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Switch rack limit!!!!!!!!

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Hugo Tigre
Trusted Contributor

Switch rack limit!!!!!!!!

i've been searching for this but i didn't find much.

Is there a limit to the amount of cascade switchs.

I know the limit of hops, and i also know that i have to check the latency of the packet, depending on my topology.

but i'm not shure if there's some kind of limit to the number of switch i can interconnect!

thanks in advance for a reply

regards,
Hugo
11 REPLIES
MIBA NSD
Advisor

Re: Switch rack limit!!!!!!!!

Hi!
If u mean stacking I read that a Stack accepts up to 16 Switches!
regards peter
Hugo Tigre
Trusted Contributor

Re: Switch rack limit!!!!!!!!

Hi peter

i don't mean stacking, i just mean having the switches conneted to one another, for exemple:

host1 sends a packet to host2, then the packet has to travel through a number of switches to reach host2. I want to know if there's a limit to this number of switches, or if the limit is equal to the limit of hops (TTL).

regards hugo
Caroline_7
Advisor
Solution

Re: Switch rack limit!!!!!!!!

Hi Hugo

I think you can chain up to 8 switches or even more,in a star configuration, after that, you have to watch what you do, as you can have problems with speed, corruption and funtionality. If your network is large enough for 8 switches not to be enough for you, then you can turn one of your patch panels into a switch by connecting it to a port on one of the switches.

good luck
caroline
Hugo Tigre
Trusted Contributor

Re: Switch rack limit!!!!!!!!

Hi caroline

Thanks for the reply, thats exactly what i wanted to know, i can't really test that in my current lab environment, but i needed to have some idea for my report on the project i'm working on.

thanks for your help

regards,
Hugo
Ardon
Trusted Contributor

Re: Switch rack limit!!!!!!!!

Hi Hugo,

You have to make a distinction here between latency and the number of Hops.

You and Caroline mention a maximum of 8 which comes from running Spanning Tree and is actually 7. This means that two distinct bridges in the network should not be more
than seven hops away the one to the other.

Now another thing is latency. Looking at micro seconds (as switching is done in hardware ASICS) that will not impose any applicational problem. Normally higher Layer protocols (TCP or the Session Layer) can buffer Packets which arrive out of order and then assemble the complete one.

Believe me. I have seen networks with hundreds of Switches although a Frame will never travel trhough all of them of course but only a few and therefore latency can be ignored.

Regards, Ardon
ProCurve Networking Engineer
Ardon
Trusted Contributor

Re: Switch rack limit!!!!!!!!

Forgot one thing.

You mention TTL. TTL is ONLY applicable when Routing comes into play (see my other posting "Number of hops can be reseted!!!!!!!!!! " )

At Layer 2 (Switching) regular Frames carrying user traffic have no Layer 3 header and therefore no TTL.

ProCurve Networking Engineer
Hugo Tigre
Trusted Contributor

Re: Switch rack limit!!!!!!!!

Hi Ardon,

thanks for the reply, your answer really help me.

I do know the difference between latency and hops! What i didn't know is what's responsible for restricting the number of cascade switches.

One thing u said i didn't quite understand:

- "You and Caroline mention a maximum of 8 which comes from running Spanning Tree and is actually 7.This means that two distinct bridges in the network should not be more
than seven hops away the one to the other."

Basicly what i think u r saying is that Spanning Tree is the one responsably for allowing a packet to only run through a total of 7 Swicthes!!!! But, Why is Spanning Tree responsible for this?

regards, hugo.
Anders_35
Regular Advisor

Re: Switch rack limit!!!!!!!!

The 7 hop limit is an IEEE recommendation, not an absolute.
It has to do with the time-out of root information (max age) in the spanning-tree.
See http://www.cisco.com/warp/public/473/122.html for more information.
Les Ligetfalvy
Esteemed Contributor

Re: Switch rack limit!!!!!!!!

When it comes to meshing, HP has a 5 hop rule.
Sergej Gurenko
Trusted Contributor

Re: Switch rack limit!!!!!!!!

Use RSTP or MSTP and you will newer face theoretical "7 switches limit" problem.
Hugo Tigre
Trusted Contributor

Re: Switch rack limit!!!!!!!!

Thank you all for your input

it really helped.