Answer, Config VM: Frame Relay IP

By certskills May 18, 2012 07:00

This blog post simply lists the answers to the earlier Config VM from a few days ago. As with all the Config Museum pieces, the goal is to give you a straightforward set of lab requirements: no guile, no tricks, just a chance to exercise. This exercise focuses on configuring IP addresses on Frame Relay interfaces, assuming the interface and subinterface configuration already exists.

Some related links:

The original problem statement

Wendell’s book (for deeper background on Frame Relay)

Configuring IP Addresses on Physical Interfaces or Subinterfaces

Often times when learning Frame Relay configuration on routers, the description shows the Frame Relay details and the IP details together. This lab separates the IP configuration, with an earlier lab including the Frame Relay configuration.  Because so many descriptions put it all together (my book included), this lab may be a bit of a challenge.

The first challenge is to decide where to put the ip address command. It can be a subcommand of the physical interface, and it can be a subcommand of a subinterface. To choose the right location in this case, you have to use the following rules:

  • Configure IP addresses on the physical or sub interface where the matching PVC is associated.
  • Routers associate a PVC with a subinterface based on either the frame-relay interface-dlci subcommand or the frame-relay map subcommand
  • Any PVCs that exist on a physical Frame Relay access link, that are not associated with a subinterface, are associated with the physical interface

In this lab, the figure shows the local DLCIs for the various PVCs, and the blog post’s text lists the configuration on each router. The config shows subinterfaces, with all the local DLCIs in the figure associated with a subinterface. So, the IP addresses should be configured on the subinterfaces in this case, and not on the physical interfaces.

Using One or Two Subnets for Frame

This lab also left you with a subnetting design choice: whether to use one or two subnets. Frame Relay supports both options. If you had started from nothing, and chosen all the configuration, you could have used either choice. However, the starting point of this lab, with the Frame Relay configuration listed, drives you to only one choice: using two subnets for the Frame Relay part of the network.

Why? Well, per the original figure, repeated here, R1 has two subinterfaces: one for the PVC connected to R2, and one for the PVC connected to R3. The configuration shows each as a “point-to-point” subinterface, which means that only one other router can be associated with the subinterface. Additionally, one a given router, only one interface/subinterface IP address can be from the same subnet. So, router R1 could not have two IP addresses from the same subnet; these two subinterfaces must instead have IP addresses from different subnets. (R1’s IOS would reject the 2nd ip address command, by the way.)

Figure 1 – Frame Relay Configuration Details


The Chosen Two Subnets

The lab also required that you make several choices about the subnets used. Specifically:

  • Use the smallest subnets possible (fewest IP addresses in each subnet)
  • Use the fewest number of subnets
  • Use the numerically-smallest subnets whose first two octets are “10.2”

The earliest discussion stated that you need two subnets. With point-to-point subinterfaces, each subnet needs only two IP addresses. So, to meet the needs of this design, you need two subnets, each with a /30 ( mask, which gives you 2 host addresses per subnet.

The two smallest such subnets that begin with 10.2 are and

Configuration Answer

Examples 4, 5, and 6 shows the configuration added to routers R1, R2, and R3, respectively, to complete this lab.


Example 1: R1 Added Config

Example 2: R2 Added Config

Example 3: R3 Added Config

Convert Masks, Drill 1
Convert Masks, Drill 1: Answers
By certskills May 18, 2012 07:00
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