ethernet ring

Unanswered Question
Jun 6th, 2009

Hi All

I have 7 x 2960G switches and 2 x 3750e switches. The switches must be connected into a ring for redundancy purposes. The diagram is as follows:



1 = 3750E

2 = 2960G

The 2960G switches will belong to the same vlan. 3750 switches will be the distribution layer switches and configured with static routing and HSRP. The link between 3750 will be a trunk(3750 will be configured with extra vlans). The traffic is mostly from a few clients on each 2960G to one of the 3750's. The Traffic in opposite direction is minimal. Has anyone had any previous experience with this type of design? What will be the latency for the worst case scenario (if there are 7 hops to the destination)? Can this design support video transmission?

Thanks for any comments

I have this problem too.
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cisco_lad2004 Sun, 06/07/2009 - 00:31

If I understand you correctly, HSRP will be on 3750. 3750s will act as GTW to 2960.

Since you have a ring, you need to consider using STP. For increasing your convergence time if failures occurs use PVRST.

Ensure your HSRP Active switch is also root switch.

With regards to Video feed, when it comes to these type of switches I prefer to run Multicast inside a dedicated VLAN (as long as it is one feed for all users).

The above are just the way I would do it.



andrew.butterworth Mon, 06/08/2009 - 02:30

If that is a ring of switches at layer-2 then it's too big for STP to deal with by default. With the default timers STP supports only seven layer-2 hops and looking at your diagram there are nine so I think STP will be screwed up?

It might be worth explaining what you need to achieve in more detail.


Ronyn____ Mon, 06/08/2009 - 17:17


The distances between switches are about 600 - 800m. That's why that kind of topology will be implemented.

I can configure four of the 2960G switches with Vlan 10 and the last tree with Vlan 20. All links will be configured as trunks. Cisco 3750E will be distribution layer switches configured with static routing and HSRP. VTP pruning will be disabled on all switches. PVST will be configured, so the diameter for vlan 10 is 5 and for vlan 20 is 4.

Thanks for any comments.

Joseph W. Doherty Mon, 06/08/2009 - 04:14

Generally, we often try to avoid such long cascaded topologies in preference to a hierarchy. This isn't possible? Also, the two 3750Es can not be stacked?

Every switch hop will add some latency, although probably minimal unless you encounter traffic congestion. (Which is more likely to happen at the "top" rather than the "bottom" of the ring. (If you're wondering how you have a top and bottom of a ring, you need a variant of STP to logically break the ring resulting in a logical stack.)

Since the traffic should be to/from a 3750, you'll want to adjust your STP link cost and root placement (probably) to divide the ring in the middle of the 2960Gs.

As Andrew notes, you're exceeding the default STP diameter, but besides adjusting timers, as Andrew also mentions, believe STP diameter is based more on time rather than actual hops, so there's a good chance you wouldn't need to adjust your timers.

"Can this design support video transmission? "

It might. Would this be a streaming or real-time video? Bandwidth? Unicast or multicast? Just one stream or N streams? If N, max? Etc.

Ronyn____ Mon, 06/08/2009 - 16:12

The two 3750E switches will be placed in different locations (the distance between them is about 1.2km). Stp configuration will block one of the port between the top 2960G switches. That kind of topology has been chosen because of distance between switches. The video transmission will be real-time about 4 or 5 unicast streams per switch. The stream bandwidth will be 2Mbps.

In the future each 3750E switch will be stacked with the second 3750E that will be connected to 3 x 2960G in the same topology.

Thanks for any comments

cisco_lad2004 Mon, 06/08/2009 - 21:52

couple of years ago, I had a Metro ring made up 17x3560 + 2x6509 as root. used MST/RSTP, carried voice and measured 3s convergence on link failure with no timers tunning. Link utilisation was approaching 80% .


Ronyn____ Tue, 06/09/2009 - 00:53


Thanks for the comment. Questions: what was the STP diameter for the metro ring? Was it at layer 2 (one vlan acorss all 3560 switches)?

sam_meftahi Tue, 06/09/2009 - 01:23

It was a L2 for management so one VLAN for all 3560 + 2x6509.

Customer traffic was also L2 from customer CPE to 6509 where it was L3 terminated. If I recal well it was over 70VLANs , but we used MSTP.

We have not done any further fine tunning on timers or STO diameter.




Where 2 are Root bridge 6509, with 2 MSTs.



Joseph W. Doherty Tue, 06/09/2009 - 04:35

1.2 Km apart, you would need really, really long stack cables. ;)

There shouldn't be an issue with your real-time video along as you enable QoS on your switches and place it into the priority-queue.

Ronyn____ Tue, 06/09/2009 - 04:51


The future expansion requires adding the second 3750E and connecting them by stackwise cables. So there will be two stacks, each including two 3750E. The distance between these two stacks is 1.2km on SM fibre.


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