Wire Speed Forwarding Rate & Maximum Forwarding Bandwidth

Nov 11th, 2008

Hi all, I am interested in finding out what these two terms mean.

Quoting information about the Cisco 2955 switch,

The 2955 ships with 12 10/100 ports and 2 fixed 10/100/1000BASE-T Copper uplink ports with a 4.8 Mpps wire speed forwarding rate and 6.4 Gbps maximum forwarding bandwidth.

If someone could explain to me what these terms mean or point me in the right direction it would be greatly appreciated.

Thank you.

Overall Rating: 5 (2 ratings)

Replies

Joseph W. Doherty Tue, 11/11/2008 - 16:28
• Super Bronze, 10000 points or more

Ethernet wire speed, for 64 bytes packets, is 148,809 pps for 100 Mbps and 1,488,095 pps for gig. 12 ports at 100 Mbps and 2 ports at gig would require a forwarding rate of (148,809 pps * 12 FastE ports = 1,785,708 pps) + (1,488,095 pps * 2 gig ports = 2,976,190) = 4,761,898 pps. Bandwidth would be (100 Mbps * 12 ports * 2 [duplex] = 2.4 Gbps) + (1 Gbps * 2 ports * 2 [duplex] = 4 Gbps) = 6.4 Gbps.

The above math shows the 2955 specs should support wire speed.

voiper_99 Tue, 11/11/2008 - 16:40

Thank you for that information, I really appreciate it. So how does the Forwarding Rate fit into things?

Joseph W. Doherty Tue, 11/11/2008 - 16:55
• Super Bronze, 10000 points or more

Forwarding rate is measured in pps (packets per second). If 100 Mbps Etherner requires about 149 Kpps for 64 byte sized packets, but the device can only provide 75 Kpps, then you won't be able to push more than about 50 Mbps.

Often many network devices overall performance, either pps and/or internal bandwidth, won't support all their ports running concurrently at full capacity. In the real world, we seldom see any device with multiple ports really needing such fall capacity performance.

User hosts often work well from 20 to 50 times oversubscription, server hosts often work well from 4 to 8 times oversubscription. However, devices that can deliver 100% wire rate avoid burst overload situations and/or make for nice bragging rights.

voiper_99 Tue, 11/11/2008 - 18:33

I see. So what your saying is that the Maximum Forwarding Bandwidth is the combined speed of all ports transmitting at their full capacity, and the Forwarding Rate is the actual bandwidth that the device has and must distribute to all of its ports. Is this correct?

By the way, just in case you don't have this formula, I've been told this is how to work out the Forwarding Rate - This example is for a 3750G-24T which has a forwarding rate of 35.7Mb:

35.7 M packets/second * (64+20) bytes/packet * 8 bits/byte / 24 ports

Joseph W. Doherty Wed, 11/12/2008 - 04:38
• Super Bronze, 10000 points or more

Not exactly but close. Device forwarding rate, in pps, is usually the total forwarding (packet switching) capacity of the device, shared by all ports. Device bandwidth, is the internal bandwidth that ties together all the ports.

The specification for a device's pps rate and (fabric) bandwidth is what the device is capable of. Since you mentioned the 3750G-24T its specifications (see attachment) are 35.7 Mpps and 32 Gbps.

Wire rate for the 3750G-24T 24 gig Ethernet ports requires 24 * 1.488095 Mpps = 35.71428 Mpps and 24 * 2 Gbps = 48 Gbps. So, this switch has the pps forwarding performance but not enough internal bandwidth to provide wire speed for all ports operating at full capacity.

If we look at the 4948, rated at 72 Mpps and 96 Gbps, for wire speed it requires 48 * 1.488095 Mpps = 71.42856 Mpps and 48 * 2 Gbps = 96 Gbps, which meets the need for both pps and bandwidth.

For calculation of Ethernet wire speed for 64 byte packets, use 1.488095 Mpps for gig, divide by 10 or 100 for 100 Mbps or 10 Mbps, respectively. For bandwidth, assuming duplex ports, double link bandwidth.

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