Troy’s checklist for preparing for the CCNA: Objective 8

July 31, 2009 at 2:09 pm | Posted in Cisco | 2 Comments
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Thanks for returning for the final installment of my review checklist for the CCNA exam. In this session we will cover the topics included in Objective 8: Implement and Verify WAN links.  Let’s get started!

You should be able to describe the differences between the categories of data transfer between physical locations. These include:

  • Cell switching – Cell switching is a WAN switching technology that is used by ATM. ATM is an International Telecommunication Union-Telecommunications (ITU-T) standard for the transmission of data, voice, or video traffic. It uses a fixed size frame of 53 bytes, known as cells. Out of these 53 bytes, the initial five bytes are header information and the rest of the 48 bytes are the payload.
  • Packet switching – Packet switching is popularly used for data transfer, as data is not delay-sensitive like voice traffic is, and it does not require real-time transfer from a sender to a receiver. With packet switching, the data is broken into labeled packets and transmitted using packet-switching networks.
  • Circuit switching – Circuit switching dynamically establishes a virtual connection between a source and destination. The virtual connection cannot be used by other callers unless the circuit is released. Circuit switching is the most common method used by the Public Switched Telephone Network (PSTN) to make phone calls. A dedicated circuit is temporarily established for the duration of call between caller and receiver. Once the caller or receiver hangs up the phone, the circuit is released and is available for other users.

You should how to configure a serial link for a WAN connection. Make sure that you know how to use these commands:

encapsulation ppp – an interface configuration mode command  used to configure encapsulation for a PPP (Point to Point Protocol) serial link on a Cisco router. PPP encapsulation provides for router-to-router and host-to-network connections over both synchronous and asynchronous circuits. Serial links are configured to use Cisco High Level Data Link Control (HDLC) encapsulation, by default, on Cisco routers. The Cisco version of HDLC is incompatible with the industry standard version used on other router brands because it contains a type field that identifies the underlying network protocol being encapsulated by HDLC. This is a beneficial feature of Cisco HDLC but makes it incompatible with other router brands.

authentication pap, authentication ms-chap, and authentication chap commands – used to apply Password Authentication Protocol (PAP), Microsoft Challenge Authentication Protocol (MS-CHAP), or Challenge Authentication Protocol (CHAP) authentication to the connection, respectively.

clock rate 640000 – will configure the clock rate to 64,000 bits per second on an interface. The clock rate command is used to configure the clock rate for hardware connections on serial interfaces. These interfaces can be network interface modules (NIMs) and interface processors. The syntax of this command is clock rate bps.

service-module 56k clock rate speed – will configure the network line speed for a 4-wire, 56/64-kbps CSU/DSU module.

description – used to provide a description to an interface. It is not a mandatory configuration. However, if you have configured the description for an interface, anyone who is working on the router can easily identify the purpose of the interface.

Following is an example of the description command:

RouterA(config)# interface s0
RouterA(config-if)# description AT&T T1 to Internet

If you need help with putting these together, look here:

You should know how to configure a Frame Relay link. Know these commands and how to use them:

frame-relay map – will create a static map of the remote IP address to the local DLCI 401 without permitting broadcasts and multicasts over the VC.

show frame-relay pvc – displays the status of each configured connection as well as traffic statistics.

show frame-relay lmi – displays LMI traffic statistics, such as LMI type, on the interface.

show frame-relay map – displays the network layer address and the associated DLCI for each remote destination.

show interfaces serial – displays information about encapsulation, LMI, and DLCI used for local management.

show controllers – provides hardware-related information useful to troubleshoot and diagnose issues with Cisco router interfaces, such as to learn the end of the cable to which the router is connected (DCE or DTE).

Make sure you know what a DLCI is, and how they relate to your connection. If you don’t understand this concept, you are NOT ready for the exam. Also know the two methods for mapping DLCIs to IP addresses:

  • Static map
  • Inverse ARP

Know the possible physical topologies of Frame Relay.

  • Full mesh – All routers have a connection to every other router. This is the most expensive topology, and also the most fault tolerant.
  • Star topology – All routers are connected to a router in the center in a hub and spoke configuration. This is the least expensive topology, as physical interfaces can be divided into subinterfaces so that a single connection can be leveraged and used to connect to multiple locations using the Frame Relay switching network.
  • Partial mesh – This is a combination of the previous two configurations. Not all sites have connections to all other sites, so complete fault tolerance is not present. It also makes use of subinterfaces to leverage physical connections.

Know that when you configure a Frame Relay link, you have some choices about certain parts of the configuration. These choices involve:

  • Encapsulation frame type – can either be Cisco or IETF. Cisco is the default, but IETF should be used when non-Cisco routers are involved.
  • LMI type – can be Cisco, ANSI or Q933a. Cisco is the default.

LMI is used for management purposes, and allows two directly connected devices to share information about the status of virtual circuits (VCs) as well as their configuration. Three different standards are defined for LMI and its interaction with a Frame Relay DTE and DCE:

ANSI: ANSI’s Annex D standard, T1.617.

Cisco: The Gang of Four, for the four companies that developed it: Cisco, DEC, StrataCom, and NorTel (Northern Telecom). This standard is commonly referred to as Cisco’s LMI.

Q933a: ITU-T’s Q.933 Annex A standard.

Know the difference between the DTE and the DCE end of a cable are and which end gets the click rate (DCE)

Know what BECN and FECN mean when viewing the output of the show frame relay command.

Know what this means when you see it in the output of the show frame relay command:

PVC STATUS = INACTIVE –  indicates a problem with the remote site.

Know why you configure subinterfaces on a physical interface to host multiple Frame relay circuits. (to avoid the effects of split horizon)

Moving beyond Frame Relay, know the other encapsulation types that can be used on serial connections:

  • PPP – use with non Cisco routers
  • HDLC – only works on Cisco routers, and is the default

Know the two sub protocols of IPsec and what IPsec is.

  • ESP
  • AH

If you don’t know IPsec look here:

Know the methods of authentication that can be used with a PPP connection:

  • CHAP – hashes the password (more secure)
  • PAP – password transmitted in clear text (not secure)

Well, that’s it for this week, and also the end of this series. If you like it or have any specific questions, please let us know!

Good luck on the Exam!


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  1. Now that I’m wrapping up my Microsoft tracks I’m thinking of going for my CCNA, I will definately be using your checklist.

    What are your thoughts on the single or two test version of this cert? Does the Kaplan CCNA simulator cover both? (Say I buy the two test study guides and the simulator but then decide to take the single test would I be covered?)

    Thanks for the series of posts on the CCNA!

    • Thanks, Les. You can use the CCNA simulator for either the one-test or two-test approach. The exercises are designed to prepare you for either method.

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