Frame-relay: Inverse ARP

Frame relay is a packet switching technology that emulates like a switch. Switch uses ARP broadcast to find out the mac address of the host so that switch can forward the frame to the destined switch port.

Frame relay however does not have broadcast ability, it uses encapsulation technique either Cisco or IETF encapsulation format.

Datalink connection identifier (DLCI) is the frame-relay address. To dynamically resolve a destination IP address to an exit DLCI, inverse arp technique is used. Inverse ARP resolves an IP address to its DLCI equivalent.

R1 Frame relay switch

frame-relay switching

interface Serial0/0
 bandwidth 2000
 no ip address
 encapsulation frame-relay IETF
 clock rate 2000000
 frame-relay lmi-type ansi
 frame-relay intf-type dce
 frame-relay route 305 interface Serial0/1 503
end

interface Serial0/1
 bandwidth 2000
 no ip address
 encapsulation frame-relay IETF
 clock rate 2000000
 frame-relay lmi-type ansi
 frame-relay intf-type dce
 frame-relay route 503 interface Serial0/0 305
end

R2 connecting frame relay switch

interface Serial0/0
 bandwidth 2000
 ip address 10.10.10.1 255.255.255.0
 encapsulation frame-relay IETF
 frame-relay lmi-type ansi
end

R3 connecting frame relay switch

interface Serial0/0
 bandwidth 2000
 ip address 10.10.10.2 255.255.255.0
 encapsulation frame-relay IETF
 frame-relay lmi-type ansi
end

Frame relay route

R1#show frame-relay route
Input Intf      Input Dlci      Output Intf     Output Dlci     Status
Serial0/0       305             Serial0/1       503             active
Serial0/1       503             Serial0/0       305             active
R1#

R2 frame relay map

R2#show frame-relay map
Serial0/0 (up): ip 10.10.10.2 dlci 305(0x131,0x4C10), dynamic,
              broadcast,
              IETF, status defined, active
R2#

R3 frame relay map

R3#show frame-relay map
Serial0/0 (up): ip 10.10.10.1 dlci 503(0x1F7,0x7C70), dynamic,
              broadcast,
              IETF, status defined, active
R3#

Local Management Interface
1. A DTE sends LMI status/enquiry message to DCE.

R2#
*Mar  1 00:11:15.375: Serial0/0(out): StEnq, myseq 67, yourseen 66, DTE up
*Mar  1 00:11:15.379: datagramstart = 0x6D01B14, datagramsize = 14
*Mar  1 00:11:15.379: FR encap = 0x00010308
*Mar  1 00:11:15.379: 00 75 95 01 01 01 03 02 43 42
*Mar  1 00:11:15.387:
*Mar  1 00:11:15.415: Serial0/0(in): Status, myseq 67, pak size 14
*Mar  1 00:11:15.415: RT IE 1, length 1, type 1
*Mar  1 00:11:15.419: KA IE 3, length 2, yourseq 67, myseq 67
R2#
*Mar  1 00:11:25.375: Serial0/0(out): StEnq, myseq 68, yourseen 67, DTE up
*Mar  1 00:11:25.375: datagramstart = 0x6D01C54, datagramsize = 14
*Mar  1 00:11:25.375: FR encap = 0x00010308
*Mar  1 00:11:25.375: 00 75 95 01 01 01 03 02 44 43
*Mar  1 00:11:25.375:
*Mar  1 00:11:25.379: Serial0/0(in): Status, myseq 68, pak size 14
*Mar  1 00:11:25.383: RT IE 1, length 1, type 1
*Mar  1 00:11:25.383: KA IE 3, length 2, yourseq 68, myseq 68
R2#

2. DCE responds by sending back LMI status to inform the connected routers about the status of the virtual circuit as well as DLCI

R1#
*Mar  1 00:12:45.731: Serial0/1(in): StEnq, myseq 75
*Mar  1 00:12:45.731: RT IE 1, length 1, type 1
*Mar  1 00:12:45.735: KA IE 3, length 2, yourseq 76, myseq 75
*Mar  1 00:12:45.735: Serial0/1(out): Status, myseq 76, yourseen 76, DCE up
*Mar  1 00:12:46.191: Serial0/0(in): StEnq, myseq 75
*Mar  1 00:12:46.191: RT IE 1, length 1, type 1
*Mar  1 00:12:46.191: KA IE 3, length 2, yourseq 76, myseq 75
*Mar  1 00:12:46.191: Serial0/0(out): Status, myseq 76, yourseen 76, DCE up
R1#
*Mar  1 00:12:55.667: Serial0/1(in): StEnq, myseq 76
*Mar  1 00:12:55.671: RT IE 1, length 1, type 1
*Mar  1 00:12:55.671: KA IE 3, length 2, yourseq 77, myseq 76
*Mar  1 00:12:55.675: Serial0/1(out): Status, myseq 77, yourseen 77, DCE up
*Mar  1 00:12:56.147: Serial0/0(in): StEnq, myseq 76
*Mar  1 00:12:56.147: RT IE 1, length 1, type 1
*Mar  1 00:12:56.151: KA IE 3, length 2, yourseq 77, myseq 76
*Mar  1 00:12:56.151: Serial0/0(out): Status, myseq 77, yourseen 77, DCE up
R1#

3. The LMI status/enquiry messages are a form of LMI keepalive message.
4. LMI holdtime is 3x the LMI interval.
5. LMI is automatically enabled when encapsulation frame-relay command is used.
6. LMI keepalives are sent every 10 seconds by default. To change the keepalive interval use keepalive command under the interface configuration mode.

R2(config-if)#keepalive ?
  <0-30>  Keepalive period (default 10 seconds)
  <cr>

R2(config-if)#keepalive

7. If LMI autosense is unsuccessful, retry scheme kicks in. Every N391 interval (default 60 seconds, which is 6 LMI keepalives for each 10 seconds interval) the LMI autosense will attempt to ascertain the LMI type and request full status update. To change LMI autosense/full status update interval use frame-relay lmi-n391dte command under interface configuration mode.

R2(config-if)#frame-relay lmi-n391dte ?
  <1-255>  event

For example if full status report interval is expected to be every 200 seconds.

R2(config-if)#frame-relay lmi-n391dte 200
R2(config-if)#

This configuration will trigger full status update every 200 seconds which is every 20 LMI keepalives sent if the keepalive interval is default 10 seconds.

Turn off inverse arp
To disable frame relay switch inverse arp use no frame-relay inverse arp command under interface configuration mode.

Static frame-relay map
To statically map destination ip address with the exit DLCI use the below example:

interface Serial0/0
 bandwidth 2000
 ip address 10.10.10.1 255.255.255.0
 encapsulation frame-relay IETF
 frame-relay map ip 10.10.10.2 305 broadcast IETF
 frame-relay lmi-type ansi
 frame-relay lmi-n391dte 60
end

Use show frame-relay map command to display the map.

R2#show frame-relay map
Serial0/0 (up): ip 10.10.10.2 dlci 305(0x131,0x4C10), static,
              broadcast,
              IETF, status defined, active
R2#