What is oversubscription?When selecting a 16 port 10 gig blade for cisco 6500 , it says oversubscription as 4:1. What will happen if I populate all 16 ports, will some ports be blocking or dropping packet?
asked May 21, 2013 at 4:35
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Did any answer help you? If so, you should accept the answer so that the question doesn't keep popping up forever, looking for an answer. Alternatively, you can post and accept your own answer.– Ron Maupin ♦Commented Jan 3, 2021 at 4:55
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3 Answers
It sounds like you're talking about the WS-X6716-10GE. Over-subscription in this context means you have a 16 x 10GE ports on the front of the LC and no more than 4x10GE at the fabric connection to the rest of the chassis.
Over-subscription will not matter if you locally switch traffic within the same linecard; however, you will only be able to send up to your fabric connection (i.e. 40Gbps) to other linecards in the same chassis. So it's possible to get blocking or drops if your links congest the fabric connections.
answered May 21, 2013 at 4:43
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Hai Mike, thanks for the answer. The thing that worries me is, when designing a collapsed core switch that needs 12 uplinks to be connected, should I go for 16 port card that you specified or go with 2*8 port cards or 4* 4port cards. Commented May 21, 2013 at 5:02
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@NAVEENGEORGE, you are asking a very different question now... what part of oversubscription is worrying you... we deal with oversubscription every day as network engineers. 4:1 at the core is not terribly bad, unless a) you think many of those links will operate at capacity... and b) you think you'll need 40G of backplane capacity. If you're really worried, use Sup2T + WS-X6908-10G-2T, which has full 80G to the fabric Commented May 21, 2013 at 5:09
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Thanks Mike Sup2T + WS-X6908-10G-2T looks like a good option. Commented May 21, 2013 at 5:24
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1@MikePennington, with 4-ports per ASIC, and the ASIC only able to handle a single 10G line-rate as I understand, doesn't the oversubscription still apply since the DFC forwarding happens above the port ASIC? My understanding is that the DFC only offloaded the MSFC/PFC functions and allowed for local switching without the SUP, but ASIC port constraints would still apply. If there was one ASIC per port with total bandwidth exceeding the 2x20Gb fabric links, then this makes sense to me that DFC can exceed 40G and have no oversubscription. Commented May 21, 2013 at 6:34
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1@MikePennington, for the life of me, I can't find the Cisco doc I saw before on this. I believe, and have seen others describe, the port group ASIC on the 6716 is 10Gb. Even with local switching and the DFC, I believe traffic still needs to travel out of the port group up to the Fabric ASIC within the module -- but not onto the Fabric crossbar and out of the module -- over that 10G port ASIC even if destined for a different port in the same port group. Commented May 22, 2013 at 5:40
In addition to @Mike's answer, the following Cisco webpages provide some context into the Cat65k fabric architecture:
answered May 21, 2013 at 4:53
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Hai Mike, thanks for the answer. The thing that worries me is, when designing a collapsed core switch that needs 12 uplinks to be connected, should I go for 16 port card that you specified or go with 2*8 port cards or 4* 4port cards. Commented May 21, 2013 at 5:01
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Hi Naveen - If your intent was to address Mike, you should have commented on his answer rather than mine. The answer to your question is going to depend on the amount of traffic you're expecting as well as your loss/latency tolerance; these are ultimately design decisions and are beyond the scope of your original question. Commented May 21, 2013 at 5:07
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1@JohnJensen Welcome @ networkengineering :). A little feedback: in the futur, if your answer is an addition to another answer, you should add it as a comment, not as a standalone answer. This gives a better oversight when multiple answers are given.– BulkiCommented May 21, 2013 at 5:51
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(adding a "self-defense" comment to note that users can't leave comments on answers that are not their own unless they get 50 rep) :-) Commented May 21, 2013 at 6:32
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Here's some additional links further to John's answer as throughput is not as simple as undersubscribing or oversubscribing interfaces; cisco.com/en/US/products/hw/switches/ps708/… supportforums.cisco.com/docs/DOC-21377 cisco.com/en/US/prod/collateral/switches/ps5718/ps708/… cisco.com/en/US/prod/collateral/switches/ps5718/ps708/… cisco.com/en/US/solutions/collateral/ns340/ns517/ns224/ns668/…– BaldrickCommented May 21, 2013 at 9:26
Oversubscription happens when your network does not have the capacity to handle the traffic if all devices start sending at maximum speed.
In many cases this is not a problem, and it is very common in networking and many other areas. When running virtual machines, the host does not have all CPU, RAM and disk storage promised to its guests. Airlines and hotel owners are also well known to overbook.
When you oversubscribe, you save money, but take the risk of having capacity problems when there is a usage peak. You should consider how much and where to oversubscribe in your network design. And always keep an eye on the meters to spot trends that require adding extra capacity to prevent structural performance issues.
