Layer 2
CISCO: https://www.cisco.com/c/ja_jp/support/docs/content-networking/keepalives/118390-technote-keepalive-00.html Apresia: https://www.apresia.jp/products/ent/np/usersguide/layer2/top.html?p=NP_loop_c_sp.html allied-telesis CentreCOM GS950:…