Over subscription

Answered Question
Sep 19th, 2010

Hi

I wanted to know if someone can expalin this to me exactly what do they mean when they say over subscription. I have been listening to a lot of pod cast and they are talking about the 4500 and 6500 and over subcription. I what to make sure that i'm getting the exact meaning of what they are talking about when they use this word.Thanks in advance and have a great day.

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Correct Answer by Jon Marshall about 6 years 2 months ago

scooter817 wrote:

Hi

I wanted to know if someone can expalin this to me exactly what do they mean when they say over subscription. I have been listening to a lot of pod cast and they are talking about the 4500 and 6500 and over subcription. I what to make sure that i'm getting the exact meaning of what they are talking about when they use this word.Thanks in advance and have a great day.

When referring to 4500 and 6500 switches oversubscription can be a big issue. A practical example might help -

a 6509 with a supervisor 720 supports up to 720Gbps switch fabric. The switch fabric is in effect the pathway between each module in the chassis and the other modules and supervisor. Each fabric enabled or fabric only module has a dedicated connection to the switch fabric ie. it does not share it's connection with any other modules in the chassis.

So lets say you have 2 modules you want to insert into the 6500, both of which have dedicated connections to the switch fabric -

WS-X6548-GE-TX  - this is a 48 port 10/100/1000Gbps module that has a single 8Gbps connection to the switch fabric

WS-X6748-GE-TX  - this is a 48 port 10/100/1000Gbps module that has 2 x 20Gbps connection to the switch fabric so 40Gbps in total.

With each module you can connect up to 48 devices each running 1Gbps NICs. But the oversubscription is very different ie.

6548 only support 8Gbps to switch fabric but you can in theory have 48Gbps coming into the module so this module is very heavily oversubscribed, hence the reason you don't use this as a server module. If you wanted to avoid oversubscription you would need to only connect 8 devices into this module.

6748 supports 40Gbps to switch fabric and in theory you can have 48Gbps coming into the module so this module is far less likely to see oversubscription.

And in both scenarios it is highly unlikely you could get all 48 devices to push out 1Gbps all at the same time so with the 6748 oversubscription is not often seen whereas it is a fairly common occurence when using the 6548 especially if you have servers connected.

When you spec up a 4500 or 6500 it is very important to pay attention to the individual modules connections to the switch fabric and the overall switch fabric available eg. the 4500 chassis only support 6Gbps per module which can severly limit it depending on what you are using it for, whereas the 4500-E chassis with a Sup 6-E can support up to 24Gbps per module.

You need to decide what the switch will be used for ie. desktop client, server farms etc. and choose accordingly.

Jon

Correct Answer by vragotha about 6 years 2 months ago

Over-utilization of ports. Trying to push more traffic than it can handle will generally result in output drops leading to over subscription

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Correct Answer
vragotha Sun, 09/19/2010 - 11:28

Over-utilization of ports. Trying to push more traffic than it can handle will generally result in output drops leading to over subscription

Correct Answer
Jon Marshall Mon, 09/20/2010 - 06:39

scooter817 wrote:

Hi

I wanted to know if someone can expalin this to me exactly what do they mean when they say over subscription. I have been listening to a lot of pod cast and they are talking about the 4500 and 6500 and over subcription. I what to make sure that i'm getting the exact meaning of what they are talking about when they use this word.Thanks in advance and have a great day.

When referring to 4500 and 6500 switches oversubscription can be a big issue. A practical example might help -

a 6509 with a supervisor 720 supports up to 720Gbps switch fabric. The switch fabric is in effect the pathway between each module in the chassis and the other modules and supervisor. Each fabric enabled or fabric only module has a dedicated connection to the switch fabric ie. it does not share it's connection with any other modules in the chassis.

So lets say you have 2 modules you want to insert into the 6500, both of which have dedicated connections to the switch fabric -

WS-X6548-GE-TX  - this is a 48 port 10/100/1000Gbps module that has a single 8Gbps connection to the switch fabric

WS-X6748-GE-TX  - this is a 48 port 10/100/1000Gbps module that has 2 x 20Gbps connection to the switch fabric so 40Gbps in total.

With each module you can connect up to 48 devices each running 1Gbps NICs. But the oversubscription is very different ie.

6548 only support 8Gbps to switch fabric but you can in theory have 48Gbps coming into the module so this module is very heavily oversubscribed, hence the reason you don't use this as a server module. If you wanted to avoid oversubscription you would need to only connect 8 devices into this module.

6748 supports 40Gbps to switch fabric and in theory you can have 48Gbps coming into the module so this module is far less likely to see oversubscription.

And in both scenarios it is highly unlikely you could get all 48 devices to push out 1Gbps all at the same time so with the 6748 oversubscription is not often seen whereas it is a fairly common occurence when using the 6548 especially if you have servers connected.

When you spec up a 4500 or 6500 it is very important to pay attention to the individual modules connections to the switch fabric and the overall switch fabric available eg. the 4500 chassis only support 6Gbps per module which can severly limit it depending on what you are using it for, whereas the 4500-E chassis with a Sup 6-E can support up to 24Gbps per module.

You need to decide what the switch will be used for ie. desktop client, server farms etc. and choose accordingly.

Jon

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