At which layer of the OSI model dose PPP perform?（）

单项选择题Which protocol is an open standard protocol framework that is commonly used in VPNs,to provid secure end-to-end communication?（）

A. L2TP
B. IPsec
C. PPTP
D.RSA

单项选择题which command is used to enable CHAP authentication whit PAP as the fallback method on a serial interface?（）

A.(config-if)#authentication ppp chap fallback ppp
B.(config-if)#authentication ppp chap pap
C.(config-if)#ppp authentication chap pap
D.(config-if)#ppp authentication chap fallback ppp

单项选择题Refer to the exhibit.Statements A,B,C,and D of ACL 10 have been entered in the shown order and appled to interface E0 inbound,to prevent all hosts(except those whose addresses are the first and last IP of subnet 172.21.1.128 28) from accessing the network.But as is,the ACL dose not restrict anyone from the network.How can the ACL statements be re-arranged so that the system works as intended?（）

A.CDBA
B.DBAC
C.ACDB
D.BADC

单项选择题Which statement is correct regarding the operation of DHCP?（）

A.A DHCP client uses a ping to detect address conflicts.(ARP)
B.A DHCP server uses a gratuitous ARP to detect DHCP clients.
C.A DHCP client uses a gratuitous ARP to detect a DHCP server.
D.If an address conflict is detected, the address is removed from the pool and an administrator must resolve the conflict.
E.If an address conflict is detected, the address removed from the pool for an amount of time configurable by the administrator.
F.If an address conflict is detected, the address is removed from the pool and will not be reused until server is rebooted.

单项选择题Refer to the exhibit. An administrator pings the default gateway at 10.10.10.1 and sees the output as shown. At which OSI layer is the problem?（）

A.data link layer
B.application layer
C.access layer
D.session layer
E.network layer