Amit Singh Thu, 04/26/2007 - 03:08

A switching fabric is the combination of hardware and software that moves data coming in to a network node out by the correct port (door) to the next node in the network. A switching fabric includes the switching units in a node, the integrated circuits that they contain, and the programming that allows switching paths to be controlled. The switching fabric is independent of the bus technology and infrastructure used to move data between nodes and also separate from the router.

Switch bandwidth is generally referred to as the aggregate bandwidth available among all the ports.

-amit singh

pavelalex1 Thu, 04/26/2007 - 04:19


Can you explain that for Catalyst 3750G as example? In brochures I saw for them 32GB as switch fabric performance.

For example I have got one 3570G-12S and one 3750G-48TS connected in stack. How many full-duplex 1Gb links (theoretically fully loaded in both directions) can be connected to them to achieve overburdening?

mohammedmahmoud Mon, 05/14/2007 - 00:49


I was doing some reading and i found this comprehensive info that might be helpful for you:

The switch fabric is essentially the backplane for all ports and modules on the switch module. When a connection is made from a port on one module to a port on another module, it is made across the switch fabric. Physically, it is the combination of silicon, plastic, and metal that enables ports to connect and pass traffic between themselves.

The switch fabric can be blocking or non-blocking. Non-blocking fabric ensures that the total bandwidth of all ports that use the switch fabric do not exceed its capacity. What this means is that the density of the ports on the switch are such that their total capacity will never be greater than that of the switch fabric. Switches operating in non-blocking mode ensure that congestion will never occur on the switch, nor will ports ever want for bandwidth between each other.

A blocking switch has a port density capacity that exceeds the total capacity of the switch fabric. Control is possible by blocking traffic flow when the switch fabric capacity is exceeded or otherwise not available.

The switch fabric resides on the SE. When a port has to communicate with another port, it has the supervisor check its tables (Content Addressable Memory [CAM] for Layer 2 addresses and Ternary CAM [TCAM] for Layer 3 addresses) to determine what slot and port it needs. The supervisor then establishes the connection between the ports.The switch fabric can also reside on its own module (such as the Switch Fabric Module 2 (WS-X6500-SFM2) and the Switch Fabric Module (WS-C6500-SFM for the Catalyst 6500 Series), which enables the available capacity to be expanded without replacing the SE, or to expand beyond the capacity of the SE.

HTH, please do rate all helpful replies,

Mohammed Mahmoud.

pavelalex1 Mon, 05/14/2007 - 03:40


Some examples of switching fabric performance:

Cat6500 Sup720 - 720Gbps;

Cat4500 SupV-10Ge - 136Gbps;

Cat3750E StackWise Plus – 126Gbps;

Cat3750 StackWise – 32Gbps;

I try to understand what these number means.

Let’s look at theoretical switch with two Gigabit Ethernet ports. How big must be its switch fabric performance to process non-blocking there two GE full-duplex ports: 2Gbps or 4Gbps?

mohammedmahmoud Mon, 05/14/2007 - 04:15


Each Catalyst chassis has a series of interconnected slots into which modules such as the SE or port modules are inserted.The pathway that interconnects these slots is called the backplane, and can be a key determinant of performance (maximum performance), accordingly i'll guess that these numbers represents the backplane/switch fabric performance.

Check this link to see all the switches performance:

HTH, please do rate all helpful replies,

Mohammed Mahmoud.

pavelalex1 Mon, 05/14/2007 - 05:17

When some equipment communicates to other one through a switch it sends a network packet, switch processes this packet and then forwards to recipient. I correctly understand that switch fabric processes it twice and to ensure 10Gbps non-blocking traffic flow from one equipment to another one switch must have at least 20Gbps switch fabric?

And in this case Cat3750 StackWise with 32Gbps switch fabric can process only a 16Gbps traffic flow?

mohammedmahmoud Mon, 05/14/2007 - 06:00


No if i understand right, then it should serve a pure 32Gbps.

But note that The 3750G and 3560G 48 port units have oversubscribed switching backplane (52-to-32 Gbps oversubscription)

52G = 48 Ethernet 10/100/1000 ports + 4 SFP-based Gigabit Ethernet ports.

HTH, please do rate all helpful replies,

Mohammed Mahmoud.

pavelalex1 Tue, 05/15/2007 - 02:01

> No if i understand right, then it should serve a pure 32Gbps.

I am not sure. I suppose Cisco is also confused.

Look to “Cisco Catalyst 3750-E Series Switches” description page

“Key features include: Wire rate performance with 68 Gbps nonblocking switching fabric”

In the same time in “Cisco Catalyst 3750-E Series Switches Data Sheet”

Cisco Catalyst 3750-E Series Primary Features: StackWise Plus for ease of use and resiliency with 64 Gbps of throughput

And below: Table 2. Descriptions and Specifications

Performance - 128-Gbps switching fabric

I found other example of performance paramenters dependence in “CCDA Exam Certification Guide”:

Cisco Catalyst 2900 Series Key Features: Switch fabric of 3.2 Gbps, a forwarding rate of more than 3.0 Mpps, and a maximum

forwarding bandwidth of 1.6 Gbps, delivering wire-speed performance across all ports.

So switch fabric is twice as bandwidth?


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