Now, i'll make some notes about the issues in Frame Relay. When we connect multiple sites through a single router interface, we'll face the routing problem caused by split horizon. Split horizon is a mechanism in routing protocol that designed to eliminate routing loop by blocking routing updates to be sent out of the router interface. See the picture below:
 |
| Split Horizon in Frame Relay |
R1 received routing updates through serial S0 from R4, but it won't be sent out to R2 or R3 because of split horizon. And how to resolved this problem? Use subinterface. There are two type of subinterfaces support by Cisco:
- Point-to-point, used when we have a separate subnet for each VC. Only one DLCI can be configured per point-to-point subinterfaced. Here's the example of the point-to-point configuration:
interface Serial1/0
encapsulation frame-relay
serial restart-delay 0
!
interface Serial1/0.102 point-to-point
ip address 10.1.1.1 255.255.255.252
frame-relay interface-dlci 102
!
- Multipoint, by default, on cisco router, the physical interfaces are the multipoint interfaces. When we created a multipoint subinterfaces under the physical interfaces, we need to specifically assign DLCI to multipoint subinterfaces. Here's the example of the point-to-point configuration:
interface Serial1/0
encapsulation frame-relay
serial restart-delay 0
!
interface Serial1/0.103 multipoint
ip address 10.1.2.1 255.255.255.248
frame-relay interface-dlci 103
frame-relay interface-dlci 104
!
Here the complete configuration for our frame relay network
 |
| Frame Relay Point-to-Point and Multipoint |
!
interface Serial1/0
no ip address
encapsulation frame-relay
serial restart-delay 0
!
interface Serial1/0.102 point-to-point
ip address 10.1.1.1 255.255.255.252
frame-relay interface-dlci 102
!
interface Serial1/0.103 multipoint
ip address 10.1.2.1 255.255.255.248
frame-relay interface-dlci 103
frame-relay interface-dlci 104
!
R2:
!
interface Serial1/0
ip address 10.1.1.2 255.255.255.252
encapsulation frame-relay
serial restart-delay 0
frame-relay interface-dlci 201
!
R3:
!
interface Serial1/0
ip address 10.1.2.2 255.255.255.248
encapsulation frame-relay
serial restart-delay 0
frame-relay interface-dlci 301
!
R4:
!
interface Serial1/0
ip address 10.1.2.3 255.255.255.248
encapsulation frame-relay
serial restart-delay 0
frame-relay interface-dlci 401
!
Verify Configuration
R1#show frame-relay map
Serial1/0.103 (up): ip 10.1.2.2 dlci 103(0x67,0x1870), dynamic,
broadcast,
CISCO, status defined, active
Serial1/0.103 (up): ip 10.1.2.3 dlci 104(0x68,0x1880), dynamic,
broadcast,
CISCO, status defined, active
Serial1/0.102 (up): point-to-point dlci, dlci 102(0x66,0x1860), broadcast
status defined, active
R1#show frame-relay lmi
LMI Statistics for interface Serial1/0 (Frame Relay DTE) LMI TYPE = ANSI
Invalid Unnumbered info 0 Invalid Prot Disc 0
Invalid dummy Call Ref 0 Invalid Msg Type 0
Invalid Status Message 0 Invalid Lock Shift 0
Invalid Information ID 0 Invalid Report IE Len 0
Invalid Report Request 0 Invalid Keep IE Len 0
Num Status Enq. Sent 436 Num Status msgs Rcvd 437
Num Update Status Rcvd 0 Num Status Timeouts 0
Last Full Status Req 00:00:23 Last Full Status Rcvd 00:00:23
R1#show frame-relay pvc
PVC Statistics for interface Serial1/0 (Frame Relay DTE)
Active Inactive Deleted Static
Local 3 0 0 0
Switched 0 0 0 0
Unused 0 0 0 0
DLCI = 102, DLCI USAGE = LOCAL, PVC STATUS = ACTIVE, INTERFACE = Serial1/0.102
input pkts 26 output pkts 93 in bytes 2284
out bytes 25942 dropped pkts 0 in pkts dropped 0
out pkts dropped 0 out bytes dropped 0
in FECN pkts 0 in BECN pkts 0 out FECN pkts 0
out BECN pkts 0 in DE pkts 0 out DE pkts 0
out bcast pkts 73 out bcast bytes 23862
5 minute input rate 0 bits/sec, 0 packets/sec
5 minute output rate 0 bits/sec, 0 packets/sec
pvc create time 01:13:19, last time pvc status changed 01:12:59
DLCI = 103, DLCI USAGE = LOCAL, PVC STATUS = ACTIVE, INTERFACE = Serial1/0.103
input pkts 27 output pkts 27 in bytes 2318
out bytes 2318 dropped pkts 0 in pkts dropped 0
out pkts dropped 0 out bytes dropped 0
in FECN pkts 0 in BECN pkts 0 out FECN pkts 0
out BECN pkts 0 in DE pkts 0 out DE pkts 0
out bcast pkts 7 out bcast bytes 238
5 minute input rate 0 bits/sec, 0 packets/sec
5 minute output rate 0 bits/sec, 0 packets/sec
pvc create time 01:13:20, last time pvc status changed 01:13:00
DLCI = 104, DLCI USAGE = LOCAL, PVC STATUS = ACTIVE, INTERFACE = Serial1/0.103
input pkts 16 output pkts 20 in bytes 1244
out bytes 1380 dropped pkts 0 in pkts dropped 0
out pkts dropped 0 out bytes dropped 0
in FECN pkts 0 in BECN pkts 0 out FECN pkts 0
out BECN pkts 0 in DE pkts 0 out DE pkts 0
out bcast pkts 10 out bcast bytes 340
5 minute input rate 0 bits/sec, 0 packets/sec
5 minute output rate 0 bits/sec, 0 packets/sec
pvc create time 01:13:21, last time pvc status changed 01:13:01
Test Ping:
R1#ping 10.1.1.2
Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to 10.1.1.2, timeout is 2 seconds:
!!!!!
Success rate is 100 percent (5/5), round-trip min/avg/max = 8/25/36 ms
R1#ping 10.1.2.2
Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to 10.1.2.2, timeout is 2 seconds:
!!!!!
Success rate is 100 percent (5/5), round-trip min/avg/max = 8/28/68 ms
R1#ping 10.1.2.3
Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to 10.1.2.3, timeout is 2 seconds:
!!!!!
Success rate is 100 percent (5/5), round-trip min/avg/max = 4/17/40 ms
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