David Sudjiman

To create a frame relay (FR) switch we need at least a router with two serial interfaces and IOS release 11.0 or later. These two interfaces will connect to two routers and create an FR cloud between these two routers. The FR switch will act as DCE and the other two routers will act as DTE. Therefore, we will need DTE-to-DCE cable.

We can use the cheapest used router like Cisco 2501 router or Cisco 2503 router for two serial interfaces or if we could buy Cisco 2522 router we will have 10 serial interfaces.

Scenario

We have three routers and want to connect those three routers to FR switch as multipoint connection shown in Figure 1.

Figure 1.

Basically we need to create Virtual Connection (VC) from serial 1 to serial 2, serial 2 to serial 3, and serial 3 to serial 1. However, to create those three VCs means to create FR router from serial 1 to serial 2 and serial 2 to serial 1, serial 2 to serial 3 and serial 3 to serial 2, and the last one is serial 3 to serial 1 and serial 1 to serial 3. We can create the frame-relay router using command:

frame-relay route inbound_DLCI interface outbound serial outbound_DLCI

and below are the complete commands to create the routes.

interface serial 1
frame-relay route 122 interface s2 221
frame-relay route 123 interface s3 321

interface serial 2
frame-relay route 221 interface s1 122
frame-relay route 223 interface s3 322

interface serial 3
frame-relay route 321 interface s1 123
frame-relay route 322 interface s2 223

To make it easy, this example will use that particular DLCI numbering to make it more sense to read and remember. For example, DLCI 122 means serial1-to-serial2, DLCI 221 means serial2-to-serial1, and so on. Of course, we can use any number for the DLCI from 16 to 1007.

However, those are not the only commands we need to create the routes in the FR switch. Additional commands are required. We need to enable the command frame-relay switching within global configuration mode.

Don’t forget that this FR switch will act as DCE and it means we need to enable the clock rate with the appropriate speed. Also, we need to define the serial interface as a DCE using command frame-relay intf-type dce.

Cisco uses cisco as the LMI messages as default. If we want to change to another LMI message type, we can use command frame-relay lmi-type {ansi|cisco|q933a}

It is also good to use command no frame-relay inverse-arp as this command disables Inverse-ARP to create mapping between Layer 3 address and Layer 2 address. Since we don’t use Layer 3 addressing on fr_switch, therefore we don’t need to use I-ARP. However, in some cases the necessity to use I-ARP on connected routers (not on the fr_switch itself) are sometimes required if we want to use the mapping feature.

Here are the complete commands to create an FR switch.

ena
conf t
hostname fr_switch
no ip domain-lookup

frame-relay switching

interface serial 1
encapsulation frame-relay
frame-relay intf-type dce
clock rate 56000
no frame-relay inverse-arp
frame-relay route 122 interface s2 221
frame-relay route 123 interface s3 321

interface serial 2
encapsulation frame-relay
frame-relay intf-type dce
clock rate 56000
no frame-relay inverse-arp
frame-relay route 221 interface s1 122
frame-relay route 223 interface s3 322

interface serial 3
encapsulation frame-relay
frame-relay intf-type dce
clock rate 56000
no frame-relay inverse-arp
frame-relay route 321 interface s1 123
frame-relay route 322 interface s2 223

Last one, we can view the routes that we’ve made using command:

fr_switch#sh frame-relay route
Input Intf      Input Dlci      Output Intf     Output Dlci     Status
Serial1         122             Serial2         221             inactive
Serial1         123             Serial3         321             inactive
Serial2         221             Serial1         122             inactive
Serial2         223             Serial3         322             inactive
Serial3         321             Serial1         123             inactive
Serial3         322             Serial2         223             inactive

Sources:
http://www.thebryantadvantage.com/Frame%20Relay%20Switch.htm

CCNP Practical Studies: Remote Access; ISBN1587200732
http://www.cisco.com