Frame Relay Physical Interface Config – Answers

By certskills December 18, 2012 09:05

Well, things went sideways, plus I thought I had posted this answer. Sorry about that… Anyway, go back to the original post for the config museum lab, which basically asked for a short a config as possible to get Frame Relay working on physical serial interfaces. Answers are listed here, with the relevant defaults.

First Task: Configure IP and Frame Relay

The lab requires practically no configuration at all. It requires an IPv4 address on each router’s serial and LAN interfaces. Plus each router needs to enable Frame Relay encapsulation on its serial interface. And that’s all. The original problem statement spelled out the particulars, like which IP addresses to use, and that you should make use of defaults when possible.

Figure 1 repeats the same figure from the original problem statement, with the three examples that follow showing the answers.

Figure 1 – Frame Relay Configuration


So, your first task is to list the configuration on all three routers, such that all three routers can ping each other’s Frame Relay IP addresses. Use physical interfaces, and use as many defaults as possible.

Example 1: R1 Config


Example 2: R2 Config


Example 3: R3 Config

Note that all three routers’ config required you to do a little subnetting work as well. I left that as an exercise for you, but feel free to ask about that topic as well.

Defaults, and Configuration that Would Make Your Answer Incorrect

The problem statement asked you to use as many defaults as possible. If you went beyond the configuration shown in Examples 1, 2, and 3, then you added too much! If this lab was morphed into an exam question, then you might have missed points, or missed it altogether. Here’s a list of some items you might have configured that were not required:

  • frame-relay interface-dlci
  • frame-relay map
  • frame-relay inverse-arp
  • frame-relay lmi-type
  • The “cisco” parameter on encapsulation frame-relay cisco
  • Subinterfaces

So, why could you ignore all these configuration steps? Because of the IOS defaults related to Frame Relay. Running down the list, in no particular order:

  1. Once configured with the encapsulation frame-relay command, IOS defaults to enable inverse ARP on the physical interface and any subinterfaces as well.
  2. Once configured with the encapsulation frame-relay command, IOS defaults to auto-sense the FR LMI Type, and use whatever LMI type the attached FR switch uses.
  3. Only subinterfaces need a frame-relay interface-dlci command, to associate a DLCI with the subinterface. IOS automatically associates all other known DLCIs with the physical interface.
  4. Because the physical interface already uses Inverse ARP, there is no need for any FR static mappings as configured with the frame-relay map command.
  5. The lab did not ask for any particular type of encapsulation, so the default of IETF (configured by not listing the “cisco” parameter on the encapsulation frame-relay command) requires less configuration.

That’s it!

Config Museum: Frame Relay Physical Interface Config
Config Museum: Frame Relay Multipoint Interface Config
By certskills December 18, 2012 09:05
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