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loop problems


we have two networks working with each other, connected with two trunk links. on our side we have cisco6509 and rstp is configured, on other side alcatel switch and mstp is configured. also on our side guard loop is configured. when loops occures on their network ours switch blocks vlans on which loop occures. but after that these vlans doesn't come up. only after restarting interface everything works. what can cause this problem,and how we can solve it


Re: loop problems

Hi shodan,

i have no idea why you would implement two different xSTP over the same infrastructure but the difference in mapping the vlan/spanning-tree instances is quite different between MSTP and RSTP. since you have all these loops , why don't you pick a standard version of STP ( as recent as you can) and deploy it across your entire infrastructure? It is very important to let match many parameters in a STP in order to properly span all your infrastructure. timers and instances in first place.

BTW, i would take away loop guard until you won't have the same STP version on both the Cisco and the alcatel. It can just misinterpretate the BPDU tarvelling between the MSTP and the RSTP.

keep in mind then that the RSTP is legacy too....

In May 2012 they even defined SPB... I don't know much yet but it looks interesting from a cost viewpoint.




loop problems

Hi Alessio,

thanks for answer

we have for nearly 2000 equipment all over the city, which connected to our central office through providers layer 2 mpls network. on all our equipment we use rstp, but our provider uses mst. we cant change configuration on all equipment.

i read that mst works with rstp without any problems

VIP Mentor

loop problems

Hi Sodan,

As per my knowledge.

The problem is that the6509 is not really running RSTP but Rapid-PVST. This is PVST with each vlan running an RSTP instance. MST was designed to interact with the IEEE version of RSTP, that only feature a single instance for all the vlans.

When doing MST/Rapid-PVST interaction, we have to fall back to plain PVST mode of operation (i.e. STP convergence type).

If you want RSTP operation between the 6509 and the MST region you can:

- run MST on the 6509, even with no configuration (a bridge running MST with no configuration is almost exactly an RSTP bridge).

- Another solution is to avoid trunking between the Rapid-PVST and the MST region (we can run RSTP between a Rapid-PVST bridge and an MST region on an access port).

or see this:

Also, there are some issues such as: topology change in one vlan on switches running pvst propagates into MST, and then appears as a topology change in ALL vlans to the pvsts on the other side; so running a bunch of old spanning trees on the edge and MST on the core can lead to issues. As far as MST and rpvst, there isn't any interoperability beyond the legacy mechanism,


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