FR Multipoint Config – Answers

certskills
By certskills January 14, 2013 12:05

This latest Frame Relay #CCNA lab painted you into, well, two different possible corners. The requirement to use as many defaults as possible, plus as few commands as possible, may have pointed you towards using the frame-relay interface-dlci command. As it turns out, the frame-relay map command would have worked just as well, within the way the question was worded. Today’s post walks through the configuration that uses the interface-dlci option, with the following post examining the static frame-relay map commands.

Overview and Comparison of the Two Configuration Options

The problem statement requires that we assume as many defaults as possible. As a result, the configuration can use the default for LMI (autosense), for the specific style of encapsulation (cisco).

Both answers must still enable Frame Relay on the physical interface, because the encapsulation defaults to HDLC. Specifically, both must have the encapsulation frame-relay subcommand on the serial interface.

Both options use the same IPv4 addresses (192.168.9.65, .66, and .67), because the solution must use the numerically-lowest IPv4 addresses in subnet 192.168.9.64/27.

The only difference between the two solutions is their use of the frame-relay interface-dlci or frame-relay map commands.  Before moving on to these differences, Examples 1, 2, and 3 show incomplete configurations for all three routers, with the commands in common with both solutions.

Figure 1 – Frame Relay Configuration

 

Example 1: R1 Config

 

Example 2: R2 Config

 

Example 3: R3 Config

Finally, on this baseline config common to both solutions, note that the specific subinterface numbers could have been any value, as long as they are unique among all subinterfaces on the same physical interface. These examples use subinterfaces .1, .2, and .3 just to make it obvious that the subinterface numbers do not have to match on the different routers.

Solution Using the frame-relay interface-dlci Command

To use subinterfaces, the router must either be told, or dynamically learn, some details about what ideas to associate with the subinterface. The router knows many facts about the PVCs that use a particular physical interface, but the router is missing information of how to connect those ideas to one subinterface or another. Specifically, a router must know the following info for each FR subinterface:

  1. The DLCIs of the PVCs to associate with the subinterface, as opposed to some other subinterface or the physical interface
  2. The next-hop IP address on the other end of each of those same PVCs (called mapping)

Cisco routers give us two methods to achieve both these requirements. One teams the frame-relay interface-dlci configuration command with the dynamic Inverse ARP process. The other option configures all the information in a command, namely, the frame-relay map command.

This post looks at only the frame-relay interface-dlci option.

The configuration for the interface-dlci is actually simple – possibly so simple that the underlying concepts remain mostly hidden. Examples 4, 5, and 6 show the additional configuration on routers R1, R2, and R3, when combined with the configuration in Examples 1, 2, and 3, complete the Frame Relay configuration.

Example 4: R1 Config

 

Example 5: R2 Config

 

Example 6: R3 Config

 

The configuration directly tells the routers which PVCs (based on their DLCIs) are associated with the subinterface. To find the mapping – the next-hop router IP address on the other end of the PVC – the router takes advantage of Inverse ARP, which is enabled by default.

That’s it!

Frame Relay Multipoint Config – Answers Part 2
Config Museum: Frame Relay Multipoint Interface Config
certskills
By certskills January 14, 2013 12:05
Write a comment

2 Comments

  1. sanjinsan February 22, 02:59

    Hi,

    do we need Inv ARP in case we are using point to point subinterfaces or multi point to multipoint subinterfaces?

    sanjinb

    Reply to this comment
View comments

Write a comment

Comment; Identify w/ Social Media or Email

Subscribe

Subscribe to our mailing list and get interesting stuff and updates to your email inbox.

Thank you for subscribing.

Something went wrong.

Search

Categories