3750 48-PS ports vs 24-PS ports

Unanswered Question
Sep 21st, 2007

Besides the number of ports, is there any major difference between 48 or 24 port 3750 Switches. I was reviewing the model comparison chart and noticed that the forwarding Rate was more than double for 48port switch. 24-port has forwarding rate 6.4mpps, and 48-port has 13.1mpps.

Does this make 24-port switch much slower when it comes to switching packets. For data only traffic it could be acceptable, but for converged traffic (Voice and Data) speed should always be the priority.

The only reason I am going for 24 port POE switch since it can supply the maximum 15.4 power to each of its ports compare to 48-port that can only support 7.7W per port.

Can't decide which one to go with as I thought that the backplane, and switching capabilites on both switches would be the same.

I have this problem too.
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wochanda Fri, 09/21/2007 - 11:03

No, there isn't any difference between the 24 port and 48 port switch architecture-wise. The 24-port switch is just a 48-port switch cut in half.

Since the packet forwarding is distributed among the different port-asics on the switch, having 1/2 the ports means we have 1/2 the port asics, and thus 1/2 of the forwarding. The actual throughput achievable between ports does not change.

Will

Joseph W. Doherty Fri, 09/21/2007 - 16:31

From http://www.cisco.com/en/US/products/hw/switches/ps5023/products_data_sheet0900aecd80371991.html

32-Gbps switching fabric

? Stack-forwarding rate of 38.7 mpps for 64-byte packets

? Forwarding rate: 6.5 mpps (Cisco Catalyst 3750-24TS, 3750-24FS, and 3750-24PS), 13.1 mpps (Cisco Catalyst 3750-48TS and 3750-48PS), 17.8 mpps (Cisco Catalyst 3750G-12S), 35.7 mpps (Cisco Catalyst 3750G-24T), 38.7 mpps (Cisco Catalyst 3750G-24TS and 3750G-24WS), 35.7 mpps (Cisco Catalyst 3750G-16TD), 38.7 mpps (Cisco Catalyst 3750G-24TS-1U), 38.7 mpps (Cisco Catalyst 3750G-24PS), 38.7 mpps (Cisco Catalyst 3750G-48TS), 38.7 mpps (Cisco Catalyst 3750G-48PS)

Your looking at the 100 Mbps POE switches, correct? For 64 byte packet wire speed, 24*100 Mbps with 2*gig needs 6,547,200 pps (and 8.8 Gbps), 48*100 Mbps with 4*gig needs 13,094,400 pps (and 17.6 Gbps). The Cisco 6.5 and 13.1 values, above, are rounded. The 48 port plus 4 SFP version offers exactly 2x the performance of the 24 port plus 2 SFP version. Either model appears wire speed capable.

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