One tiny bit of information: a ping command in IOS with a size of 9000 will calculate the ICMP payload so that the total IP packet is 9000 Bytes in length. As the link between those two routers runs a 1500 MTU, this bad boy has to be fragmented. Errr, what? Worry not, it shall all be explained below! What's the capture about?įirst thing's first, the screenshot above shows a capture of a ping between two routers in GNS3 with a size of 9000. You can see a bunch of fragments, which it says are Reassembled in #7, but packet number 7 has a size of 134. It always looked dodgy to me and I didn't make the effort to make some sense out of it. Up until recently, I have to shamefully admit, I had no idea how to read a Wireshark capture of fragmented packets. It also might cause engineers to lose their sanity while troubleshooting weird problems. It's what happens when a big packet spawns a lot of smaller baby packets because the MTU is not big enough, be it anywhere in transit (IPv4) or only at the source (IPv6).
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