Frame Relay Implementation


            Once the Washington School District WAN is installed and functioning, the final step will be to connect it to the global Internet.  The sole link to the Internet

will be from the District Office Data Center, one of the three regional hubs.  Although many options exist, the District has requested that this link be a T1 data line

using Frame Relay.
 

            Frame Relay has a number of features that have made it desirable for the District's Internet link, many concerned with relieving network congestion.  Frame

Relay has methods for handling congestion through Forward Explicit Congestion Notification (FECN) and Backward Explicit Congestion Notification (BECN).

When a network is congested, it sends a BECN packet back to the source router telling it that the network is congested and that it should stop sending so many

packets.  Upon receipt of the BECN packet, the router reduces the rate at which it sends packets by 25% for a while.  Frame Relay also reduces congestion

through Discard Eligibility (DE).  Packets being sent through a Frame Relay network have a DE bit; if this bit is set, the packet is discarded and resent later when

the congestion is relieved.  If the links weren't able to handle congestion, packets would just keep coming and coming, making the congestion worse and slowing

down the transmission. Because the links in a Frame Relay network are prepared to handle congestion, overall communication is faster.
 

            Frame Relay is also desirable because of its reliability features.  Frame Relay is connection-oriented, so it establishes a link, known as a virtual circuit,

before sending data.  Secondly, a company offering Frame Relay service agrees to give the customer a certain amount of bandwidth at any time; this is referred to

as the Committed Information Rate (CIR).  Thanks to the CIR, a Frame Relay customer always knows the minimum speed available, so network planning is

simplified.  Frame Relay also makes use of the Local Management Interface (LMI) to manage the virtual circuits.  LMI keeps track of which virtual circuits still

exist, thus preventing routers from transmitting over nonexistent paths.  LMI also transmits keepalive packets across virtual circuits so that the virtual circuit isn't shut

down as a result of prolonged inactivity; the circuit will only shut down after communication is finished and link termination is requested.  The establishment of virtual

circuits, LMI's ability to keep track of these virtual circuits, and the promise of a minimum bandwidth all make Frame Relay technology reliable, and therefore make

it desirable for the Washington School District's connection to the global Internet.
 

            With the entire WAN installed, Frame Relay implementation will need little more.  One of the serial interfaces on the high end router at the District Office

Data Center will be connected to a CSU/DSU, which will in turn connect to a T1 line purchased from the phone company.  From here, the T1 line will go through

an unknown number of Frame Relay switches until it reaches the next hop router on the Internet.  The area past this router is all beyond the scope of the Washington

School District WAN; it is the Internet.  The Frame Relay link will run over a T1 line, so it will need to be capable of using this medium's speed.  Therefore, a CIR

of at least 1.544 Mbps is required.

            With all the necessary hardware installed, the final step in implementing Frame Relay will be writing the command sequences on the District Office Data

Center's router.  To configure Frame Relay, the following steps must be taken:
 

  1. Login to the router through the console port, auxilary port, or Telnet.

    2.  
  2. Enter privileged mode with the enable command.

    3.  
  3. Enter global configuration mode with the config t command.

    4.  
  4. Since the Internet connection is to go out interface Serial 0, Frame Relay must be configured on that interface.  Enter interface configuration mode with the int s0 command.

    5.  
  5. The IP address and subnet mask should have already been configured by this point.  If not, enter them.

    6.  
  6. Since this is a Frame Relay link, the encapsulation should be set as such.  Enter the command encapsulation frame-relay.  For simplicity, it is assumed that the next hop router is a Cisco router.  If not, ietf would simply be added on to the end of the previous command.

    7.  
  7. Configure bandwidth for the Frame Relay link.  This is important for things such as routing protocols that use bandwidth as a metric.  Enter the bandwidth kilobits command.

    8.  
  8. Enable inverse ARP with the command frame-relay ip 100, with 100 as the  DLCI of the Serial 0 interface on the high-end router at the District Office     Data Center.

    9.  
  9. Exit out of configuration mode by hitting Ctrl-Z.

    10.  
  10. Copy the newly modified configuration to NVRAM with the copy run start command.

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