Answers: MLPPP 1

Wendell
By Wendell September 15, 2016 09:10

Configuring multilink PPP (MLPPP) uses a large number of commands for a single feature. However, many of the commands are repetitive on several interfaces. The trick is to figure out which commands are repeated on both the physical and multilink interface, and which ones are configured only on one or the other type of interface. Do the lab on your own first, and then check here for my answer and explanation.

Answers

Figure 1: MLPPP Topology

 

Example 3: Router R1 Config

 

Example 4: Router R2 Config

 

Commentary

You can think of MLPPP configuration as having two major ideas, even though the configuration does not make these two ideas obvious. The first is that IOS requires a virtual interface, called a multilink interface, to act as the layer 3 interface for MLPPP. Each MLPPP multilink interface is assigned an integer interface number. The second big idea is that each physical serial link needs to be associated with that MLPPP group through the multilink interface concept, by virtue of a reference to the multilink interface number.

Keeping those ideas in mind, the configuration of the first big idea focuses on these two commands found on router R1 (along with the similar commands on R2):

 

These commands create the multilink interface on router R1, and enable IPv4 on the link, respectively. Router R2 has similar commands, setting address 172.16.12.2. As a result, the concept of a single MLPPP link between the two routers is configured.

The second big idea, which correlates the physical interfaces with the multilink interface on each router. To do so, each router uses the same interface subcommand on the physical serial interfaces and the multilink interface itself: the ppp multilink group 1 command.

Finally, the configuration must also enable PPP as well as the MLPPP feature with two separate commands, again on both the physical and multilink interfaces, as done with these two commands.

 

To summarize the key points, an MLPPP configuration includes:

  • The same three commands on the physical and multilink interfaces (encapsulation ppp, ppp multilink, ppp multilink group x)
  • The no ip address command configured (which is the default) on serial interfaces
  • The ip address command configured on the multilink interfaces

Finally, note that the lab asked you to consider possible changes to the initial configuration. In this lab, no such changes are needed. Here’s why:

  • The OSPF configuration already matches all interfaces whose addresses begin with 172.16, so the new multilink interface is already matched and added to OSPF area 0.
  • The existing IP addresses do not overlap with the new IP subnet used on the multilink between the routers, so no changes are needed to IP addressing.
SNMPv3 2
Multilink PPP 1
Wendell
By Wendell September 15, 2016 09:10
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