Understanding Linux Network Internals

Changing State on a Bridge Port

A bridge port is either active or inactive: the associated states are BR_STATE_FORWARDING or BR_STATE_BLOCKING. However, while the BR_STATE_BLOCKING state can be assigned right away to a port, the BR_STATE_FORWARDING state is reached only after first going through the intermediate states introduced in the section "Port states" in Chapter 15.

The BR_STATE_FORWARDING and BR_STATE_BLOCKING states are assigned with the br_make_forwarding and br_make_blocking routines, respectively. The same two routines are used regardless of whether the bridge device hosting the port is running the STP.

static void br_make_blocking(struct net_bridge_port *p)
{
    if (p->state != BR_STATE_DISABLED &&
        p->state != BR_STATE_BLOCKING) {
        if (p->state == BR_STATE_FORWARDING ||
            p->state == BR_STATE_LEARNING)
            br_topology_change_detection(p->br);

        p->state = BR_STATE_BLOCKING;
        br_log_state(p);
        del_timer(&p->forward_delay_timer);
    }
}

static void br_make_forwarding(struct net_bridge_port *p)
{
    if (p->state == BR_STATE_BLOCKING) {
        if (p->br->stp_enabled) {
            p->state = BR_STATE_LISTENING;
        } else {
            p->state = BR_STATE_LEARNING;
        }
        br_log_state(p);
        mod_timer(&p->forward_delay_timer, jiffies + p->br->forward_delay);
    }
}

Note that you cannot assign a port any of the intermediate states between BR_STATE_BLOCKING and BR_STATE_FORWARDING, which is why br_make_forwarding returns if asked to change a port that is not in the BR_STATE_BLOCKING state to BR_STATE_FORWARDING. However, an intermediate state ...