experiment procedure:
Step 1: Pre-configure
ATM1(config)#username libo password cisco on ATM1
//and configure the username and password
ATM1(config)#int f3/0
ATM1(config-if)#pppoe enable
//Enable pppoe
ATM1 on the interface (config-if)#no sh
Step 2: Configure vpdn
ATM1(config)#vpdn enable
//Enable vpdn function
ATM1(config)#vpdn-group libo
//Define the group name of vpdn
ATM1(config-vpdn)#request-dialin
//Request-dial
ATM1( config-vpdn-req-in)#protocol pppoe
//Enable the pppoe protocol, (pppoe is an implementation of ppp, which is essentially a bridge architecture. The typical bridge implementation has a lot of security vulnerabilities. Adding the ppp architecture to the Ethernet bridge function, using pap or chap authentication can solve these problems to a certain extent,
ATM1(config-vpdn-req-in)#exi
Step 3: Configure bba-group
ATM1(config)#bba-group pppoe global
ATM1(config-bba-group)#virtual-template 1
//Configure virtual template
ATM1(config-bba-group)#exit
Step 4: Configure the virtual-template interface:
ATM1(config)#int virtual-template 1
//Configure the virtual template
ATM1(config-if)#ip add 10.0.0.2 255.255.255.0
//Configure the ip address of the virtual template
ATM1( config-if)#en ppp
//Package type
ATM1(config-if)#ppp authentication pap
//Authentication method.
ATM1(config-if)#peer default ip address pool libo
ATM1(config-if)#exi
ATM1(config)#ip local pool libo 10.0.0.10 10.0.0.20
//Define the dial-up address pool, the address pool and virtual template are in Same network segment
Step 5: Dial and check the status 
Step 6: During the dialing process, we use wireshark to capture packets. 
Step 7: View PPPoe debugging information on
ATM1 ATM1#sh ip local pool
Pool Begin End Free In use Blocked
libo 10.0.0.10 10.0.0.20 10 1 0
ATM1#debug pppoe events
PPPoE protocol events debugging is on
ATM1#
Jan 4 20:40:26.451: PPPoE 2: I PADT R:0200.4c4f.4f50 L:caff.0a20.0054 Fa3/0
Jan 4 20:40:26.455: [2]PPPoE 2: Destroying R:0200.4c4f.4f50 L:caff.0a20.0054 Fa3/0
Jan 4 20:40:26.459: PPPoE: Returning Vaccess Virtual-Access1.1
Jan 4 20:40:26.467: [2]PPPoE 2: AAA account stopped
ATM1#
Jan 4 20:40:32.111: PPPoE 0: I PADI R:0200.4c4f.4f50 L:ffff.ffff.ffff Fa3/0
Jan 4 20:40:32.115: Service tag: NULL Tag
Jan 4 20:40:32.119: PPPoE 0: O PADO, R:caff.0a20.0054 L:0200.4c4f.4f50 Fa3/0
Jan 4 20:40:32.119: Service tag: NULL Tag
Jan 4 20:40:32.147: PPPoE 0: I PADR R:0200.4c4f.4f50 L:caff.0a20.0054 Fa3/0
Jan 4 20:40:32.151: Service tag: NULL Tag
Jan 4 20:40:32.151: PPPoE : encap string prepared
Jan 4 20:40:32.155: [3]PPPoE 3: Access IE handle allocated
Jan 4 20:40:32.155: [3]PPPoE 3: pppoe SSS switch updated
Jan 4 20:40:32.159: [3]PPPoE 3: AAA unique ID allocated
Jan 4 20:40:32.159: [3]PPPoE 3: No AAA accounting method list
Jan 4 20:40:32.163: [3]PPPoE 3: Service request sent to SSS
Jan 4 20:40:32.167: [3]PPPoE 3: Created, Service: None R:caff.0a20.0054 L:0200.4c4f.4f50 Fa3/0
Jan 4 20:40:32.187: [3]PPPoE 3: State NAS_PORT_POLICY_INQUIRY Event MORE_KEYS
Jan 4 20:40:32.187: [3]PPPoE 3: O PADS R:0200.4c4f.4f50 L:caff.0a20.005
ATM1#4 Fa3/0
Jan 4 20:40:32.199: [3]PPPoE 3: State PPP_START Event DYN_BIND
Jan 4 20:40:32.203: [3]PPPoE 3: data path set to PPP
Jan 4 20:40:32.407: [3]PPPoE 3: State LCP_NEGOTIATION Event PPP_LOCAL
Jan 4 20:40:32.411: PPPoE 3/SB: Sent vtemplate request on base Vi1
Jan 4 20:40:32.487: [3]PPPoE 3: State VACCESS_REQUESTED Event VA_RESP
Jan 4 20:40:32.487: [3]PPPoE 3: Vi1.1 interface obtained
Jan 4 20:40:32.503: [3]PPPoE 3: State PTA_BINDING Event STAT_BIND
Jan 4 20:40:32.503: [3]PPPoE 3: data path set to Virtual Acess
Jan 4 20:40:32.507: [3]PPPoE 3: Connected PTA
ATM1#sh ip route
…………
10.0.0.0/8 is variably subnetted, 2 subnets, 2 masks
C 10.0.0.10/32 is directly connected, Virtual-Access1.1
//You can see the route of PPPoE Client on ATM1
C 10.0.0.0/24 is directly connected, Virtual-Access1.1
ATM1#debug pppoe packets
PPPoE control packets debugging is on
//Debug ppp0e packets
Jan 4 20:44:56.951: PPPoE 3: I PADT R:0200.4c4f.4f50 L:caff.0a20.0054 Fa3/0
CA FF 0A 20 00 54 02 00 4C 4F 4F 50 88 63 11 A7
00 03 00 00
ATM1#
Jan 4 20:45:03.379: PPPoE 0: I PADI R:0200.4c4f.4f50 L:ffff.ffff.ffff Fa3/0
FF FF FF FF FF FF 02 00 4C 4F 4F 50 88 63 11 09
00 00 00 10 01 01 00 00 01 03 00 08 04 00 00 00 ...
Jan 4 20:45:03.387: PPPoE 0: O PADO, R: caff. 0a20.0054 L:0200.4c4f.4f50 Fa3/0
Jan 4 20:45:03.391: Service tag: NULL Tag
02 00 4C 4F 4F 50 CA FF 0A 20 00 54 88 63 11 07
00 00 00 2C 01 01 00 00 01 03 00 08 04 00 00 00 ...
Jan 4 20:45:03.439: PPPoE 0: I PADR R:0200.4c4f.4f50 L:caff.0a20.0054 Fa3/0
CA FF 0A 20 00 54 02 00 4C 4F 4F 50 88 63 11 19
00 00 00 24 01 01 00 00 01 03 00 08 04 00 00 00 ...
Jan 4 20:45:03.459: [4]PPPoE 4: O PADS R:0200.4c4f.4f50 L:caff.0a20.0054 Fa3/0
ATM1#
02 00 4C 4F 4F 50 CA FF 0A 20 00 54 88 63 11 65
00 04 00 24 01 01 00 00 01 03 00 08 04 00 00 00 ...
ATM1#debug ppp authentication
PPP authentication debugging is on
//查看ppp认证信息。
Jan 4 20:47:31.943: ppp6 PPP: Using *** set call direction
Jan 4 20:47:31.943: ppp6 PPP: Treating connection as a callin
Jan 4 20:47:31.947: ppp6 PPP: Session handle[E4000008] Session id[6]
ATM1#
Jan 4 20:47:33.931: ppp6 PPP: Authorization required
Jan 4 20:47:34.007: ppp6 PAP: I AUTH-REQ id 12 len 15 from "libo"
Jan 4 20:47:34.007: ppp6 PAP: Authenticating peer libo
Jan 4 20:47:34.027: ppp6 PPP: Sent PAP LOGIN Request
Jan 4 20:47:34.043: ppp6 PPP: Received LOGIN Response PASS
Jan 4 20:47:34.135: Vi1.1 PPP: Sent LCP AUTHOR Request
Jan 4 20:47:34.139: Vi1.1 PPP: Sent IPCP AUTHOR Request
Jan 4 20:47:34.151: Vi1.1 LCP: Received AAA AUTHOR Response PASS
Jan 4 20:47:34.155: Vi1.1 IPCP: Received AAA AUTHOR Response PASS
Jan 4 20:47:34.159: Vi1.1 PAP: O AUTH-ACK id 12 len 5
Problem analysis:
PADT situation:
if the user name or password entered on the computer is wrong, the padt interface listed above will appear on the router
May 10 13:58:11.263: PPPoE 3: I PADT R:0200.4c4f.4f50 L:caff.0bc8.0054 Fa3/0
will appear in the following situations on the pc (691 error)
