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PPPoE Questions

August 4th, 2017 in ROUTE 300-101 Go to comments

Question 1

Explanation

PPPoE provides a standard method of employing the authentication methods of the Point-to-Point Protocol (PPP) over an Ethernet network. When used by ISPs, PPPoE allows authenticated assignment of IP addresses. In this type of implementation, the PPPoE client and server are interconnected by Layer 2 bridging protocols running over a DSL or other broadband connection.

PPPoE is composed of two main phases:
+ Active Discovery Phase: In this phase, the PPPoE client locates a PPPoE server, called an access concentrator. During this phase, a Session ID is assigned and the PPPoE layer is established.
+ PPP Session Phase: In this phase, PPP options are negotiated and authentication is performed. Once the link setup is completed, PPPoE functions as a Layer 2 encapsulation method, allowing data to be transferred over the PPP link within PPPoE headers.

Reference: http://www.cisco.com/c/en/us/td/docs/security/asa/asa92/configuration/vpn/asa-vpn-cli/vpn-pppoe.html

Question 2

Explanation

PPP Session Phase: In this phase, PPP options are negotiated and authentication is performed. Once the link setup is completed, PPPoE functions as a Layer 2 encapsulation method, allowing data to be transferred over the PPP link within PPPoE headers.

Reference: http://www.cisco.com/c/en/us/td/docs/security/asa/asa92/configuration/vpn/asa-vpn-cli/vpn-pppoe.html

Question 3

Explanation

The “dialer persistent” command (under interface configuration mode) allows a dial-on-demand routing (DDR) dialer profile connection to be brought up without being triggered by interesting traffic. When configured, the dialer persistent command starts a timer when the dialer interface starts up and starts the connection when the timer expires. If interesting traffic arrives before the timer expires, the connection is still brought up and set as persistent. An example of configuring is shown below:

interface Dialer1
ip address 12.12.12.1 255.255.255.0
encapsulation ppp
dialer-pool 1
dialer persistent

Question 4

Explanation

The “vpdn enable” command is used to enable virtual private dialup networking (VPDN) on the router and inform the router to look for tunnel definitions in a local database and on a remote authorization server (home gateway). The following steps include: configure the VPDN group; configure the virtual-template; create the IP pools.

Question 5

Explanation

There are three authentication methods that can be used to authenticate a PPPoE connection:

+ CHAP – Challenge Handshake Authentication Protocol
+ MS-CHAP – Microsoft Challenge Handshake Authentication Protocol Version 1 & 2
+ PAP – Password Authentication Protocol

In which MS-CHAP & CHAP are two encrypted authentication protocol while PAP is unencrypted authentication protocol.

Note: PAP authentication involves a two-way handshake where the username and password are sent across the link in clear text; hence, PAP authentication does not provide any protection against playback and line sniffing.

With CHAP, the server (authenticator) sends a challenge to the remote access client. The client uses a hash algorithm (also known as a hash function) to compute a Message Digest-5 (MD5) hash result based on the challenge and a hash result computed from the user’s password. The client sends the MD5 hash result to the server. The server, which also has access to the hash result of the user’s password, performs the same calculation using the hash algorithm and compares the result to the one sent by the client. If the results match, the credentials of the remote access client are considered authentic. A hash algorithm provides one-way encryption, which means that calculating the hash result for a data block is easy, but determining the original data block from the hash result is mathematically infeasible.

Question 6

Explanation

A PPPoE session is initiated by the PPPoE client. If the session has a timeout or is disconnected, the PPPoE client will immediately attempt to reestablish the session. The following four steps describe the exchange of packets that occurs when a PPPoE client initiates a PPPoE session:
1. The client broadcasts a PPPoE Active Discovery Initiation (PADI) packet.
2. When the access concentrator receives a PADI that it can serve, it replies by sending a PPPoE Active Discovery Offer (PADO) packet to the client.
3. Because the PADI was broadcast, the host may receive more than one PADO packet. The host looks through the PADO packets it receives and chooses one. The choice can be based on the access concentrator name or on the services offered. The host then sends a single PPPoE Active Discovery Request (PADR) packet to the access concentrator that it has chosen.
4. The access concentrator responds to the PADR by sending a PPPoE Active Discovery Session-confirmation (PADS) packet. At this point a virtual access interface is created that will then negotiate PPP, and the PPPoE session will run on this virtual access.

If a client does not receive a PADO for a preceding PADI, the client sends out a PADI at predetermined intervals. That interval is doubled for every successive PADI that does not evoke a response, until the interval reaches a configured maximum.

If PPP negotiation fails or the PPP line protocol is brought down for any reason, the PPPoE session and the virtual access will be brought down. When the PPPoE session is brought down, the client waits for a predetermined number of seconds before trying again to establish a PPPoE.

Reference: http://www.cs.vsb.cz/grygarek/TPS/DSL/pppoe_client.pdf

Question 7

Question 8

Question 9

Explanation

The picture below shows all configuration needed for PPPoE:

PPPoE_Topology_with_config.jpg

As we can see from the PPPoE Client configuration, to get the IP address assigned from the PPPoE server the command “ip address negotiated” should be used. For more information about PPPoE configuration please read our PPPoE tutorial.

Question 10

Explanation

According to this link: http://www.cisco.com/c/en/us/td/docs/ios-xml/ios/bbdsl/configuration/xe-3s/bba-pppoe-client.html

The PPPoE client does not support the following:
+ More than ten clients per customer premises equipment (CPE)-> This means a CPE can support up to 10 clients so answer A is correct.
+ Coexistence of the PPPoE client and server on the same device -> answer C is not correct

In the above link there is a topology shows “DMVPN Access to Multiple Hosts from the Same PPPoE Client” -> Answer B is correct.

Question 11

Question 12

Comments
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  9. Jav
    February 20th, 2018

    Q7
    Refer to the exhibit.
    interface Ethernet 0
    pppoe-client dial-pool-number 5
    pppoe-client ppp-max-payload 1500
    interface Dialer 1
    ip address negotiated
    dialer pool 5
    mtu 1492

    isn’t this configuration missing the following commands:
    *encapsulation ppp (on Dialer 1)
    *pppoe enable (on Ethernet 0)

    anyone?

  10. Marcus
    March 4th, 2018

    Can anyone provide an explanation for Q7?

  11. CCNP_ROUTE
    March 8th, 2018

    Q10: Answer is B,C
    https://www.cisco.com/c/en/us/td/docs/ios-xml/ios/bbdsl/configuration/xe-3s/bba-xe-3s-book/bba-pppoe-client-xe.pdf
    Restrictions for PPP over Ethernet Client
    The PPPoE client does not support the following:
    • More than ten clients per customer premises equipment (CPE)
    • Quality of service (QoS) transmission with queueing on the dialer interface
    • Dial-on-demand
    • Easy VPN
    • Native IPv6
    • PPPoE client over ATM permanent virtual circuit (PVC)
    • Coexistence of the PPPoE client and server on the same device
    • Multilink PPP (MLP) on dialer interfaces
    • Nonstop forwarding (NSF) with stateful switchover (SSO)

  12. EIGRP Stub
    March 9th, 2018

    @CCNP_ROUTE
    as you wrote in your link
    The PPPoE client does NOT support the following:
    – More than ten clients per customer premises equipment (CPE) (so 8 client are permitted)
    ..
    – PPPoE client over ATM permanent virtual circuit (PVC) [Not supported]
    – Coexistence of the PPPoE client and server on the same device [ Not supported]

    correct answer: AB

  13. Anonymous
    March 22nd, 2018

    Q10

    Is there a wording issue ? The explanation says “A” is correct. Is it or isn’t it ? And if it is, why isn’t it the answer ?

    What configurations does PPPoE allow? (Choose two)

    A. Client can be installed on the same network devices as server
    B. 8 clients can be configured on 1 CPE
    C. Clients can connect to multiple hosts over DMVPN
    D. Client connecting over ATM PVC
    E. Client installed on native IPv6 network

    Answer: B C

    Explanation

    According to this link: http://www.cisco.com/c/en/us/td/docs/ios-xml/ios/bbdsl/configuration/xe-3s/bba-pppoe-client.html

    The PPPoE client does not support the following:
    + More than ten clients per customer premises equipment (CPE)-> This means a CPE can support up to 10 clients so answer A is correct.
    + Coexistence of the PPPoE client and server on the same device -> answer C is not correct

    In the above link there is a topology shows “DMVPN Access to Multiple Hosts from the Same PPPoE Client” -> Answer B is correct.

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    June 21st, 2018

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