Cisco Support Community
cancel
Showing results for 
Search instead for 
Did you mean: 
Community Member

Oversubscribed switch blade

Please advise me the technical cons of the sentence in layman's terms:

"Cisco's WS-X6148-GE-TX on Core C6509 is heavily oversubscribed (8:1). Also it connects only to the 32Gbps shared bus."

1 ACCEPTED SOLUTION

Accepted Solutions
Hall of Fame Super Blue

Re: Oversubscribed switch blade

Hi Narayan

Many thanks for the compliment and rating.

I'll try and explain the 8:1 ratio in a bit more detail. The WS-X6148-GE-TX has an 8:1 oversubscription ratio because it groups together 8 ports into a single ASIC. This ASIC (called a pinnacle asic) transfers data between the switching fabric and the network ports. There is a 1Gbps uplink from the ASIC to the switching fabric ie. the aggregate throughput of the 8 port group cannot exceed 1Gbps.

So you have 8 network ports that can each have a device supporting 1Gbps which only have a 1Gbps connection to the switch fabric. This is why you are oversubscribed and that's why the WS-X6148-GE-TX is primarily for the wiring closet where you expect some level of oversubscription.

Contrary to what i said before it does look like the port groups of 8 are contiguous ie. 1 -> 8, 9 -> 16 etc.

HTH

Jon

14 REPLIES
Hall of Fame Super Blue

Re: Oversubscribed switch blade

Hi

There are two things to cover here.

1) Oversubscription. The WS-X6148-GE-TX blade is a 48 port 10/100/1000 blade. So you can connect 48 devices at 1Gbps speed to this module. But because the module is oversubscribed each device will not get 1Gbps throughput. An 8:1 oversubscription means that each group of 8 ports supports 1Gbps throughput. So if you only connected 6 devices at 1Gbps into this module there would be no oversubscription ie. 48 % 8 = 6.

Note that the 8 port groups are not necessarily continguous ie. the first group of 8 = ports 1 -> 8, the second group = 9 -> 16. It's not as simple as this, you need to read the docs to find out how the groups are formed.

2) 32Gbps shared bus. When the 6500 was originally released it only supported a shared bus ie. all the modules in the 6500 used the same shared bus connection to the switch fabric.

With a Supervisor 2 you could insert an extra module called a Switch Fabric module (SFM) which allowed "fabric enabled modules" to have a dedicated connection to the switch fabric.

With a Supervisor 720 there is an inbuilt SFM (hence the reason the supervisor goes into slots 5 & 6 on a 6509) so fabric enabled modules have a dedicated connection to the switch fabric. Note that different fabric enabled modules have different connections to the switch fabric eg.

The WS-X6548-GE-TX has a single 8Gbps connection to the switch fabric.

The WS-x6748-GE-TX which only works with the Sup720 supports 2 x 20Gbps connections to the switch fabric.

So the WS-6148-GE-TX is a non fabric enabled "classic" linecard which only has a connection to the shared 32Gbps bus. It would work with a Supervisor720 and a Supervisor 2 with an SFM but it will still only connect to the 32Gbps bus ie. it does not support dedicated connections to the switch fabric.

HTH

Jon

Community Member

Re: Oversubscribed switch blade

Thanks for the reply Jon. Makes things easier to understand. You mentioned that "So if you only connected 6 devices at 1Gbps into this module there would be no oversubscription ie. 48 % 8 = 6. "

Does this mean "If I attach six devices to each of the identified group of 8 ports, I will get no oversubscription, or Six devices ONLY on the available 48 ports, I would break even?

Also, What do you suggest I should go for on my C6509E chassis, in order to get the best deal for my Server farm connectivity?

And finally, is it a good idea to connect my Server farm directly to my Core or is there any other 'best practise'?

You advise had been most helpful.

Hall of Fame Super Blue

Re: Oversubscribed switch blade

Hi Fahim

It means you can only attach 6 devices in total to that module before you get oversubscription

The server farm module depends on which supervisor you have in your chassis. Attached is a link to Cisco doc on 6500 10/100/1000 Mbps modules. If you scroll down to the tables there is a column "Primary Application" which suggests what the modules primary purpose is. Look at the ones "Data centre and Server farm". As mentioned previously be aware that the 67xx modules will only work with a Sup720.

http://www.cisco.com/en/US/products/hw/switches/ps708/products_data_sheet0900aecd8017376e.html

Where to connect the serverfarm. Generally speaking it is best practice not to connect servers directly into the core switches but to have access-layer switches which then connect back to the core. This is for standalone servers.

There are also HP / IBM blade chassis systems which are a group of servers in an enclosure with a switch on either side of the enclosure. These switches can be directly connected to the core switches.

Finally best practices are a good guide but they do not need to be followed slavishly, a lot depends on your particular requirements.

HTH

Jon

Re: Oversubscribed switch blade

Hi,

I am not sure about the 8:1 ratio source, but the oversubscribtion comes from the fact that all the modules uses the 32Gbps shared bus, despite each card needs 48Gbps to be fully utilized.

[edit] Very nice explaination from Jon.

HTH,

Mohammed Mahmoud.

Community Member

Re: Oversubscribed switch blade

That's what I thought Mahmoud but seems like it's not so simple, for if you divide 48/32, the ratio that comes out is 1.5:1, which is quite acceptable. 1:8 is not.

I guess we'll have to go with jon. :)

Re: Oversubscribed switch blade

Indeed a very good explanation by Jon

deserves a rating..

but still i am confused with the 8:1 oversubcription ratio

Narayan

Hall of Fame Super Blue

Re: Oversubscribed switch blade

Hi Narayan

Many thanks for the compliment and rating.

I'll try and explain the 8:1 ratio in a bit more detail. The WS-X6148-GE-TX has an 8:1 oversubscription ratio because it groups together 8 ports into a single ASIC. This ASIC (called a pinnacle asic) transfers data between the switching fabric and the network ports. There is a 1Gbps uplink from the ASIC to the switching fabric ie. the aggregate throughput of the 8 port group cannot exceed 1Gbps.

So you have 8 network ports that can each have a device supporting 1Gbps which only have a 1Gbps connection to the switch fabric. This is why you are oversubscribed and that's why the WS-X6148-GE-TX is primarily for the wiring closet where you expect some level of oversubscription.

Contrary to what i said before it does look like the port groups of 8 are contiguous ie. 1 -> 8, 9 -> 16 etc.

HTH

Jon

Re: Oversubscribed switch blade

That makes things clear..

deserves rating again :-)

btw, is this info available in the CCO

Narayan

Hall of Fame Super Blue

Re: Oversubscribed switch blade

Narayan

The info is spread around in a few separate docs. The ones i found particularly useful

WS-X6148-GE-TX / WS-X6548-GE-TX specific information

http://www.cisco.com/en/US/products/hw/switches/ps700/products_tech_note09186a00801751d7.shtml#topic3

6500 Buffers / queues

http://www.cisco.com/en/US/products/hw/switches/ps708/products_white_paper09186a0080131086.shtml

6500 architecture

http://www.cisco.com/en/US/products/hw/switches/ps708/products_white_paper0900aecd80673385.shtml

Have fun :-)

Jon

Re: Oversubscribed switch blade

Hi Jon,

Agree with Narayan, deserves rating indeed :), you have really collected the info together perfectly.

BR,

Mohammed Mahmoud.

Hall of Fame Super Blue

Re: Oversubscribed switch blade

Mohammed

Comments and rating very much appreciated.

Thanks

Jon

Community Member

Re: Oversubscribed switch blade

JOn..reply appreciated and duly rated.

One query remains still..

What's the alternative that I have to push into my C6509 chassis? I need to connect this blade to my server farm. Also, I have heard that jumbo packets is on it's way in, so should I have something compatible both at my Edge and Core?

Hall of Fame Super Blue

Re: Oversubscribed switch blade

Fahim

Apologies for delay in replying. Could you clarify what you are looking for.

Can you also specify what type of suprevisor and exsiting modules you already have in your chassis.

Jon

Community Member

Re: Oversubscribed switch blade

Jon and others, I would appreciate your help at this post:

Just search for "Server farm block"

Thank you

1393
Views
28
Helpful
14
Replies
CreatePlease to create content