CCNA Skills Lab 3: Part 2

Part 2 of this lab examines how to configure Frame Relay. Per the lab’s listed requirements, you can take as many defaults as you like, so the config avoids such issues as LMI types, DLCI mapping, and encapsulation. But it does include the basics, along with the creation of point-to-point subinterfaces. And the biggest learning point? An opportunity to think through the global DLCIs in the lab’s original network diagram and then derive the local DLCI values, which will be needed for the configuration. And as usual, it makes sense to have read the lab exercise before looking at this solution.

First, look at Figure 4, which repeats (unchaged) Figure 1 from the original lab exercise. The figure shows DLCIs, so the first question you should ask is whether the DLCIs listed are global DLCIs or local DLCIs.

Figure 4: FR Topology with Global DLCIs

So, how do you know whether the figure shows local or global DLCIs? Find a router with more than one PVC connected to it, and ask yourself whether the figure lists just one DLCI, or one per PVC. Figure 4 shows just one DLCI, which implies global DLCIs.
The global DLCI defines the local DLCI used on the other end of the PVCs. In this case, for example, R1’s listed global DLI of 101 means that R2, R3, and R4 each use DLCI 101 as their local DLCI for their respective PVCs connected to R1. It’s the local DLCI that gets configured in any router’s config. If you want to read more about how all this works, look back to this earlier post on global DLCIs, and for even more on Frame Relay DLCIs, look at the three posts just before that post.
Armed with those facts, you can now start to create the configuration. The key facts to keep in mind in this case are:

1. You must enabled FR on the physical serial interface using the encapsulation frame-relay command
2. You need to create the point-to-point subinterfaces. The subinterface numbers do not have to match on both endpoint routers, and they do not have to match the DLCIs
3. You assign the WAN IP addresses to the point-to-point subinterfaces (which was already shown in the previous post)
4. You must associate the correct local DLCI to the intended subinterface using the frame-relay interface-dlci subcommand

Compared to the configuration shown in the previous post, which showed the IP configuration, the first three of the four items above have already been configured.  The previous post had to show those steps to be realistic in relation to the IP configuration. Examples 5 through 8 expand those configurations, showing the new commands in bold.

Example 5: FR Added to IP Configuration, R1

interface F0/0
 ip address 172.23.1.1 255.255.255.192
!
interface serial 0/0/0
 encapsulation frame-relay
!
interface S0/0/0.1 point-to-point
 ip address 172.23.0.1 255.255.255.192
 frame-relay interface-dlci 102
!
interface s0/0/0.2 point-to-point
 ip address 172.23.0.65 255.255.255.192
 frame-relay interface-dlci 103
!
interface s0/0/0.3 point-to-point
 ip address 172.23.0.129 255.255.255.192
 frame-relay interface-dlci 104

Example 6: FR Added to IP Configuration, R2

interface F0/0
 ip address 172.23.2.1 255.255.255.192
!
interface serial 0/0/0
 encapsulation frame-relay
!
interface S0/0/0.1 point-to-point
 ip address 172.23.0.62 255.255.255.192
 frame-relay interface-dlci 101

Example 7: FR Added to IP Configuration, R3

interface F0/0
 ip address 172.23.3.1 255.255.255.192
!
interface serial 0/0/0
 encapsulation frame-relay
!
interface S0/0/0.1 point-to-point
 ip address 172.23.0.126 255.255.255.192
 frame-relay interface-dlci 101

Example 8: FR Added to IP Configuration, R4

interface F0/0
 ip address 172.23.4.1 255.255.255.192
!
interface serial 0/0/0
 encapsulation frame-relay
!
interface S0/0/0.1 point-to-point
 ip address 172.23.0.190 255.255.255.192
 frame-relay interface-dlci 101