Understanding Linux Network Internals

The Big Picture

Figure 16-11 shows the key routines that the bridging code uses to process ingress and egress frames (both data frames and BPDUs).

Figure 16-11. The big picture

In particular, note that:

• There are as many hooks as there are at the IP layer (see Figure 18-1 in Chapter 18). One more hook in br_handle_frame (NF_BR_BROUTING), not shown in the figure, is used by ebtables and is described in the section "Data Frames Versus BPDUs."

• Ingress data frames may go through netif_receive_skb twice. netif_receive_skb is described in Chapter 10. See also the section "Processing Data Frames," later in this chapter.

• Ingress frames are passed by netif_receive_skb to the bridging subsystem when a call to handle_bridge indicates it is necessary, or to the upper-layer protocol handlers otherwise (as described in Chapter 13).

• Ingress frames may be dropped by the bridging code, for example, because the port is disabled.

• Ingress data frames are dropped by br_forward when the receiving port has been blocked by the STP. Egress frames do not need to be transmitted out of any port by br_deliver when the destination address is local to the host. In both cases, unneeded transmissions are filtered by should_deliver.

• Egress data frames go through dev_queue_xmit twice. dev_queue_xmit is described in Chapter 11. See also the section "Transmitting on a Bridge Device" later in this chapter.

• The br_flood function ...