What is forwarding bandwidth : 2960-S switch

Unanswered Question
May 19th, 2010

According to catalog of 2906-S, Fowarding Bandwidth is 88 Gbps.

I think switching bandwidth may means switch backplane but I have no idea of forwarding bandwidth.

Woud you please let me know what is forwarding bandwidth or referece document ?


I have this problem too.
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Michael Hutt Tue, 06/01/2010 - 01:07

I was looking at this myself.  I'm no expert, but it seems like the term "Switching bandwidth" refers to the amount of data that can be pumped through the switching plane, and that "Forwarding Bandwidth" actually refers to the amount of data that can be forwarded out interfaces.  These numbers are probably based on optimal conditions.

Maybe a full stack of 2960-S switches can achieve 176 Gbps total switching bandwidth (FDx), but each switch can only forward up to 32 Gbps out its interfaces.  The number given for forwarding bandwidth isn't specified full or half duplex, so I assume it's measured only as half duplex.  Each stack member would have a 20 Gbps link with the rest of the stack, which would limit cross-stack forwarding to ~20 Gbps.

Maybe someone else can tell me if that's on the mark or not, I'd like to know the real story behind these numbers too...

P.S. sorry bout the Necropost, but this post came up on google and I thought it was worth it to find an answer to this

p-miranda Mon, 02/21/2011 - 09:33

Hi friends ...

This topic is very ethereal and datasheets from Cisco are only marketing, words are very tricky ... don´t let them confuse you.

Till now, Cisco have not answered this question.

I have investigated much about this, and from my point of view, when you speak about switchfabric you are talking about full capacity (full duplex - In and Out) of ports. For instance, if you have a switch with 48 ports 10/100 Mbps and 2 uplinks 1000 Base T, you have 6.80 Gbps required and if you consider ports working Full duplex, you require 13.60 Gbps of capacity in you Switchfabric in order not to have oversubscription. That means that your switchfabric should be at least 13.60 in order not to have an ideal blocking situation.

The 6.80 Gbps I think is what is called "Forwarding Bandwidth" (traffic sent only OUT, or only IN, not both at once).

If we take a look at your case, the most powerful model in 2960-S Family is the Cisco Catalyst 2960S-48TD-L: 48 10/100/1000 ports, 2 10GE ports, and 2 stack ports that cosume 10Gbps each one. You have 48x1Gbps+2x10Gbps+2x10Gbps = 88Gbps, so you have 88Gbps as forwarding bandwidth, and your minimum switchfabric for this model in particular, should be 176 Gbps (2x88Gbps) in order not to have ideal blocking.

You can see in the datasheet that for this family, the maximun switching capacity (also called in the datasheet "switching bandwidth") is 176 Gbps. But it seems that this is for all family, no matter what model you are talking about, all models have 176 Gbps of switching bandwidth, but, is that an advantege to have such capacity, and cannot use it in a small family model (I mean 2960S-24PD-L)?.

I think Cisco should standardize terms. Forwarding Bandwidth and Switching Bandwidth depend on the model and the type of ports and uplinks you can have on your switch (the real quantity and type). Switch fabric should be related to the backplane capacity of switch in order to determine if the architecture is blocking or non-blocking (over-suscripted) and should be at minimum, twice the real forwarding bandwidth of the switch.




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