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PPP Authentication Configuration - Authentication allows a device to ensure that it is communicating with the correct device. Authentication is normally being initiated by the called party. - The 2 authentication protocols that can be used on PPP links are Password Authentication Protocol (PAP) and Challenge Handshake Authentication Protocol (CHAP). - PAP sends the username and password in clear text (very insecure) across a PPP link in the first message during the link establishment phase. - During the PAP authentication phase, the calling device will sends its username (or hostname) and password to the called device. The called device compares this information with a list of locally stored usernames and passwords and will reply with an accept or reject message accordingly. PAP is a two-way handshake mechanism. - CHAP does not send passwords directly; CHAP passwords are exchanged as MD5 hash values. The authentication phase takes place after the link establishment phase. CHAP begins with a challenge message, which states a random number. The challenged router (2nd router) replies with a calculated Message Digest 5 (MD5) one-way hash result based on the password and the random number. The 1st router calculates the same hash result with the same set of password and random number, and compares its result with the result replied from the challenged router. CHAP is a three-way handshake mechanism. - PPP authentication is defined as a one-way authentication method (the ppp authentication {pap | chap} command can be just issued on either end of a PPP link). However, two-way authentication can be achieved by issuing the ppp authentication command on both ends. - one-way authentication is the minimum possible authentication. The called party verifies the identity of the calling party. Also known as unidirectional authentication. two-way authentication is where the calling party can also verify the identity of the called party. Also known as bidirectional authentication. - If both PAP and CHAP are enabled with the ppp authentication {chap pap | pap chap} interface subcommand, the 1st method specified is used. If another peer suggests the 2nd method or refuses the 1st method, the 2nd method is used. - For one-way PAP authentication, the username {remote-hostname} password {passwd} statement is only required on the called device to verify the username and password sent by the calling device; whereas for two-way PAP authentication, it is required on both devices. - The username password statement is required on both devices for both unidirectional and bidirectional CHAP authentication. In unidirectional CHAP authentication (a local device authenticating a remote device), it is first used by the remote device (RT2) to response to the challenge generated by the local device (RT1), and then used by the local device (RT1) to verify the response from the remote device (RT2). - In PPP authentication configuration, the hostname of the remote router will be configured as the username on the local router, and the same shared secret password will be configured on both routers. Note: Passwords are case sensitive but usernames are not case-sensitive. 155 Copyright © 2008 Yap Chin Hoong yapchinhoong@hotmail.com
- The function of the username {remote-username} password {passwd} statement is different for PAP and CHAP. With PAP, it is only used to verify that an incoming username and password; whereas CHAP uses it to generate the response to a challenge (on remote router) and verify a response (on local router). - Bidirectional (two-way) CHAP Authentication configuration on RT1: RT1#conf t Enter configuration commands, one per line. End with CNTL/Z. RT1(config)#username RT2 password cisco123 RT1(config)#int s0/0 RT1(config-if)#ppp authentication chap 00:15:21: %LINEPROTO-5-UPDOWN: Line protocol on Interface Serial0/0, changed state to down RT1(config-if)#^Z RT1# - Bidirectional (two-way) CHAP Authentication configuration on RT2: RT2#conf t Enter configuration commands, one per line. End with CNTL/Z. RT2(config)#username RT1 password cisco123 00:16:01: %LINEPROTO-5-UPDOWN: Line protocol on Interface Serial0/0, changed state to up RT2(config)#int s0/0 RT2(config-if)#ppp authentication chap RT2(config-if)#^Z RT2# - Below shows the output of the debug ppp authentication privileged command on RT2 when the serial link was disconnected and connected back again. It shows the inner scenes behind the PPP link establishment and authentication phases. Note: With bidirectional authentication, each router sends and receives CHAP challenges. RT2#debug ppp authentication PPP authentication debugging is on RT2# 00:21:35: %LINK-3-UPDOWN: Interface Serial0/0, changed state to up 00:21:35: Se0/0 PPP: Using default call direction 00:21:35: Se0/0 PPP: Treating connection as a dedicated line 00:21:35: Se0/0 CHAP: O CHALLENGE id 40 len 24 from "RT2" 00:21:35: Se0/0 CHAP: I CHALLENGE id 9 len 24 from "RT1" 00:21:35: Se0/0 CHAP: O RESPONSE id 9 len 24 from "RT2" 00:21:35: Se0/0 CHAP: I RESPONSE id 40 len 24 from "RT1" 00:21:35: Se0/0 CHAP: O SUCCESS id 40 len 4 00:21:35: Se0/0 CHAP: I SUCCESS id 9 len 4 00:21:35: %LINEPROTO-5-UPDOWN: Line protocol on Interface Serial0/0, changed state to up RT2# - An O indicates an outgoing message while an I indicates an incoming message. 156 Copyright © 2008 Yap Chin Hoong yapchinhoong@hotmail.com
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CCNA Complete Guide 2nd Edition Yap
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CCNA Complete Guide 2nd Edition Cop
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Upper Layers Lower Layers Applicati
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Cisco Hierarchical Model - Defined
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Header Length (4) Source Port (16)
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- Socket is a communication channel
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Physical Layer - The physical layer
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- Another way to reduce emissions i
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Figure 3-7: Single-Mode and Multimo
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Wireless AP Switch Figure 3-8: 802.
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- Below lists the common IOS CLI er
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RT1#show running-config interface F
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- Cisco IOS treats a mistyped comma
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- Below describes the STP convergen
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- Refer back to Figure 5-1B, assume
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- A switch running RSTP only need 6
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Port Security Configuration - Port
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SW2# 00:25:00: STP: VLAN0001 new ro
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- Both protocols utilize a 12-bit V
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Layer 4 Switching (Content Switchin
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VLAN Trunking Protocol Configuratio
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- VLAN information will not be prop
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Cisco Discovery Protocol (CDP) c250
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Troubleshooting IP - Internet Contr
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- Firstly, PC1 purposely sends out
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- Below describes the operation of
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Maximum Hop Count Hop count metric
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- Invalid Timer specifies how long
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MISC IP Routing Commands - The pass
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- The show ip ospf database EXEC co
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- The configuration above defines 2
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cisco Systems, Inc. 170 West Tasman
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Catalyst Switch IOS Upgrade Procedu
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Catalyst Switch IOS Upgrade Procedu
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Cisco Router Password Recovery Proc
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Catalyst Switch Password Recovery P
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Constructing a Compound Frame Relay
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Decimal Hex Binary Decimal Hex Bina
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MISC Data Link Layer Notes - Ethern
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Ethernet Autonegotiation and Duplex
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The TCP Connection States timeout s
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- Q: How does a client application
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TCP Selective Acknowledgment (SACK)
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- Local Proxy ARP is used when ther
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Spanning Tree Protocol Port ID - Wh
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The Problem of Router-on-a-Stick Co
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- Sample Subnet Zero configuration:
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The permanent keyword in the Static
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RIPv1 and VLSM RT1 RT4 Figure A6-9:
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- Below shows the NAT operations on
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Bidirectional (2-Way) NAT PC1 172.1
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- Note: A router does not require a
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The Access Control List established
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Switch Port Access Control Lists -
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- Below shows a sample Dynamic Acce
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Router(config)#ip access-list exten
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Optional PPP Commands - The compres
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- Below shows the output of the PPP
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- Below shows the output of the PPP
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- Below configure the username and
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IEEE 802.11 Standards and Specifica
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IEEE 802.11 Types and Subtypes Type
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Version Header Length Time To Live
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- The authentication pre-share ISAK
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Troubleshooting ISDN E1/T1 Physical
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- Below addresses ISDN E1/T1 contro
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Troubleshooting ISDN Layer 2 - Unde
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Troubleshooting ISDN Layer 3 - Only
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ISDN Loopback Test Call - In a loop
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locking state, STP, 39 BNC, 27 bool
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show vlan, 57 show vtp status, 58 s
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framing, 22 MAC addresses, 22 stand
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PAT (Port Address Translation), 127
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star topologies, 25 startup-config
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