[Bridge] STP with only 2 switches

Stephen Hemminger shemminger at osdl.org
Thu Aug 11 10:26:22 PDT 2005

On Thu, 11 Aug 2005 16:33:01 +0530
"sameer" <sameer.neugui at tatainfotech.com> wrote:

> Hi all,
> We are facing a problem with STP implementation. We have only 2
> switches. Thats the constraint, but we feel STP should work.
> Background:
> We have developed a layer 2 switch. This uses Cirrus logic's ep9301
> processor, embedded Linux 2.4.21 kernel as the OS and zarlink's switch
> controller.
> The observation:
> 1)we have 2 switches, switch A and switch B
> 2)port 1 and port 2 of switch A are connected to port 1 and port 2 of
> switch B
> 3)1 PC is connected to port 0 of switch A and another to COM port of
> switch A to observe STP

	PC-A  ---0 switch A 1 ---- 1 switch B
			    2 ---- 2

> 4)We restart the 2 switches and observe that one of the links is
> blocked and the other in forwarding state. STP is always enabled on
> reboot

That is good.

> 6)We start to ping switch A and the switches and the two PC hang.

How are you setting up switch A? what are the equivalent bridge
commands and ip configuration?  Are you assigning IP to eth device
or br device?

Define hang? is network not responsive? system not responsive?
Did one of the switches produce a OOPS on console?
> 7)the LEDs on the switch device that signify the activity are seen
> continuously glowing.

You may have created a packet loop because of STP problem.
Simple test is to just unplug the port 2 connection.

> 8)switch A and B, PC 1 and 2 cannot be accessed at all.
> 9) We remove one of the links and the STP reconfigures and the switch
> A can be pinged.
> 10)Most of the time we have observed, although not a confirmed
> pattern, that if we remove the blocked link, the STP just disables
> the port on switch A (we have not observed the counter part on the
> switch B
> 11)Neither the switches nor the PCs were on the rest of the company
> network
> Its not a hang exactly as upon some topology change the system is
> back to working. I also checked on the internet. On cisco site I
> found something called unicast flooding.
> 1)Is this the reason for the system to blog down?
> 2) How do I find which version of STP I am using? The person who has
> originally implemented has quit.
> what would be the fix or some pointer in this regard?

You could start by using a newer kernel.  Using 2.6 might be impossible
but linux-2.4.21 is old. You might compare the current 2.4 net/bridge
directory with your kernel source.

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