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Switch buffering between Gigabit inbound, 10/100 outbound.

Hi all. This is my first post here so hello to everyone. I'm quite new to networking, just about to graduate but am currently on a year out from my degree, working in a LAN team on a large, multi-site Cisco network.

My question is this... Say, for example, I have a server connected to a switch in my network via Gigabit, and it needs to file transfer to another machine, connected to another switch in my network but running at 10 or 100, how does the 'receiving' switch cope with it? If the receiving switch has a gigabit uplink to the network so is receiving at that speed, how does it reliably control and transfer the switching of the conversation through the access level port which may only be transmitting at 10%, or even 1% of the speed the switch is receiving the information at??

Any explanations gratefully received.

Thanks, Niall.

2 REPLIES
Green

Re: Switch buffering between Gigabit inbound, 10/100 outbound.

Remember that most of this kind of traffic is (usually) acknowledged at some level: the second packet / group of packets / segment(s) won't be sent until the receiving station has acknowledged the proper receipt of the first packet / group of packets / segment(s). With acknowledged / connection-oriented traffic, the sender is paced by the responses received by the receiver.

The speed / clocking differential is handled with buffers.

In the case of UDP traffic (unacknowledged) and no other hardware / layer one "flow control," the traffic that can't be handled / accepted will be dropped. The use of UDP assumes that a higher-level protocol or process will verify that the traffic was properly received ... tftp would be a good example.

In the case of UDP traffic that's not error-checked (usually because there's no point of re-transmission, like in "real time") then the traffic is just dropped. If it's critical traffic (like voice or video) some process, local to the receiver, is usually instituted to "cover" the effect of the dropped traffic (usually some kind of logical blending in the case of audio or video).

That's it in a nutshell. As usual, there's a ton of details and caveats not covered, if there's something more specific needed, just ask....

Scott

New Member

Re: Switch buffering between Gigabit inbound, 10/100 outbound.

Hi Scott, thanks for your reply.

Being quite new to this, I'm obviously not as experienced or knowledgeable as people like yourself so please feel free to point out any basic concepts you think I should be aware of.

TCP windowing is something I am familiar with though, and my question is not related to that directly. I'll try and explain better...

Say, for example, I have a server transmitting on Gigabit to a switch on full duplex, then this connects to a router on 100/Full, which then goes into a 16/Half token ring, then finally to a receiving machine. Now, I understand that the server connection will have a path established, along with an MTU, before transmission occurs. SO, assume that the MTU is agreed to be 1500 bytes, and the server has a file to send of 15,000 bytes. My question is what happens at the switch (or router) if the buffers are filled or how does each device cope with having more data coming into it than it is able to forward? As I understand, the buffer of the switch may well be full... due to the fact that the server will have transmitted it's full window size, may not have received acknowledgements from the receiving device, and all the while other devices have been inputting data into the switch (and therefore its buffers).

I looked into this and I'm wondering, is this whole process controlled by the buffering process in each device? As a thought, I'm wondering if, on our network, there may be a lot of instances whereby buffers are filling, data is being dropped and re-transmitted, and we're oblivious to it all!

I hope that's a bit clearer. All help/suggestions much appreciated.

Many thanks.

Niall.

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