Switch Fabric Throughput Calculation

[rhadden1950]

I need to know how to calculate the fabric throughput for either a 3560 or 3750 switch? We are delivering video multicast streams from four IP transmitters, between 4 to 8 MBps per stream, and 12 streams per transmitter. Each transmitter would be pumping 12 multicast streams into each 100Mb port on the switch. The multicast packets would be switched or blocked, depending on which multicast is being joined by the down-stream IP receiver. There are 10 IP-video receiver doing IGMP joins to any of the 48 streams, but only at one time. We need to know which switch models will be able to handle the multicast packet switching without dropping packets? So, if a multicast stream is not joined by any receiver, then is it just dropped at the switch input? If a single multicast stream is joined by 10 output receivers, then do you count the input stream bit-rate alone, or do you count the output stream bit-rate times 10 ?

[scottmac]

Did you really mean "MB" (usually consiidered to mean MegaBYTES, versus MegaBITS)?

Are the transmitters on Gig Ports?

Are there any routers in the path(s)?

Properly configured, unused streams would be black-holed by the receiving switch, but they would still take processor time and buffers,

Multiple clients joining a single stream would only consume the bandwidth of the transmitted stream. Sending the transmitted stream to a / some / many ports does not consume much of the switch's resources. I think it is the "Earl" ASIC that accommodates and controls those functions ... you may be able to find some more docs on the main Cisco site.

Recent Cisco switches are engineered such that they are non-blocking (every port full and functional with no bottleneck in the switching fabric).

Good Luck

Scott

[rhadden1950]

YES! It should be 4 to 8 Mbps (bis/sec) for SDTV, and up to 19Mbps for HDTV.

Sorry - that was a typing mistake on my part.

2 - There are no routers in the network design. Everything will be through the switch.

3 - We have not purchased the switch yet, so that is the reason for the discussion. Do we need Gig ports, or can we just use 100bT ?

[scottmac]

Gigabit links to the servers would be strongly suggested.

If, for no other reason, bandwidth requirements tend to go up over time. While you may be able to accomplish your needs with 100BASE, having Gig will give your system "longer legs" and provide many more options over the long haul.

Good Luck

Scott

[rhadden1950]

Bandwidth is what I'm trying to calculate so just buying al Gig ports does not help me understand the switching fabric.

If I'm sending 8Mbps on an input port, with multicast IP packets, and I join that Multicast from one output port, then I would assume my output bandwidth is 8Mbps. What happens if I joint that multicast from 2 or more output ports. Does the switch still deliver 8Mbps to both output ports, or does it only deliver 4Mbps to each of the 2 output ports? How does the Catalyst switch count multicast packets going to multiple output ports? If there were 2 output ports, then is the single input multicast packet buffered, copied, and then delivered to both output ports?

[scottmac · Accepted solution]

Each of the output ports would deliver the entire bandwidth of the input multicast stream.

The switch should be able to handle full load to/from every port ... e.g. the switch should be able to deliver the entire mcast bandwidth to every other port in the switch concurrently ... one in to one out, or one in to 24 out .... shouldn't make any difference .... as long as you stay within the capacity of the port.

As a matter of fact, the default for a layer two switch is to send a multicast out every port (like a broadcast).

What makes the "intellegent" switch intellegent, is that it can recognize which ports are not "listening" for the multicast and remove that port from participation.

The general architecture of the switch is that each port gets a copy of the frame, then all except the ports that should have that frame are purged (shared bus & buffered output).

Let us know if you have any other questions

Good Luck

Scott

(thanks for the ratings!)