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How many access switches can be configured in a single ring topology in layer 2 access design?

rajdhingra84
Level 1
Level 1

Hi,

In one of the network design presented to me by the consultant for a manufacturing plant environment, the no. of access switches coming in a single ring toplogy is exceeding 20 and at some places even 40 also.

Does anybody have any experience or recommended design guidelines which can be referred while thinking of deploying so many access switches like 2960 in a single ring topology in layer 2 access design.

Assuming the uplink bandwidth is adequate to take care of the traffic loads from downlink end device ports, is it practically possible to have 40 or 50 access switches in a single ring topology over a single mode fiber media?

From application perspective, it would be IP CCTV cameras which would get terminated on these access switches and would send live videos to control room.

Any help would be of great help.

Thanks !!

5 Replies 5

Vivek Ruhil
Cisco Employee
Cisco Employee

Hi

If the ring is layer 2, then how does the consultant wish to take care of broadcast in the network.

I mean with 20 access switches, if every switch is sending traffic of 10Mb it will become 200Mb for the entrie ring because the traffic will be braodcasted from the originating access switch right upto the uplink. and not to mention that every switch will process that packet.

I have seen networks with as low as 5 access switches in a Layer 2 design go bust if there is a L2 loop happening.

Perhaps, if you would share more details about the switches, about their config etc from multicast perspective I can comment more.

Hi Vivek,

To avoid such problems only, I want to avoid this kind of topology and want to reduce the number of switches in a single ring topology.

I agree to your point also that broadcasting can make even a two switch network bog down in matter of seconds.

The design that I discussed in my question is just a base idea from consultant which he has put forward. I'm seeking community's help to gather some good technical facts which I can use to bring to design team's knowledge so that we can remove any design flaws well in advance.

Apart from broadcast issue, do you feel there can be other caveats which strictly discourages this ring design. I have heard but not sure that we can have only 7 switches connected in a cascaded fashion. I"m still trying to find something concrete on this limitation.

Joseph W. Doherty
Hall of Fame
Hall of Fame

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If an actual ring will be constructed, you'll need "something" to break the L2 loop.  Often the "something" is STP, which by default, assumes a diameter of no more than 7.

oaziz
Level 1
Level 1

Hi!

Were you able to find an answer to your question?  I am also interested to know how many switches can we have a in a ring network.  

Regards

haroldlink
Level 1
Level 1

The answer is 16.

With more than 16 switches in a ring topology the RSTP protocol start to fail due to IEEE defined timings, default values and dropping STP packets.

Between 8 and 16 you need to modify  STP Max_Age and Forward_Delay to cope with your installation.

From real factory experience, between 17 and 22 your start to have two STP root in your ring and you're not protected against failure. Above 22 the ring just failed. This was with the Cisco SG300 switches.

Alternative for more than 16 is to design a multi ring topology, still layer 2, and check that the max network diameter is less than 16.

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