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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?

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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.

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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. – NAVEEN GEORGE May 21 '13 at 5:02
  • @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 – Mike Pennington May 21 '13 at 5:09
  • Thanks Mike Sup2T + WS-X6908-10G-2T looks like a good option. – NAVEEN GEORGE May 21 '13 at 5:24
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    @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. – generalnetworkerror May 21 '13 at 6:34
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    @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. – generalnetworkerror May 22 '13 at 5:40
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In addition to @Mike's answer, the following Cisco webpages provide some context into the Cat65k fabric architecture:

https://supportforums.cisco.com/docs/DOC-18210

http://www.cisco.com/en/US/prod/collateral/switches/ps5718/ps708/prod_white_paper0900aecd80673385.html

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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.

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