Configuring a Cisco Router as a Frame Relay Switch

A Cisco router now has the capability to simulate a Frame Relay switch. Although this appendix will not show you how configure all of the features that a true Frame Relay switch will have, it will show you how to configure a Cisco router to propagate DLCIs and match them to the appropriate outgoing interfaces to make the router act as a Frame Relay switch.

The most important thing to remember about a Frame Relay switch in the lab is that the DLCI is sent through LMI out a particular interface, and then that DLCI and interface are mapped to an outgoing DLCI and interface.

Physical Connections to the Frame Relay Switch (Cisco 2523)

Before you start configuring the Frame Relay switch, take a look at how the lab routers are connected to the Cisco 2523 acting as the Frame Relay switch. Figure B-7 illustrates the physical connections.

bfig07.gif

Figure B-7 Frame Relay Physical Connections

As you can see from Figure B-7, R3 will be the "hub" site and requires two PVCs and DLCIs to be configured on interface S0 of the Cisco 2523. R2 and R4 need only one PVC and one DLCI. Begin by configuring the router as a Frame Relay switch.

Steps for Configuring a Cisco Router as a Frame Relay Switch

Table B-1 defines the steps to configuring a Cisco router as a Frame Relay switch.

Table B-1. Configuring a Cisco Router as a Frame Relay Switch

Step Description Command

Step 1

Enable Frame Relay switching. frame-relay switching

Step 2

Configure Frame Relay encapsulation, Frame Relay LMI type, Frame Relay DCE interface mode, and clock rate on individual interfaces. encapsulation frame-relay frame-relay lmi-type ansi frame-relay intf-type dce clock rate 64000 [1]

Step 3

Configure DLCI to interface mappings on individual interfaces. frame-relay route {local-dlci} interface {outgoing interface and number} {outgoing-dlci}

The first thing is to connect to the Cisco 2523's console port. There is no configuration on the router at this point. You should be in setup mode or at the Router> prompt. If you are in setup mode, just exit this mode (Ctrl-c).

When you are into the router, give it a host name of Frame-Switch.

Router>en
Router#config t
Enter configuration commands, one per line.  End with CNTL/Z.
Router(config)#hostname Frame-Switch

Do not worry about passwords and Telnet connectivity. This router will be a standalone Frame Relay switch. If you need to access it, you will connect to the console port.

Begin with the first step documented in Table B-1, and enable Frame Relay switching on the router. Example B-1 illustrates this configuration step.

Example B-1. Enable Frame Relay Switching

Frame-Switch(config)#frame-relay switching
Frame-Switch(config)#

After the Frame Relay switching process has been started, configure the individual interfaces for the Frame Relay switch. This includes changing the encapsulation type to frame-relay and changing the LMI type to ANSI. Because all interfaces on the Frame Relay switch are DCEs (refer to the Figure B-7), they will need to be changed to the Frame

Relay type DCE and must have the clock rate command issued as well. Example B-2 demonstrates these commands for interface Serial0.

Example B-2. Frame-Relay Commands for Serial0

Frame-Switch(config)#int serial0
Frame-Switch(config-if)#encapsulation frame-relay
Frame-Switch(config-if)#frame-relay lmi-type ansi
Frame-Switch(config-if)#frame-relay intf-type dce
Frame-Switch(config-if)#clock rate 64000

Now that all the Frame Relay commands have been set, you need to map the local DLCI of this interface to the outgoing DLCI and port. Because the Serial0 interface has two PVCs, it needs two mappings. Example B-3 shows the commands.

Example B-3. DLCI-to-Interface Mappings for Serial0

Frame-Switch(config-if)#frame-relay route 100 interface serial 2 
      101
Frame-Switch(config-if)#frame-relay route 200 interface serial 1 
      201
Frame-Switch(config-if)#no shutdown

From Figure B-7, you know that Serial 0 has two PVCs, one to R2 and one to R4. The first highlighted portion of lines 1 and 2 in Example B-3 point out the local DLCI that will be advertised out Serial 0. Therefore, R3 will see DLCI 100 and DLCI 200 because R3 is connected to the Frame Relay switch on Serial0. The second portion of highlighting in lines 1 and 2 marks the outgoing interface to which each DLCI is mapped. Therefore, anything coming from R3 on DLCI 100 will be sent to Serial2, and anything coming from R3 on DLCI 200 will be sent to Serial1. The last portion of highlighting in lines 1 and 2 indicates the DLCI assigned to the outgoing port. So, anything coming from R3 on DLCI 100 will go out Serial2 to DLCI 101, and anything coming from R3 on DLCI 200 will go out Serial1 to DLCI 201. Don't forget to remove the interfaces from shutdown mode.

The next thing you need to do is perform a similar mapping statement on interfaces Serial1 and Serial2, except that the numbers will be reversed. See Example B-4.

Example B-4. Frame Relay Commands and DLCI-to-Interface Mappings for Serial1

Frame-Switch(config)#interface serial1
Frame-Switch(config-if)#encapsulation frame-relay
Frame-Switch(config-if)#frame-relay lmi-type ansi
Frame-Switch(config-if)#frame-relay intf-type dce
Frame-Switch(config-if)#clock rate 64000
Frame-Switch(config-if)#frame-relay route 201 interface serial 0 
      200
Frame-Switch(config-if)#no shutdown

The highlighted portion of the configuration shows the local DLCI (201), the outgoing interface (Serial0), and the outgoing DLCI (200). Next, do the same for interface Serial2. See Example B-5.

Example B-5. Frame-Relay Commands and DLCI-to-Interface Mappings on Serial2

Frame-Switch(config)#interface serial2
Frame-Switch(config-if)#encapsulation frame-relay
Frame-Switch(config-if)#frame-relay lmi-type ansi
Frame-Switch(config-if)#frame intf-type dce
Frame-Switch(config-if)#clock rate 64000
Frame-Switch(config-if)#frame-relay route 101 interface serial 0 
      100
Frame-Switch(config-if)#no shut
Frame-Switch(config-if)#

The highlighted portion of the configuration shows the local DLCI (101), the outgoing interface (Serial0), and the outgoing DLCI (100). At this point, you have a functional Frame Relay switch. You will be able to verify the connections in Chapter 7, "Router Interface Configuration," but for now, take a look at the configuration and do a show frame-relay route to verify that the configuration matches the lab diagram. Example B-6 shows the running-config file. Notice where the commands are located in the configuration file.

Example B-6. Output from show running-config

Frame-Switch#show running-config
Building configuration...

Current configuration:
!
version 11.2
no service password-encryption
no service udp-small-servers
no service tcp-small-servers
!
hostname Frame-Switch
!
!
frame-relay switching
!
interface Serial0
 no ip address
 encapsulation frame-relay
 clockrate 64000
 frame-relay lmi-type ansi
 frame-relay intf-type dce
 frame-relay route 100 interface serial2 101
 frame-relay route 200 interface Serial1 201
!
interface Serial1
 no ip address
 encapsulation frame-relay
 clockrate 64000
 frame-relay lmi-type ansi
 frame-relay intf-type dce
 frame-relay route 201 interface Serial0 200
!
interface Serial2
 no ip address
 encapsulation frame-relay
 clockrate 64000
 frame-relay lmi-type ansi
 frame-relay intf-type dce
 frame-relay route 101 interface Serial0 100
!
interface Serial3
 no ip address
 shutdown
!
interface Serial4
 no ip address
 shutdown
!
interface Serial5
 no ip address
 shutdown
!
interface Serial6
 no ip address
 shutdown
!
interface Serial7
 no ip address
 shutdown
!
interface Serial8
 no ip address
 shutdown
!
interface Serial9
 no ip address
 shutdown
!
interface TokenRing0
 no ip address
 shutdown
!
interface BRI0
 no ip address
 shutdown
!
no ip classless
!
!
line con 0
 exec-timeout 0 0
line aux 0
line vty 0 4
 login
!
end

Frame-Switch#

The highlighted portions illustrate all the Frame Relay configuration tasks that you completed. Notice that none of the interfaces has IP addresses, nor do any of the interfaces need them. You are only mapping DLCIs to interfaces. This is a Layer 2 function, not a Layer 3 function, therefore, no IP address are needed.

The show frame-relay route command is a useful command in determining that your configuration is correct. Example B-7 shows the output from this command.

Example B-7. Output from show frame-relay route Command

Frame-Switch#show frame-relay route
Input Intf      Input Dlci      Output Intf     Output Dlci     Status
Serial0         100             Serial2         101             inactive
Serial0         200             Serial1         201             inactive
Serial1         201             Serial0         200             inactive
Serial2         101             Serial0         100             inactive
Frame-Switch#

From this output, you can see that the Input Dlci matches the correct interfaces from the lab diagram in Figure B-7. You also can see that Output Intf and Output Dlci match to the correct interfaces and DLCIs as well. From here, you can assume that everything is configured correctly. The status will be inactive until you configure the Frame Relay interfaces on R2, R3, and R4 and remove them from shutdown mode.

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