and automatically adds entries to the table accordingly. These entries are
. You can also add entries to the table manually, and these
are known as
. Entering static entries is useful if you have network
devices that do not advertise their MAC addresses. The system does not automatically update
The BIG-IP system does not always need to use the L2 forwarding table to find an interface for frame transmission. For instance, if a VLAN has only one interface assigned to it, then the BIG-IP system automatically uses that interface.
Occasionally, the L2 forwarding table does not include an entry for the destination MAC address and its corresponding BIG-IP system interface. In this case, the BIG-IP system floods the frame through all interfaces associated with the VLAN, until a reply creates an entry in the L2 forwarding table.