Linux Bridge 的 MTU 設定不如一般網卡簡單設定，其 MTU 預設情況下會自動調整，會自動使用所有 slave 網卡上最小的值來取代 以下列程式碼來看，剛有任何 slave 網卡加入到 bridge 上後
int br_add_if(struct net_bridge *br, struct net_device *dev,
struct netlink_ext_ack *extack)
{
struct net_bridge_port *p;
int err = 0;
unsigned br_hr, dev_hr;
bool changed_addr, fdb_synced = false;
/* Don't allow bridging non-ethernet like devices. */
if ((dev->flags & IFF_LOOPBACK) ||
dev->type != ARPHRD_ETHER || dev->addr_len != ETH_ALEN ||
!is_valid_ether_addr(dev->dev_addr))
return -EINVAL;
/* No bridging of bridges */
if (dev->netdev_ops->ndo_start_xmit == br_dev_xmit) {
NL_SET_ERR_MSG(extack,
"Can not enslave a bridge to a bridge");
return -ELOOP;
}
/* Device has master upper dev */
if (netdev_master_upper_dev_get(dev))
return -EBUSY;
/* No bridging devices that dislike that (e.g. wireless) */
if (dev->priv_flags & IFF_DONT_BRIDGE) {
NL_SET_ERR_MSG(extack,
"Device does not allow enslaving to a bridge");
return -EOPNOTSUPP;
}
p = new_nbp(br, dev);
if (IS_ERR(p))
return PTR_ERR(p);
call_netdevice_notifiers(NETDEV_JOIN, dev);
err = dev_set_allmulti(dev, 1);
if (err) {
br_multicast_del_port(p);
netdev_put(dev, &p->dev_tracker);
kfree(p); /* kobject not yet init'd, manually free */
goto err1;
}
err = kobject_init_and_add(&p->kobj, &brport_ktype, &(dev->dev.kobj),
SYSFS_BRIDGE_PORT_ATTR);
if (err)
goto err2;
err = br_sysfs_addif(p);
if (err)
goto err2;
err = br_netpoll_enable(p);
if (err)
goto err3;
err = netdev_rx_handler_register(dev, br_get_rx_handler(dev), p);
if (err)
goto err4;
dev->priv_flags |= IFF_BRIDGE_PORT;
err = netdev_master_upper_dev_link(dev, br->dev, NULL, NULL, extack);
if (err)
goto err5;
dev_disable_lro(dev);
list_add_rcu(&p->list, &br->port_list);
nbp_update_port_count(br);
if (!br_promisc_port(p) && (p->dev->priv_flags & IFF_UNICAST_FLT)) {
/* When updating the port count we also update all ports'
* promiscuous mode.
* A port leaving promiscuous mode normally gets the bridge's
* fdb synced to the unicast filter (if supported), however,
* `br_port_clear_promisc` does not distinguish between
* non-promiscuous ports and *new* ports, so we need to
* sync explicitly here.
*/
fdb_synced = br_fdb_sync_static(br, p) == 0;
if (!fdb_synced)
netdev_err(dev, "failed to sync bridge static fdb addresses to this port\n");
}
netdev_update_features(br->dev);
br_hr = br->dev->needed_headroom;
dev_hr = netdev_get_fwd_headroom(dev);
if (br_hr < dev_hr)
update_headroom(br, dev_hr);
else
netdev_set_rx_headroom(dev, br_hr);
if (br_fdb_add_local(br, p, dev->dev_addr, 0))
netdev_err(dev, "failed insert local address bridge forwarding table\n");
if (br->dev->addr_assign_type != NET_ADDR_SET) {
/* Ask for permission to use this MAC address now, even if we
* don't end up choosing it below.
*/
err = dev_pre_changeaddr_notify(br->dev, dev->dev_addr, extack);
if (err)
goto err6;
}
err = nbp_vlan_init(p, extack);
if (err) {
netdev_err(dev, "failed to initialize vlan filtering on this port\n");
goto err6;
}
spin_lock_bh(&br->lock);
changed_addr = br_stp_recalculate_bridge_id(br);
if (netif_running(dev) && netif_oper_up(dev) &&
(br->dev->flags & IFF_UP))
br_stp_enable_port(p);
spin_unlock_bh(&br->lock);
br_ifinfo_notify(RTM_NEWLINK, NULL, p);
if (changed_addr)
call_netdevice_notifiers(NETDEV_CHANGEADDR, br->dev);
br_mtu_auto_adjust(br);
br_set_gso_limits(br);
kobject_uevent(&p->kobj, KOBJ_ADD);
return 0;
err6:
if (fdb_synced)
br_fdb_unsync_static(br, p);
list_del_rcu(&p->list);
br_fdb_delete_by_port(br, p, 0, 1);
nbp_update_port_count(br);
netdev_upper_dev_unlink(dev, br->dev);
err5:
dev->priv_flags &= ~IFF_BRIDGE_PORT;
netdev_rx_handler_unregister(dev);
err4:
br_netpoll_disable(p);
err3:
sysfs_remove_link(br->ifobj, p->dev->name);
err2:
br_multicast_del_port(p);
netdev_put(dev, &p->dev_tracker);
kobject_put(&p->kobj);
dev_set_allmulti(dev, -1);
err1:
return err;
}
其中上述的重點是 br_mtu_auto_adjust，該 function 的內容如下，基本上就去找出最小ＭＴＵ並且設定
void br_mtu_auto_adjust(struct net_bridge *br)
{
ASSERT_RTNL();
/* if the bridge MTU was manually configured don't mess with it */
if (br_opt_get(br, BROPT_MTU_SET_BY_USER))
return;
/* change to the minimum MTU and clear the flag which was set by
* the bridge ndo_change_mtu callback
*/
dev_set_mtu(br->dev, br_mtu_min(br));
br_opt_toggle(br, BROPT_MTU_SET_BY_USER, false);
}