Ring Topology

mosama · May 3, 2011

Dear all,

What is the problem if I used unmanaged switch in ring topology, and if a managed switch is necessary, should all the switches be managed or only one switch ??

Thank you

robertkjonesjr · May 3, 2011

Depending on the ring management protocol used, some unmanaged switches may not support that particular protocol and may not forward the link data packets required for the protocol along. In other cases, the presence of the unmanaged switch may not hinder the ring protocol, but will not participate in rapid recovery so it may be a significant amount of time before the ring fully recovers while the forwarding table times out.

Suggestion: fastest recovery would be with ALL switches supporting the same ring protocol that way they can both send management exceptions to the other ring members if they detect failure and also they can respond to management frames when it is time to update the topology (e.g. flush the forwarding table when an event occurs)

mosama · May 3, 2011

Thanks robert for your response, but what is meant by ring management protocol? is this a protocol that implemented inside the switch?

robertkjonesjr · May 3, 2011

mosama said:
T... but what is meant by ring management protocol? is this a protocol that implemented inside the switch?

Yes. It seems every vendor has their own, and then support for some more-or-less standard ones:

MRP
STP
rSTP
Hiper Ring
DLR
REP
RapidRing
Turbo Ring
N-ring

and others... (if I left yours off the list, sorry!)

Here is a standard that defines some of them: IEC 62439

In some of the simplest cases, a protocol is used to disable one of the two switch ports at the ring manager or supervisor to avoid the loop (and subsequent network crash!) when the switch loop is created.

harryting · May 3, 2011

I thought Spanning Tree is the standard:

http://en.wikipedia.org/wiki/Spanning_Tree_Protocol

Hydro Power Guy · May 3, 2011

harryting said:
I thought Spanning Tree is the standard:

http://en.wikipedia.org/wiki/Spanning_Tree_Protocol

Spanning tree is typically for systems that are much larger than the typical control system. Also, recovery time on a spanning tree is much longer. Some ring topologies offer recovery times less than 30 ms.

Binaural · May 3, 2011

Hydro Power Guy said:
Spanning tree is typically for systems that are much larger than the typical control system. Also, recovery time on a spanning tree is much longer. Some ring topologies offer recovery times less than 30 ms.

Additionally, it's a great big pain to set up manually. If you need redundancy, choose one of the proprietary ones mentioned above that come with the switch. There are a few "which switch?" threads on the forum that have some good guidance on this. Moxa is easy to set up for redundancy, likewise Hirschmann.

mosama · May 4, 2011

Thanks again robert, and thank you all, but should all the switches in the network be managed or only one switch should be managed and the others unmanaged ??

tragically1969 · May 4, 2011

mosama said:
Thanks again robert, and thank you all, but should all the switches in the network be managed or only one switch should be managed and the others unmanaged ??

They will all need to be managed when in a ring, you can put unmanaged ones out on a spur from the ring.

You cant go far wrong looking the way of Moxa

robertkjonesjr · May 4, 2011

robertkjonesjr said:
Suggestion: fastest recovery would be with ALL switches supporting the same ring protocol that way they can both send management exceptions to the other ring members if they detect failure and also they can respond to management frames when it is time to update the topology (e.g. flush the forwarding table when an event occurs)

As tragically1969 said, it is best that they all support the same protocol. You can find some protocols and some unmanaged switches that will work - and will likely recover - but in general no vendor will support you or guarantee the ring recovery times they quote if you use non-managed switches or switches that don't support the protocol. The issue is timing: if you want the layer 2 ring recovery time quoted by the vendor they all need to match. This is really an application question: if you don't care, then put in whatever you want and it might work (suggest you validate and test first, though).

Example:
We have an install of some IO devices that are dual port -they don't support a ring protocol, but we looped them anyway and used one of the proprietary ring protocols on one Managed Switch as the ring manager. This protocol has a guaranteed ring recovery time of something like 200ms. My application testing showed that often times we met this criteria, but in about 10% of the test cases, I got a ring recovery time of about 3sec. In my control world, there is big difference between 200ms and 3s. However, in this case, we could live with up to a 5sec delay for these inputs so we decided that this architecture would work for us and knowing the worst case recovery time, I was able to account for this timing and absorb this type of fault without a process upset.

However, in other places where we need to go faster, we specify the dual port STB I/O devices from Schneider Electric (STB2311) that support rSTP out of the box, and use this with a Schneider managed switch (also running rSTP) and the recovery times are fantastic because everything supports the same protocol.

mosama · May 4, 2011

Again, many thanks. My regards.

Binaural · May 5, 2011

robertkjonesjr said:
As tragically1969 said, it is best that they all support the same protocol. You can find some protocols and some unmanaged switches that will work - and will likely recover - but in general no vendor will support you or guarantee the ring recovery times they quote if you use non-managed switches or switches that don't support the protocol. The issue is timing: if you want the layer 2 ring recovery time quoted by the vendor they all need to match. This is really an application question: if you don't care, then put in whatever you want and it might work (suggest you validate and test first, though).

Example:
We have an install of some IO devices that are dual port -they don't support a ring protocol, but we looped them anyway and used one of the proprietary ring protocols on one Managed Switch as the ring manager. This protocol has a guaranteed ring recovery time of something like 200ms. My application testing showed that often times we met this criteria, but in about 10% of the test cases, I got a ring recovery time of about 3sec. In my control world, there is big difference between 200ms and 3s. However, in this case, we could live with up to a 5sec delay for these inputs so we decided that this architecture would work for us and knowing the worst case recovery time, I was able to account for this timing and absorb this type of fault without a process upset.

However, in other places where we need to go faster, we specify the dual port STB I/O devices from Schneider Electric (STB2311) that support rSTP out of the box, and use this with a Schneider managed switch (also running rSTP) and the recovery times are fantastic because everything supports the same protocol.

One other thing worth noting on top of this excellent post - there are different levels of managed switch. If you just want to get your network back up fast after the loss of one leg of your network, then most of the industrial managed switches on the market will suit nicely. The next level up is when you start partitioning the traffic on your network - for example to reduce the network traffic on parts of the network that need to run fast, such as I/O. These sort of switches tend to be a bit bigger and more complicated.

nwboson · May 6, 2011

Ntron has some real good documentation of ring topologies for their switches: try poking around on their web site for more insight and examples.

Although documentation can be challenging for Cisco switches, I love that they make switches with Rapid Spanning Tree protocol and channel bonding/trunking. Then you can have redundant dual wires between each switch (double reliability plus double bandwidth) and create a ring topology so that even if a full pair of trunked wires is severed (or a switch in the ring dies), everything keeps working. Even the cost effective Cisco 2000/2010 series offer these features.

mosama · May 7, 2011

Thank you

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