Answers: MLPPP 2

Wendell
By Wendell October 14, 2016 09:10

This lab asks you to do a basic migration: move from using two parallel serial links as independent links to instead treating them as one layer 3 link using MLPPP. This lab requires new configuration as well as analysis of the existing configuration. Do the lab on your own first, and then check here for my answer and explanation.

Answers

Figure 1: MLPPP Topology – Before Migrating to MLPPP

Example 3: Router R1 Config

 

Example 4: Router R2 Config

 

Commentary

This lab has a rather long configuration answer. This commentary starts by working through the MLPPP configuration, and then discusses some of the migration choices, particularly to do with OSPF and IP addressing.

The requirements asked you to use MLPPP multilink group 2. As a result:

  • Both routers create a multilink interface with the interface multilink 2 command
  • Both routers use the ppp multilink group 2 subcommand on their physical serial interfaces and on the multilink interface itself

For the layer 3 details, note that the configuration on each router:

  • Adds an IPv4 address to each multilink interface, for example, with the ip address 172.20.12.1 255.255.255.0 subcommand under multilink 2 on router R1.
  • Both routers remove the IP address configuration from the physical interfaces, because they are no longer needed, with the no ip address interface subcommand.

Next, as normal, the physical serial interfaces plus the multilink interface, on each router, need the same three subcommands. These commands (respectively) enable PPP, enable the MLPPP feature of PPP, and reference the MLPPP multilink group:

encapsulation ppp

ppp multilink

ppp multilink group 2

Finally, note that the OSPF configuration needed to be updated. The initial configuration happened to show OSPF configuration that matched based on all 32 bits of each interface IP address. The MLPPP configuration in this case moved away from using the addresses in subnets 172.20.3.0/24 and 172.20.4.0/24, now using subnet 172.20.12.0/24. The end of Examples 3 and 4 show the addition of one OSPF network command on each router, matching each router’s new IP address in subnet 172.20.12.0/24. It also shows the removal of the two now unneeded network commands.

Multilink PPP 2
Wendell
By Wendell October 14, 2016 09:10
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5 Comments

  1. ProfSo4c October 14, 12:21

    Thanks Wendell, your posts have been really helpful. However some WAN configurations are not working on PT 7.0. e.g. Int Multilink 2 doesn’t work. Using notepad has been my gateway of last resort

    Reply to this comment
  2. cisco.cruz86 December 6, 19:21

    What happens with the clock rate command? I dont see it in any side of the link.

    Reply to this comment
    • certskills December 9, 19:58

      Actually, the clock rate command is supplied automatically when the interface has a DCE cable connected, at least by IOS at the later releases. Frankly I ignored it, probably because I didn’t have to code it. I don’t recall what IOS – maybe 15.2M or so?
      Anyway, for this lab, there was no implication that the routers were in a lab with a short cable, rather than a real serial link, so it seems ambiguous for the lab whether a clock rate command was required. Either way… if you expect it to be in a lab, with a back-to-back serial cable, make sure there’s a clock rate command on the DCE side.
      Thanks for the note…

      Reply to this comment
  3. Tim April 9, 21:29

    If using a simulator instead of real hardware try GNS3 it runs real Cisco IOS firmware images in a VM and thus supports things like PPP Multilink.

    Reply to this comment
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