Answers: Serial Link 1

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.



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



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 command, with router Branch1 using the ip address command, with the other IP address in the same subnet.


Serial Link 1
OSPF Network Config 2
By certskills January 9, 2016 13:10
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  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 and branch 3 is
    But on the this answer page, you seem to give replies as if branch 2 is the network and branch 3 is the 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.

      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.

      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 ?
      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
  3. Edgardo Riccobon January 5, 05:29

    Hello Wendell!

    I thought one important point suggested in the question was to make sure to set the “router rip -> no auto summary” option, since we are looking into a configuration with multiple contiguous classfull networks and we had to make sure every router can successfully ping through the networks.

    Let’s say HQ needs to route a packet to : how can he know which one of the 3 interfaces to use, if all of them route to

    Am I predicting the output of a “show ip route” correctly, is there anything I’m missing?

    Any of your insights would be, as usual, extremely valuable. Going for that 200-125 in february!

    Reply to this comment
    • certskills Author January 7, 11:18

      Hi Edgardo,
      I agree that any use of RIP requires you to think about auto-summary and no auto-summary. That said, the process begins with the question of whether the design uses one or more classful networks, and in this case, only one is used: Class B network All the subnets in the drawing show subnets of that one network. As a result, the auto-summary setting, either way, has no impact on the routes advertised by RIP.

      If instead some had been from, some from, others from – all different class B networks – then yes, a situation as you describe might occur, and “no auto-summary” would be useful.
      Hope this helps!

      Reply to this comment
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