What does the linkloop command test?
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router activity
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Maximum Transmission Unit (MTU) size
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MAC address connectivity between network cards
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loopback address connectivity between network cards
The linkloop command tests MAC address connectivity between network cards by sending packets with the source MAC set to the destination MAC, testing if the card can receive its own transmissions. It doesn't test router activity, MTU size, or IP-level loopback (ping does that).
linkloop is a low-level HP-UX diagnostic that sends link-layer (802.3) frames directly to a target's MAC address and listens for the echoed response, verifying that the network interface cards themselves can communicate at the data-link layer. It doesn't test IP-layer routing, MTU negotiation, or the loopback interface — it isolates NIC-to-NIC connectivity below the IP layer, which is why it's used to rule out cabling/NIC issues before troubleshooting higher layers.