WhichthreestatementsaboutclassificationmarkingoftrafficatLayer2aretrue？（）

A.aFrameRelayheaderincludesa1-bitdiscardeligible（DE）bittoprovidetheclassofservice（CoS）
B.anATMheaderincludesa1-bitDEfieldtoprovidetheCoS
C.anMPLSEXPfieldisinsertedintheLayer3IPprecedencefieldtoidentifytheCoS
D.theCoSfieldonlyexistsinsideEthernetframeswhen802.1QorInter-SwitchLink（ISL）trunkingisused
E.intheIEEE802.1pstandard，threebitsareusedtoidentifytheuserprioritybitsfortheCoS
F.intheIEEE802.1qstandard，sixbitsareusedtoidentifytheuserprioritybitsfortheCoS

【参考答案】

A, D, E