Understanding Linux Network Internals

Bridge Device Setup Routine

Details about how device drivers use the setup routines when initializing their devices can be found in the section "Device Type Initialization: xxx_setup Functions" in Chapter 8. Bridge devices use the br_dev_setup setup routine. The following snapshot shows the interesting part:

void br_dev_setup(struct net_device *dev)
{
    memset(dev->dev_addr, 0 , ETH_ALEN);
    ether_setup(dev);
    ...
    dev->do_ioctl = br_dev_ioctl;
    dev->hard_start_xmit = br_dev_xmit;
    dev->open = br_dev_open;
    dev->change_mtu = br_change_mtu;
    dev->stop = br_dev_stop;
    dev->tx_queue_len = 0 ;
    dev->set_mac_addr = NULL ;
    dev->priv_flags = IFF_EBRIDGE;
}

Bridge devices do not implement queuing by default. They let their enslaved devices take care of it, which explains why tx_queue_len is initialized to 0. However, the administrator can configure tx_queue_len with ifconfig or ip link.

When the Maximum Transmission Unit (MTU) on a bridge device is changed, the kernel must ensure that the new value is no bigger than the smallest MTU value among the enslaved devices. This is ensured by br_change_mtu.

The bridge MAC address dev_addr is cleared because it will be derived by the MAC addresses configured on its enslaved devices with br_stp_recalculate_bridge_id (see the section "Bridge IDs and Port IDs"). For the same reason, the driver does not provide a set_mac_addr function.

The IFF_EBRIDGE flag is set so that kernel code can distinguish bridge devices from other types of devices when needed.

The br_dev_ioctl ...