Understanding Linux Network Internals

Adding Ports to a Bridge

In the current implementation of bridging, there is a one-to-one relationship between NICs and bridge ports, as shown in Figure 16-6. Some commercial bridges allow an administrator to add an NIC to multiple bridge devices and assign traffic to a particular bridge based on user-chosen criteria, but Linux does not.

Bridge ports are added to a bridge device with br_add_if. The routine internals are shown in Figure 16-7. The routine does not care whether the STP is enabled on the bridge device.

The routine starts with a set of sanity checks. The operation is aborted if any of the following conditions is met:

• The device to be associated with the port is not an Ethernet device (or the loopback device).

• The device to be associated with the port is a bridge. As you can see in Figure 16-6, bridge ports must be assigned to real devices (or to virtual devices that are not bridge devices).

• The bridge port is already assigned to a device (i.e., dev->br_port is not NULL).

Figure 16-7. br_add_if function

When these checks are passed, the new bridge port is allocated and partially initialized with new_nbp. In particular, that function:

• Assigns a port number to the bridge port. See the section "Bridge IDs and Port IDs."

• Assigns a default priority to the port.

• Computes the port ID by combining the port number and priority using br_make_port_id. Out of the 16 bits in the port ID, 10 ( ...