switching oversubscription

Answered Question
Aug 11th, 2013
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Hi,


I have been reading about oversubscription rates in certain documents, but i cant get an exact idea what does it mean.

What exactly does it indicate when we say oversubscription rate is 1:2 or so.


It would be helpful if this can be explained to get the idea.


Thanks in advance

Correct Answer by Reza Sharifi about 4 years 6 days ago

Hi,


Have a look at table-1 in this link.  Lets look at the 8 port 10Gig module.  The switch fabric connection for the 6500 to this module is at 40Gbps.  If you use all 8 ports at 10Gig speed (line rate) you need an 80Gig switch fabric connection.  So, since you have 40Gbps backplane and 8 port 10Gig, the module is 2:1 over subscribed.

8X10=80 but the switch fabric is 40.




http://www.cisco.com/en/US/prod/collateral/switches/ps5718/ps708/product_data_sheet09186a00801dce34.html



Does it make scene?


HTH

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Correct Answer
Reza Sharifi Sun, 08/11/2013 - 19:34
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Hi,


Have a look at table-1 in this link.  Lets look at the 8 port 10Gig module.  The switch fabric connection for the 6500 to this module is at 40Gbps.  If you use all 8 ports at 10Gig speed (line rate) you need an 80Gig switch fabric connection.  So, since you have 40Gbps backplane and 8 port 10Gig, the module is 2:1 over subscribed.

8X10=80 but the switch fabric is 40.




http://www.cisco.com/en/US/prod/collateral/switches/ps5718/ps708/product_data_sheet09186a00801dce34.html



Does it make scene?


HTH

Joseph W. Doherty Mon, 08/12/2013 - 02:35
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Oversubscription is when it's possible for "offered" bandwidth to exceed "available" bandwidth.  The difference in bandwidths can be compared in a ratio.


For example.


An 8 port FE copper switch with a single FE fiber would be oversubscribed - 8:1 as the 8 copper ports could all want to send 100 Mbps out the switch across the single FE fiber port.  However, if the fiber uplink was gig, then the edge ports would be oversubscribed 10:1, as the single gig fiber port could send to any one copper FE port.

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