switch fabric(backplane) & pps

Unanswered Question
Oct 26th, 2009

What is the difference or actual meaning of backplane ( fabric) & pps ability.

Some of the higher switches are seen to have lower pps than some like 2960.what does this factor imply & how does it impact performance.

Thanks!

I have this problem too.
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Joseph W. Doherty Tue, 10/27/2009 - 06:00

Backplane (fabric), as I suspect you're using the terms, refers to bandwidth capacity between between edge ports and/or line cards. Ideally, internal bandwidth would support all edge bandwidth's full capacity but often this is not the case. When it doesn't, it can limit a device's maximum forwarding capacity.

Perhaps one way to understand this, consider two separate 9 port switches that you intend to interconnect. Each with 8 host ports and a dedicated uplink port. If the uplink port was the same bandwidth as the host ports, you would have an 8:1 oversubscription between the switches. I.e., forwarding between switches would be limited by the uplink bandwidth capacity.

If the uplink were 10x the bandwidth of the host ports, your uplink would not be a bandwidth bottleneck between the two switches. Ratios can vary per design. For example, assume you still have the 10x uplink port but now have 24 or 48 host ports. Once again the uplink can limit performance between the two switches, although the oversubscription is not as severe (2.4:1 and 4.8:1 vs. 8:1).

(NB: full capacity internal bandwidth is 2x port bandwidth assuming duplex ports.)

Since network traffic is segmented into frames/packets, a switch or router needs to examine such and decide what to do with each frame/packet as it enters the device. Since frames/packets often are of variable size, for the same bandwidth, the number of packets per second (PPS) increases as the frame/packet size decreases for the same bps rate. For example, for gig Ethernet, minimum (64 bytes) sized packets will arrive at the rate of 1,488 Kpps but maximum (1500 bytes) sized packets will arrive at the rate of 81 Kpps. To support line-rate or wire-speed, the switch's or router's performance must be able to support the necessary packet arrival rate. If it doesn't packets are either slowed when forwarded or dropped.

Like internal device bandwidth, a switch's or router's PPS capacity often does not support all its ports running at full rate especially for minimum sized frames/packets.

It normal networks, devices often do not need to support every port running at full capacity, so we can match a (less expensive) device's capacity against expected usage. (BTW, wire-speed devices seldom are necessary, but they make for nice "bragging".)

(NB: for wire-speed PPS, you need to account for duplex, but on a network device, one port's in is another port's out, so full PPS is 1x sum of port capacities.)

As example of a wire-speed/line-rate L3 switch is the 4900 series such as "Catalyst 4948 is a wire-speed" - "using a 96-Gbps switching fabric with a forwarding rate of 72 million packets per second (mpps)". Since there are 48 Gbps ports, we need 2x 48 Gbps or 96 Gbps internal bandwidth and 48x 1.488 Mpps or 71.424 Mpps both of which are documented performance numbers.

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