Testing MTU With Ping and the DF Bit
Find the path MTU with nothing but ping, from Linux and IOS XE, against a real 1400-byte link. The 28-byte math and the PMTUD black holes that hang TCP.
Cisco
Platform-level material spanning multiple Cisco product lines. Licensing, hardware behaviour, boot and upgrade paths, and the details that differ from the documentation.
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Cisco Extended Ping and Response Characters Explained
Source interfaces, the DF bit, repeat counts, and the response characters (! . U M) that tell you exactly what failed. Extended ping on Cisco IOS XE, captured live.