问答题 whatisABR？
单项选择题 Youaredeployingtwocoreswitches，oneineachbuilding，50kmawayfromeachother.Thecross-connectionbetweenthemwillbeaLayer22-gigabitEtherChannelwithan802.1Qtrunk.Youconfigureditcorrectlybutthelinkdoesnotcomeup.Theportisinthe"adminup"state，andthelineprotocolisinthe"down"state.ThefiberlinkisOK.Whatwouldbethemostlikelyreasonforthelinknottocomeup？（）
问答题 MQC的3个步骤？