cascaded switch

cascaded switch

[Ran Shalit]

Hello,

I am trying to understand the concept of cascaded switch.
I haven't find much information on this topic.

Can anyone please explain the general concept, when is it used, and
why does the device tree need to know about cascaded switch ?

Thank you,
ranran

Re: cascaded switch

[Andrew Lunn]

On Fri, May 18, 2018 at 09:35:38PM +0300, Ran Shalit wrote:
> Hello,
> 
> I am trying to understand the concept of cascaded switch.
> I haven't find much information on this topic.
> 
> Can anyone please explain the general concept, when is it used, and
> why does the device tree need to know about cascaded switch ?

Hi Ran

I think you first need to define what you mean by cascaded switches.

  Andrew

Re: cascaded switch

[Ran Shalit]

On Fri, May 18, 2018 at 10:13 PM, Andrew Lunn <andrew@lunn.ch> wrote:
> On Fri, May 18, 2018 at 09:35:38PM +0300, Ran Shalit wrote:
>> Hello,
>>
>> I am trying to understand the concept of cascaded switch.
>> I haven't find much information on this topic.
>>
>> Can anyone please explain the general concept, when is it used, and
>> why does the device tree need to know about cascaded switch ?
>
> Hi Ran
>
> I think you first need to define what you mean by cascaded switches.
>
Hi,

I mean the same terminology used in marvell's switch.(I don't think
there is more than one terminology for this, please correct me if
wrong).
Anyway, I can see examples how it is done, but I don't understand the
benefit of this constellation, and why device tree needs to be
familiar with it.

<   switch 1  >---port10--------port10- <  switch 2 >
 | ....|         |                                             | ....|    |
port 1-9     |                                      port 1-9     |
                 |                                                        |
                 |                                                        |
<cpu>--mdio----------------------------------------------

The term "cascaded switches" is also used in dsa documentation in device tree:
https://www.kernel.org/doc/Documentation/networking/dsa/dsa.txt


Regard,
Ranran

>   Andrew

Re: cascaded switch

[Andrew Lunn]

> Hi,
> 
> I mean the same terminology used in marvell's switch.(I don't think
> there is more than one terminology for this, please correct me if
> wrong).
> Anyway, I can see examples how it is done, but I don't understand the
> benefit of this constellation, and why device tree needs to be
> familiar with it.
> 
> <   switch 1  >---port10--------port10- <  switch 2 >
>  | ....|         |                                             | ....|    |
> port 1-9     |                                      port 1-9     |
>                  |                                                        |
>                  |                                                        |
> <cpu>--mdio----------------------------------------------

Your ASCII art is all messed up, but i get what you mean.

This is the D in DSA. You would use this when a single switch does not
have enough ports for your use case. So you use two switches.

You need to tell each switch what links are used to get to other
switches. There is an internal routing table. So you need to describe
these links in device tree.

      Andrew

Re: cascaded switch

[Ran Shalit]

On Fri, May 18, 2018 at 11:29 PM, Andrew Lunn <andrew@lunn.ch> wrote:
>> Hi,
>>
>> I mean the same terminology used in marvell's switch.(I don't think
>> there is more than one terminology for this, please correct me if
>> wrong).
>> Anyway, I can see examples how it is done, but I don't understand the
>> benefit of this constellation, and why device tree needs to be
>> familiar with it.
>>
>> <   switch 1  >---port10--------port10- <  switch 2 >
>>  | ....|         |                                             | ....|    |
>> port 1-9     |                                      port 1-9     |
>>                  |                                                        |
>>                  |                                                        |
>> <cpu>--mdio----------------------------------------------
>
> Your ASCII art is all messed up, but i get what you mean.
>
> This is the D in DSA. You would use this when a single switch does not
> have enough ports for your use case. So you use two switches.
>
> You need to tell each switch what links are used to get to other
> switches. There is an internal routing table. So you need to describe
> these links in device tree.
>

I understand, thanks,
So, it is used so that the 2 switch will behave as if it is one big switch.

Yet, how does it change the way the ports appears in "ifconfig" ?
Is it that if they were separate switch I wouldn't see incremental
numbers in "lanX" in ifconfig  (as is probably the result in cascaded
switch) ?

Regards,
ranran

>       Andrew

Re: cascaded switch

[Andrew Lunn]

> So, it is used so that the 2 switch will behave as if it is one big switch.

Yes. This particularly important with offloading. When your offload a
bridge, you don't need to care which switch the ports or on. If
traffic needs to go from one switch to the other, it will. If you
modelled it as two switches, you would need to manually setup that
cross switch connection.

      Andrew