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IANA's list of IEEE 802 Ethertypes includes an entry for Loopback (0x9000). However, I cannot find any specification detailing exactly how this is implemented. Do frames sent with Ethertype 0x9000 simply return to the NIC, regardless of the destination MAC address? Are they actually sent over the wire in Ethernet? In 802.11, are they sent over the air (when I inject a Wi-Fi frame with the 0x9000 Ethertype, it doesn't appear that adapters in monitor mode receive it)?

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That EtherType is (was) used for the Ethernet configuration testing protocol (CTP) which is somewhat similar to IP's ping (ICMP echo) facility but for the data link layer. I don't think it ever gained wide support though.

https://en.wikipedia.org/wiki/Ethernet_Configuration_Testing_Protocol

http://decnet.ipv7.net/docs/dundas/aa-k759b-tk.pdf

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    Cisco uses it to the day; if you enable "keepalive" on any ethernet interface, it sends loopback protocol frames.
    – Ricky
    Nov 14, 2022 at 21:36
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The Ethernet ring network (commonly known as the ring network) is [1] a ring topology composed of a group of IEEE 802.1-compatible Ethernet nodes, and each node communicates with the other two nodes through a ring port based on 802.3 Media Access Control (MAC). The Ethernet MAC can be carried by other service layer technologies (such as SDHVC, MPLS Ethernet pseudowire, etc.), and all nodes can communicate directly or indirectly.

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  • Your answer doesn't seem to address the actual question which is about a specific EtherType.
    – Zac67
    Nov 16, 2022 at 13:58

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