Serial Link 1

By certskills January 7, 2016 13:10

Ever set up serial links for your #CCENT or #CCNA lab? Here’s a short lab to get you started with the basics. The lab asks for all the commands required to get the links working, plus a few related commands, to the point where you should be able to ping all the IP addresses on all the routers in a topology. As usual, it should take you 5-10 minutes.


No muss, no fuss – just configure the serial links between the HQ router and the branches so that each of the branches are able to ping each other. Some of the configuration already exists, so make sure to check out the initial configuration. This lab focuses mostly on the serial links themselves. The specific rules for this lab are:

  • The serial links in the figure are in a lab, created with back-to-back serial cables, rather than serial links created by a Telco. That is, there is no CSU/DSU, either internal or external, used in this lab, on any of the links.
  • The HQ router connects to the DCE end of the cable for all three links.
  • Configure the serial links between the HQ router and the first two branch routers with a speed of 512 kbps.
  • Configure the serial link between the HQ router and the third branch router with a speed of 768 kbps.
  • Configure the bandwidth setting on the links connecting to Branch1 and Branch2 matches the actual speed of the link
  • Configure the bandwidth setting on the link connecting to Branch3 to 1,024 kbps, a speed different than the actual link speed.
  • Configure the serial IP addresses/masks based on the figure, using the lower IP address value in each subnet for the HQ router.


Figure 1: Simple Serial Hub and Spoke Topology


Initial Configuration

Examples 1, 2, 3 and 4 show the initial configuration state of HQ, Branch1, Branch2 and Branch3.

Example 1: HQ Config


Example 2: Branch1 Config


Example 3: Branch2 Config


Example 4: Branch3 Config


Answer on Paper, or Maybe Test in Lab

Next, write your answer on paper. Or if you have some real gear, or other tools, configure the lab with those tools.

To test your solution if you happen to try it with real gear, to check this cable the devices back-to-back with HQ connected with the DCE side of the cable; if using an older version of IOS set the clock rate of the interface on the HQ router. From here configure the commands required to met the requirements listed.

To verify the correct IP configuration, ping the other routers’ IP addresses. To verify the correct interface speeds, use the show controllers command, keep in mind that the clock rate (speed) only needs to be configured on the DCE side of the connection which in this case is on HQ only. To verify the correct bandwidth, use the show interfaces command.

Answers: OSPF Interface Config 2
Answers: Serial Link 1
By certskills January 7, 2016 13:10
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  1. Hexploit October 12, 08:23

    Is this type of question in ICND1 100-105 exam scope?

    Reply to this comment
    • CCENTSkills October 12, 10:07

      My reading of the new exam topics puts this one in ICND2, not ICND1, with the new exam topics published May 2016.

      Reply to this comment
  2. PedroT July 27, 10:27

    Hi, I’m a bit confused: In the other comment you said this would be a ICND2 topic, but this is in chapter 17 section of the ICND1 blog category. Is this a good exercise for ICND1 then?

    Reply to this comment
    • CCENTSkills July 28, 16:30

      Hi Pedro,
      Never hurts to learn something new! 🙂 Seriously though, I don’t see the word “serial”, or “WAN”, anywhere in the ICND1 exam topics. But as an approach, it doesn’t hurt to learn some ICND2 topics along the way.

      Also, I don’t pay quite as much attention to such details here in the blog, which is a bit more informal, as compared with the books – FYI.

      Reply to this comment
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