Cat 3560 Backplane BW question

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Jun 3rd, 2008
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I have 16 GB trunks (From Access switches) terminating on WS-C3560G-24TS. Also 4Gb L3 port-channels to 6509 and also 4 GB trunks to another pair of WS-C3560G-24TS. I understand the total backplane bandwidth is 35Gbps. I'm i oversubscribing? Should i expect any performance issues?


How is the 32Gbps bandwidth allocated on the switch?




Thanks.


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glen.grant Tue, 06/03/2008 - 15:30
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Just do a "show controllers utilization" and it will tell you exactly how busy it is including the switch fabric

Joseph W. Doherty Tue, 06/03/2008 - 15:56
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I believe a 3560's fabric bandwidth and forwarding rate will support line rate for all ports on the 24 gig port model (not true for the 48 gig port model).

Amit Singh Tue, 06/03/2008 - 21:19
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Francisco,


If your all the ports are sending on the line-rate then you might be oversubscribing the switch backplane. The 32Gbps switch fabric is a full duplex number and If I calculate the traffic based on the information above, you need atleast 48Gbps full-duplex switching fabric.


It might not happen that at any given time, your all the ports will sending line-rate traffic, which is OK and will suffice your need.


HTH,

-amit singh

Joseph W. Doherty Wed, 06/04/2008 - 03:35
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Amit, you may be correct because it's never been clear to me, in all cases, what Cisco's fabric bandwidth represents.


For example, 24 gig duplex ports can only send 24 gig across the "backplane". (Remember the 24 gig sent in, also becomes 24 gig sent out, so although the 24 gig duplex ports appears to be moving 48 gig, only 24 crosses the fabric.)


The necessary Ethernet line rate for 24 gig, is 35.71428 Mpps, and the 24 and 48 port 3560G models are rated as "38.7 Mpps". So we're covered for wirespeed for the 24 gig port models. But as you believe, if the 32 GB fabric is a duplex value, you're correct we would need 48 GB duplex to provide the 24 gig that actually crosses the fabric. (What makes this so complex, I believe although there is a true fabric, it's often not to each Ethernet port but to groups of ports controlled by some ASIC.)

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