Answers: Serial Link 1

certskills
By certskills January 9, 2016 13:10

The first key to getting serial links working in a lab is to make sure you have a good understanding of the back-to-back cable, the DCE end of the cable, and two competing commands: the clock rate and bandwidth commands. Exercise those commands by taking a look at the lab exercise, creating your own answer configs, and checking your answers here.

Answers

 

Figure 1: Simple Serial Hub and Spoke Topology (CL151.jpg)

 

Example 5: HQ Config

 

Example 6: Branch1 Config

 

Example 7: Branch2 Config

 

Example 8: Branch3 Config

 

Commentary

The knowledge of how to configure a serial interface is getting more and more rare as many new network engineers work mostly with Ethernet-based WAN links. When using a serial interface to connect to a networking provider, the link requires a CSU/DSU, which clocks the router serial interface. In lab, to avoid the extra expense of an internal or external CSU/DSU, you can use a back-to-back serial cable, but it requires that you know to configure the clock rate on the interface connected to the DCE end of the back-to-back cable.

With this lab you were tasked with performing a number of tasks that control the speed and bandwidth utilized by a device. The first task was to configure the speed of the link between the HQ router and the Branch1 and Branch2 routers to 512kbps; this is done with the clock rate 512000 command. You were also tasked with configuring the bandwidth on these links to match the clock rate, this is done with the bandwidth 512 command. (Notice that the clock rate is set in bits/second, and the bandwidth in kilobit/second.) Note that the bandwidth command, which documents the speed but does not set or control the speed, can be set on both routers, but the clock rate is needed only on the router with the DCE cable end connected to it, in this case the HQ router.

Another requirement asked us to configure the speed of the link between the HQ router and the Branch3 router to 768kbps, but the bandwidth as 1024 Kbps. The lab uses these different speeds just to make the point that the bandwidth command is independent from the speed at which the interface actually transmits bits. The commands for these requirements: the clock rate 768000 command (on the HQ router), and the bandwidth 1024 command (on both routers).

The lab also asks us to configure IP addresses for each of the previously configured serial interfaces based on the information shown in the figure. For this you were also tasked to using the lowest IP addresses for each subnet on the HQ router. Each of the serial subnets in the figure support two usable IP addresses, so the answer configuration shows the lower IP address on the HQ router in each case. For instance, the HQ router uses on its Serial1/0 interface the ip address 172.16.0.1 255.255.255.252 command, with router Branch1 using the ip address 172.16.0.2 255.255.255.252 command, with the other IP address in the same subnet.

 

Serial Link 1
OSPF Network Config 2
certskills
By certskills January 9, 2016 13:10
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5 Comments

  1. Ruben October 30, 15:35

    Hello again Wendell!

    I need to ask you if there is a typo in this answer. In this question’s associated image, branch 2 is 172.16.0.8/30 and branch 3 is 172.16.0.4/30.
    But on the this answer page, you seem to give replies as if branch 2 is the 172.16.0.4/30 network and branch 3 is the 172.16.0.8/30 network.
    Thank you for your time!

    Reply to this comment
    • CCENTSkills November 2, 10:00

      Hi Ruben,
      Yep, you are (were) right. I flipped the answers vs the figure. I just fixed the post so future readers won’t be confused, so… anyone reading this, note that what Ruben noted isn’t true any more! Thanks Ruben for the notice.
      Wendell

      Reply to this comment
  2. Bav July 2, 09:53

    I tried to lab this in PT and got an error when setting clock rate for that value. Is that a PT quirk?

    Reply to this comment
    • CCENTSkills July 5, 09:29

      Hi Bav,
      Couldn’t say – maybe ask your Networking Academy instructor? I don’t use it much, as it’s a tool intended for Academy use only.
      Wendell

      Reply to this comment
    • Marcelo Venegas November 4, 16:14

      Hi Bav, it seems that PT only allows certain values of clock rate. And I tried with different models of routers there, in all of those I couldn’t 512000 nor 768000.

      If you issue HQ(config)# clock rate ?
      1200
      2400
      4800
      9600
      19200
      38400
      56000
      64000
      72000
      125000
      128000
      148000
      250000
      500000
      800000
      1000000
      1300000
      2000000
      4000000
      Choose clockrate from list above

      I don’t know if with real gear we are going to have the same restrictions.

      Reply to this comment
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