Multilink PPP 2

By Chris October 12, 2016 09:05

Multilink PPP (MLPPP) makes multiple parallel links act like one link from a layer 3 perspective. In this next lab, the lab begins with a working configuration that does not use MLPPP, with two parallel serial links between two routers. Your job: reconfigure the two routers to use MLPPP, so that instead of balancing traffic using layer 3 balancing, MLPPP then load balances the traffic. This lab requires you to both configure MLPPP and consider any changes to the existing configuration that might be required as a result of the changes.


Configure MLPPP on both router R1 and R2 in the figure, so that the routers have a single subnet ( used between the two routers. Note that the figure shows two serial links between the routers, with two different subnets in use. As you will see when you look at the initial configurations, these routers begin in a state where they are using both links independently, so that each router would learn layer 3 routes that use each link as the outgoing interface. Your job: convert the configuration to treat the two links as a single layer 3 link using MLPPP, so that the routing protocol (OSPF) learns one route to remote subnet, with an outgoing interface that refers to the MLPPP multilink interface.

Follow these requirements:

  • Configure the two serial links to use MLPPP:
    • Use MLPPP interface 2
    • Use subnet
    • Give R1 an IPv4 address that ends with .1, and R2 an address that ends with .2
    • Do not use any PPP authentication
  • As for the transition from the existing configuration, examine the initial configurations shown in this lab exercise, and decide whether or not any existing configuration needs to be updated as a result:
    • Check the OSPF configuration, and make changes as needed, so that both routers still become OSPF neighbors and still exchange routes for their LAN subnets
    • Check the IP addressing configuration to remove any IP address overlaps or remove any unnecessary IP addresses.
  • As for the topology:
    • Assume all interfaces shown in the figure are up and working

Figure 1: MLPPP Topology – Initial State Before You Make Your Configuration Changes


Initial Configuration

Examples 1 and 2 show the initial configurations on routers R1 and R2, respectively.

Example 1: Router R1 Initial Configuration


Example 2: Router R2 Initial Configuration


Answer on Paper, or Maybe Test in Lab

To answer on paper, or in a text editor, just write your answers. The next post will list my suggested solution. And with this particular lab, as worded, there should be only a single correct answer.

To test in lab, you cannot use VIRL, because this lab uses serial interfaces. However, you can use Packet Tracer or real hardware. Feel free to do the lab in either environment. You can use any serial interface numbers that you want to use.

If you do implement this lab, try these show commands to get some insights into your solution:

show ppp multilink

show interfaces multilink 2

show ip interface brief

show ip ospf interface

Answers: IPv6 Extended ACLs 1
Answers: MLPPP 2
By Chris October 12, 2016 09:05
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  1. Tim April 9, 19:59

    I was under the impression that the clock rate is only set on one side of a serial interface. You yet your configs show it being set on both sides. Am I missing something here?

    Reply to this comment
    • certskills April 10, 23:01

      Hi Tim,
      Indeed, the clock rate command is only on the end with the DCE serial cable connected. That was a mistake; it’s fixed now. FYI, when blabbing, I’ll often configure the clock rate on both sides when I don’t know which end has the DCE cable installed, because that sets the clock rate on the DCE end, with the DTE end just ignoring the clock rate commands. Regardless, it’s fixed. Thanks, Wendell

      Reply to this comment
  2. JoskeV August 13, 14:02

    Just some things i noticed:
    – The subnet ( mentioned in the image is never used (should be
    – On both routers interface S0/0/0 is configured twice, and S0/0/1 is forgotten

    Reply to this comment
    • certskills September 4, 08:50

      Joske – thanks! Will get it fixed. It’s queued up for a re-draw, and I fixed the config already. Thanks again.

      Reply to this comment
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