Cisco CCNA Certification Training Case Study:
Frame Relay And DLCI Mappings
By Chris Bryant, CCIE #12933
Frame Relay is a major part of your CCNA certification test prep, and for good reason. Frame Relay is found in just about every Wide Area Network (WAN), mainly because it's cheap and dependable - a great combination!
In this Cisco certification case study, we're going to take a look at the Data Link Connection Identifier, or DLCI, and how DLCIs are applied in a typical Frame Relay router configuration.
DLCIs (pronounced "del-see") do just what the name says they do - they identify a connection at the Data Link layer of the OSI model.
One major difference between DLCIs and most of the other protocols and services you'll run into during your CCNA studies is that there's only that one single number in a DLCI header. There is no specified source or destination.
Frame Relay DLCIs are not advertised to any other router, making DLCIs locally significant only. That means that if we were assigned the same DLCI number by our Frame Relay service provider to use on multiple routers in the same network, that would not affect network operations.
A Frame Relay network can be configured to use the same DLCI for every router in a network to reach the same network. On Cisco routers, this is called global addressing, but does not affect the basic Frame Relay rule that DLCIs are locally significant only.
Since DLCIs are not advertised, the routers have no idea that other routers are using the same DLCI. In the following example, three spoke routers are all using the same DLCI to reach the hub router.
Cisco Routers, Frame Relay, And Frame Map Statements
You might be wondering how those spoke routers could ever communicate with the hub by using a DLCI that only the local router knows about. Frame Relay communication is achieved by mapping this DLCI to the remote router's IP address, and this can be done either dynamically or statically.
Those of you who have read my books and other Cisco training tutorials know that generally I'm all for using dynamic methods. There's an exception to every rule, and this is a huge exception to my "don't use static methods" rule.
Dynamic Frame Relay mappings rely on Inverse ARP, a protocol that isn't all that reliable. If you do have dynamic mappings, they will be seen with as dynamic when you run the show frame map command. Here's an example of a Cisco router with two frame map statements created by Inverse ARP.
R1#show frame map
Serial0 (up): ip 18.104.22.168 dlci 122(0x7A,0x1CA0), dynamic, broadcast,, status defined, active
Serial0 (up): ip 22.214.171.124 dlci 123(0x7B,0x1CB0), dynamic, broadcast,, status defined, active
The preferred and more reliable method is to use static frame map statements. It's also a good idea to disable Inverse ARP before creating static maps, as dynamic and static frame map statements tend not to play well together on a reload.
To disable Inverse ARP, run no frame-relay inverse-arp BEFORE opening the interface. If you configure Frame Relay on an interface and then open it, Inverse ARP runs by default.
R1(config-if)#no frame-relay inverse-arp
R1(config-if)#frame map ip 126.96.36.199 122 broadcast
R1(config-if)#frame map ip 188.8.131.52 123 broadcast
The command show frame map confirms that these map statements are static.
R1#show frame map
Serial0 (up): ip 184.108.40.206 dlci 122(0x7A,0x1CA0), static, broadcast, CISCO, status defined, active
Serial0 (up): ip 220.127.116.11 dlci 123(0x7B,0x1CB0), static, broadcast, CISCO, status defined, active
This is just part of what you'll need to know about Frame Relay to earn your Cisco CCNA certification. By understanding how to configure both dynamic and static Frame Relay mappings, you're on your way to mastering this topic and earning your certification!
You'll find plenty of additional Frame Relay tutorials and practice exam videos on my CCNA Tutorials page - and while you're at it, seize this exclusive opportunity..
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