Re: [Bridge] Link fail over - bridge slow learning !?!

[Alex Zeffertt <ajz@cambridgebroadband.com>]

Alexander Indenbaum wrote:
> Hello,
> 
> I'm playing with dual-port NIC driver level link failover:
> * Driver exposes single network interface to the OS and operates both
> ports in active-passive failover mode.
> * Upon Link down event on active port, driver switches active and
> passive ports transparently for the OS.
> 
> I'm testing the driver using Linux bridge module: "failover" dual-port
> NIC connected with two cables back-to-back to eth0 and eth1 which are
> part of br0 bridge.
> 
> I simulate link fail with following scenario:
> * At t0 both eth0 and eth1 port links are UP, traffic is accepted by
> eth0 and forwarded to br0
> * At t1 I manually unplug eth0 cable, causing link to go down.
> "Failover" driver switches the traffic immediately from eth0 to eth1,
> while using the same MAC address.
> * From t1 till t1+12 secs packets are accepted by eth1 but dropped by
> bridge and not forwarded to br0.
> * At t1+12 secs bridge starts forwarding packets from eth1 to br0
> 
> Hmm... I would expect that eth0 link down event would flush from
> bridge's table any MAC address associated with the port and that the
> bridge would start forwarding packets from eth1 to br0 immediately.
> 
> Why does it take ~12 secs for bridge to learn that MAC address moved
> from eth0 to eth1 in the described scenario?
> 

It may be spanning tree, rather than MAC address learning that takes
so long.  Bridges spend a while just listening before forwarding, to
avoid becoming part of a bridging loop.

Experiment with:

   brctl showstp br0
   brctl stp br0 off
   brctl setfd br0 1

Alex