Switching

Hi,

we're running a network with 2 core switches (EX4500),  and  edge switches (EX4200) with dual attachements to the core (1 giga link toward each core membre).

rSTP is running on all switches interfaces. 

Core-1 and Core 2 have a back-to-back LAG attachment.  One of the Core switches is root of the rSTP topology, the second core switch is the second best choice and appears as the alternate bridge on the edge switches.  Each edge switch as its "root" port forwarding toward core-1 and its "alternate" port toward core-2 in blocking mode. 

So far so good, all is running smoothly. 

On the edge switches, rSTP is running on every edge interface in porfast mode. 

I was wondering what would happen if, by mistake, someone plugged a low-cost switch with no STP  on 2 ports of an edge switch. 

=> Will the edge switch detect a loop because the low-cost switch will sent back to him a BPDU that the edge switch originated ? 

=> Will the low-coast switch simply drop any BPDU it receives from the Edge switch ... and here we go, ethernet loop and broadcast storm due to double attachment, rSTP on edge switch being of no help in that case ?

I've attached a jpeg (ethloop.jpg) showing the topology, to make my case clear.

Hello,

If the low-cost switch passes BPDUs, then edge switch is able to detect a loop.

If it does not, then use "mac-move-limit" with action "block" or "shutdown"

http://www.juniper.net/documentation/en_US/junos12.1/topics/reference/configuration-statement/mac-move-limit-port-security.html

HTH

Thanks

Alex

Thanks :

- if the low-cost switch forward BPDU the EX4200 Edge switch will  block one of its ports and consider it as a backup port, that's how RSTP works I guess (dual attachment to the same segment), 

- if the low-cost switch drop the BPDU, limiting the mac moves can be helpful to limit / resolve the broadcast storm, indeed. Thanks for the tip !!