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At wire speed?

Can someone please explain this to me. I struggle to understand it.

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level 1
CCNA R&S6 points · 2 months ago

"At wire speed" is terminology used to indicate how fast a device is capable of forwarding packets. "Wire speed" refers to the maximum negotiated speed of the medium used to connect two devices.

For example, if I connect two switches together with GigabitEthernet interfaces, then "wire speed" would be 1Gbps. Let's say that both switches are not very powerful, and I start pumping a bunch of traffic into one of them. That switch may only be able to effectively switch packets at a speed lower than wire speed (1Gbps) - let's say 200Mbps. Now, let's say I swap those switches out for a pair that are more powerful and pump the same amount of traffic into them. Those new switches will be capable of forwarding that traffic at the maximum speed that the physical medium allows - in our case, 1Gbps. These switches would be forwarding traffic "at wire speed."

Another way to think of the term is that the "bottleneck" in the flow of your traffic is the physical medium that connects your devices together - the speed of the wires. If you upgraded the link between the two from a GigabitEthernet link to a TenGigabitEthernet link, then wire speed would be 10Gbps, and the switches may or may not be able to forward traffic at wire speed.

I hope this answers your question - let me know if you're still confused!

level 2
Just 'cause it ain't in my flair doesn't mean I don't have certs4 points · 2 months ago

To build on this, know that on a given port, a device can only ever communicate at wire speed. It's not possible for a device to transmit data at a rate of 200Mbps on a 1Gbps port.

In the case of QoS, this is achieved (ELI5 version) by only transmitting for 20% of the time, which averages to 200Mbps, but at any given moment you're either transmitting at 1Gbps, or at 0.

In the case of the backplane of a switch, if you exceed its capacity thae frame are being dropped. This would still mean that the frames that make it are sent out a given port at whatever its operational speed is (100Mbps, 1Gbps, whatever), but the overall amount of data going through is less.

level 1

Full bandwidth of specified physical medium. Ie: gigabit or 1000mbs. Fast Ethernet 100mbps

Some switches (read cheap) do not switch at the full wire speed due to hardware. For example a 4 port gigabit switch, You would expect having all 4 ports occupied and transferring gigabit speeds on all ports, for combined total of 4gbps, however due to hardware it may only switch at 2gbps, meaning if all ports are used then the max speed of 500mbps per port.

level 1
Original Poster2 points · 2 months ago

Thank you for your explanations. I noticed switch is being mentioned multiple times. Am I safe to then think wire speed is L2 terminology?

level 2
A+, N+, L+, CCNA (RS)2 points · 2 months ago

Layer 1, it has to deal with the physical specifications and electrical signals.

level 2
CCNA R&S1 point · 2 months ago

Wire speed applies to all devices with physical connections. Whether it's a router, switch, firewall, appliance, or host, the speed of the wire is a potential bottleneck for transmission.

level 3
Original Poster1 point · 2 months ago

🤔 How come the wire is the potential bottle-neck? I thought it's the hardware because that's what I understood from previous comments, not because the interface is configured as 1/10 Gbps it means that it's capable of meeting those speed unless it's a beefy device! Now I am more confused.

level 4
CCNA R&S1 point · 2 months ago

As always, the answer is the dreaded "it depends"!

Let's say we purchase a Cisco Catalyst 3750-24TS. According to the datasheet, this device has a forwarding rate of 6.5 mpps (million packets per second) and is equipped with 24 10/100 FastEthernet interfaces and 2 SFP (GigabitEthernet) interfaces.

Let's say that the MTU (Maximum Transmission Unit - the largest defined size that a packet can be) of our network is 1,500 bytes. This is 12,000 bits, since each byte is made up of 8 bits. Let's say that a large file is being transferred between two computers off of Fa1/0/1 and Fa1/0/2, respectively. Each packet is 1500 bytes in size, and the maximum transmission rate of the interface is 100Mbps (100,000,000 bits per second). Given the size of our packets, this interface is capable of transmitting 8,333 1500-byte packets per second. This number falls well below our 6.5 million packets per second forwarding rate, so the hardware responsible for making forwarding decisions inside of the switch can process those packets without an issue - however, the physical hardware responsible for putting those packets on the wire has reached its maximum limitation, so wire speed is our bottleneck.

Now, let's consider an extremely noisy network where all interfaces (including our GigabitEthernet uplink interfaces) are forwarding small, 64 byte (512 bit) packets. Each FastEthernet interface can forward 195,312 packet per second, and each of our two GigabitEthernet interfaces can forward 1,953,125 packets per second. In total, this would result in 8,593,738 forwarding decisions per second, which is higher than our switch's forwarding rate of 6,500,000 packets per second. In this case, our bottleneck is the forwarding engine of the switch, and the hardware of our interfaces will occasionally be waiting on the forwarding engine to send it packets that it can put on the wire.

One thing that a lot of people misunderstand is that a network device is like any other computer. It is not a single box where a frame goes in, magic happens, and a frame comes out somewhere else. The box has multiple individual hardware components that work together to properly forward that frame towards its decision. Each of these individual components has its own maximum capabilities, and not all of the maximum capabilities are exactly the same. From the outside, it looks like "the switch" is not capable of forwarding all of the traffic - in reality, there is a specific component within the switch that is acting as the bottleneck for the traffic. At the end of the day, your limiting reagent is "the hardware", but understanding which piece of hardware is the exact issue helps separate the good network engineers from the great network engineers.

Hope this helps! Keep the questions coming if you have them!

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