User talk:Zac67/Archives/2023/August


Fragment Free Switching

Hi,Zac67, sorry for writing here, I read an answer of yours on "Stack Exchange", but for some reason I don't know I cannot create a count and login "Stack Exchange", but I'm really want to get some help from you about the fragment free switching mechanism(please excuse me for my silly question, I'm a new guy for ethernet, and learning and testing about the switching mechanism).On "Stack Exchange"([1]https://networkengineering.stackexchange.com/questions/60049/what-exactly-is-fragment-free-switching), you mentioned". There is no integrity check for the first 64 bytes in Ethernet. On L2, there's only FCS for the whole frame, nothing else. Depending on the actual L1 PHY, there may be additional PCS level checks or FEC, but these are on line symbols or code groups. For IPv4 (L3) there's an IP header checksum that falls into the first 64 bytes, but IPv6 and other L3 protocols don't have that. Some transport-layer protocols (L4) also use header checksums but those differ as well. – Zac67♦ Jun 26, 2019 at 17:13 ", So my question is if a fragment free switch reviewed a frame more than 64 bytes(e.g. 128bytes), but with a CRC error, then what will be the behavior of the switch? To my understanding, the switch will forward the frame, until it monitored that there is a "CRC Error", that means the switch will "cut off" a part of the frame, and forward a part of the frame, am I right? And for the forwarded frame, is there a "CRC" 4 bytes in it? As a results of my test, there is a CRC on in the forwarded frame, and it is a Error CRC, that makes me confuse. I hope I didn't disturb you and hope to receive your reply. Best Regards 111.118.203.20 (talk) 01:55, 29 August 2023 (UTC)

I don't think this is the right place, but in short: in cut-through mode the switch has already started forwarding and the FCS hasn't been received yet. When the FCS is received it's too late to stop forwarding, so the frame is forwarded in full, including the bad FCS. In store-and-forward mode (with a previous FCS error rate above a certain threshold) the frame is dropped for failing FCS check. --Zac67 (talk) 07:01, 29 August 2023 (UTC)
Thank you so much for your reply. Yes, I think the "cut-though" mode is just like what you said, but what about "fragment free"? To my understanding, it is between "store forward" and "cut-though", "fragment free" will check the received frame to make sure it is not a runt frame, and then forward it, and what if the switch received a normal size frame(more than 64B) but with a bad CRC, will the switch cut off a part of the frame? Or the switch will be just like "cut-though" mode? 111.118.203.24 (talk) 01:29, 30 August 2023 (UTC)
Short follow up: I've tried to find out whether SE blocks account creation per IP address but couldn't find that. Perhaps it's the Great Firewall blocking you. --Zac67 (talk) 16:55, 29 August 2023 (UTC)
Yes, maybe that's the reason.:/
Thanks again and holp I didn't bother you too much. 111.118.203.24 (talk) 01:31, 30 August 2023 (UTC)