# Switch Backplane

Answered Question
Nov 13th, 2008

What does it mean when they say the switch is 100Mbps but it has higher or lower backplane. Or the switch is 1000Mbps but it has higher/lower backplane.

0 votes
Correct Answer by Joseph W. Doherty about 7 years 11 months ago

Mpps is how many packets the switch can forward per second. It's usually given for the minimum packet size of 64 bytes packets. For Ethernet, 100 Mbps requires 148,809 pps. Divide by 10 for 10 Mbps, multiply by 10 for gig.

The switch fabric capacity, is the internal bandwidth that connects all the ports.

An an example, a wire rate switch for eight 100 Mbps ports, should have 148,809 pps * 8 = 1,190,472 (or about 1.2 Mpps) and 8 * 100 Mbps * 2 (duplex) = 1.6 Gbps internal bandwidth.

You'll find many switches are unable to run all ports concurrently at full speed (they run short of pps and/or bandwidth), but again, seldom is this necessary.

Overall Rating: 5 (4 ratings)

## Replies

Joseph W. Doherty Thu, 11/13/2008 - 05:12

Backplane bandwidth is the available bandwidth between the device's ports.

To better understand this, consider two external 8 port switches that you interconnect with a single uplink. If the all ports were 100 Mbps, the uplink would become a bottleneck for traffic between the two switches since it too is only 100 Mbps. If the uplink were gig, it would offer more bandwidth than the combined capacity of all eight 100 Mbps ports. If the uplink were gig and there were ten 100 Mbps ports, it would exactly provide the necessary bandwidth between switches.

Likewise, internal backplanes or fabric bandwidths provide the bandwidth between ports within the same device, and can provide more or less or even the exact amount of bandwidth between the device's ports.

PS:

Exactly how a backplane or fabric provides bandwidth to the device's ports can be important too for how well the device operates.

new_networker Thu, 11/13/2008 - 06:40

Thanks.

So before ordering a Cisco switch, how can I determine the internal backplane capacity. What term should I look for in the Cisco product sheet.

So in your example, it will be ideal to have 1 Gig internal backplane (between 10 ports) as well. Is it Right ?

Joseph W. Doherty Thu, 11/13/2008 - 12:35

The attachment will note both forwarding capacity (pps) and internal bandwitdh for many Cisco switches.

In my example of 10 ports, each 100 Mbps, you would normally look for 2 gig bandwidth (2x port bandwidths assuming ports are duplex) if you wanted to insure all ports could communicate at full speed at the same time.

BTW: Not all switches provide the capacity to handle all ports full bandwidth, but then again, real traffic often isn't that demanding.

Attachment:
new_networker Thu, 11/13/2008 - 13:08

Really good document.

Could you please explain the terminology 'Switch Performance (Mpps)and Switch Fabric (Gbps)',

Correct Answer
Joseph W. Doherty Thu, 11/13/2008 - 13:33

Mpps is how many packets the switch can forward per second. It's usually given for the minimum packet size of 64 bytes packets. For Ethernet, 100 Mbps requires 148,809 pps. Divide by 10 for 10 Mbps, multiply by 10 for gig.

The switch fabric capacity, is the internal bandwidth that connects all the ports.

An an example, a wire rate switch for eight 100 Mbps ports, should have 148,809 pps * 8 = 1,190,472 (or about 1.2 Mpps) and 8 * 100 Mbps * 2 (duplex) = 1.6 Gbps internal bandwidth.

You'll find many switches are unable to run all ports concurrently at full speed (they run short of pps and/or bandwidth), but again, seldom is this necessary.

new_networker Fri, 11/14/2008 - 01:11

Beginners' question.

When we learn that the switch is fast ethernet, does it mean that 100Mbps is the bandwidth of one-way traffic only and not two-way. In other words does the aggregate bandwidth per port become 200Mbps for a fastethernet switch.

Jon Marshall Fri, 11/14/2008 - 01:13

If the connection is 100Mbps full duplex then yes you have 100Mbps in each direction.

Jon