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Spanning tree in ring topology

Occasional Visitor

Spanning tree in ring topology

I have five procurve 2824 placed in a ring. This ring is connected to another, faster, ring consisting of six Alcatel Omniswitch 6850 with 10Gbit fiber interfaces. Both rings forms a figure 8. On both rings there are branches of other switches. I have about 10 VLAN's in the network, but have so far opted for flat STP.

I have enabled STP on all switches.
Allmost everything seems to work out of the box. No packet storms so far. One strange thing occurs when i disable a link in the HP ring. There is then a brief disruption of nework traffic on the Alcatel ring. This makes all IP-phones on the network loose connection with their PBX, and start sending out DHCP reqests. All servers (DHCP, PBX etc) are located on the Alcaltel ring so no traffic should be affected by the broken link in the hp ring.
Could there be a problem with the elected STP route? Should i set this manually to the alcatel switch that is a member of both rings? Which STP variant is the most suitable for my network layout?
Mohieddin Kharnoub
Honored Contributor

Re: Spanning tree in ring topology


There are 3 Rings in your topology, HP one, Alcatel One and the Whole one.

Each switch is a part of this Spanning Tree topology, so if you unplugged one switch, the Whole process has to be redone to elect the root, and during that, ports will blocked for few seconds.

- Manual Select and configure a Root Switch from the Alcatel side.
- Manually select and configure a Backup root switch from the HP Side.
- Configure IP Phones ports as Edge Ports, and so it will not participate in the Spanning-Tree Root Election.

Good Luck !!!
Science for Everyone
Jarret Workman
Frequent Advisor

Re: Spanning tree in ring topology

Hello Innov,

From the description of the behavior, a spanning-tree topology change is detected when one of the links is disabled. This is normal operation of the spanning-tree and does impact the whole spanning-tree network as mentioned by the previous individual.

It sounds like the Alcatel ring may be running 802.1d spanning-tree, which by default, blocks traffic on ports for about 30 seconds while the new spanning-tree topology is being sorted out.

In addition to looking at which device is your root bridge, I would also recommend looking the Alcatel's to see if they support RSTP as this blocking behavior is modified where ports still pass traffic during a topology change.