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  • Cisco CCNA R&S Addition – Serial Multilink Done on GNS3

    If you are getting psyched for the newly added stuff in CCNA R&S v3, I have a little piece of it here for you today. It is called MLP (Multilink PPP) and it comes down to bundling multiple real links into one virtual link. It is one of the new topics and there is even lab time given for it, so I wanted to see if it would also work in GNS3.

    Spoiler alert – it does work successfully in GNS3, but let’s see how it is done. The topology is shown in figure 1:

    001-cisco-ccna-serial-multilink-done-on-gns3

    Figure 1

    As you can see, there are two real interfaces connected on each router, Serial 0/0 to Serial 0/0 and Serial 0/1 to Serial 0/1. The purpose here is to bundle those real interfaces into a virtual link, so the real interfaces are configured with encapsulation ppp, no ip addresses, and the no shutdown command.

    Here are some of the configuration steps:

     

    R1(config)# interface multilink 1

    R1(config-if)# ip address 172.16.1.1 255.255.255.0

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    R1(config-if)# ppp multilink

    R1(config-if)# ppp multilink group 1

     

    You notice after the first command is issued that a new interface is created, much like crating a loopback interface or a tunnel interface.

    Now do the same on R2, allowing for the different IP address:

     

    R2(config)# interface multilink 1

    R2(config-if)# ip address 172.16.1.2 255.255.255.0

    R2(config-if)# ppp multilink

    R2(config-if)# ppp multilink group 1

     

    All that remains now is to assign interfaces to the multilink bundle. Enter these commands on both R1 and R2:

     

    R1(config)# interface Serial1 0/0

    R1(config-if)# ppp multilink

    R1(config-if)# ppp multilink group 1

    R1(config)# interface Serial 0/1

    R1(config-if)# ppp multilink

    R1(config-if)# ppp multilink group 1

     

    And R2:

     

    R2(config)# interface Serial1 0/0

    R2(config-if)# ppp multilink

    R2(config-if)# ppp multilink group 1

    R2(config)# interface Serial 0/1

    R2(config-if)# ppp multilink

    R2(config-if)# ppp multilink group 1

     

    Once these commands have been issued, a useful show command is show ppp multilink

    Figure 2 shows the output of that command on R1:

    002-cisco-ccna-serial-multilink-done-on-gns3

    Figure 2

     Figure 3 shows the same command output on R2:

    003-cisco-ccna-serial-multilink-done-on-gns3 

    Figure 3

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     One thing to note is that R1 says its endpoint discriminator is R2 and vice versa on R2. Figure 4 shows if we can ping across this virtual link:

    004-cisco-ccna-serial-multilink-done-on-gns3

    Figure 4

    That is pretty cool, plus if a single real link fails, the multilink will remain up and functioning. My main goal was to test this concept in GNS3, and per usual, GNS3 performed admirably. This is just one of the additions to the v3 of the CCNA R&S track, stay tuned for more exposure to the updates in future blogs. I would be happy to hear any questions or comments regarding this blog, so feel free to post.

    Until next time.

    Mark Jacob
    Cisco and CompTIA Network + Instructor – Interface Technical Training
    Phoenix, AZ

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