EIGRP: Spoke-to-spoke neighbor relationship through hub

Rack1R1#show running-config interface s0/0
Building configuration...

Current configuration : 227 bytes
!
interface Serial0/0
 ip address 180.1.123.1 255.255.255.0
 encapsulation frame-relay
 no fair-queue
 frame-relay map ip 180.1.123.3 103 broadcast
 frame-relay map ip 180.1.123.2 102 broadcast
 no frame-relay inverse-arp
end

Rack1R1#show running-config | section eigrp
router eigrp 420
 network 180.1.123.1 0.0.0.0
 no auto-summary

Rack1R2#show running-config interface s0/0
Building configuration...

Current configuration : 217 bytes
!
interface Serial0/0
 ip address 180.1.123.2 255.255.255.0
 encapsulation frame-relay
 no fair-queue
 frame-relay map ip 180.1.123.3 201
 frame-relay map ip 180.1.123.1 201 broadcast
 no frame-relay inverse-arp
end

Rack1R2#show running-config | section eigrp
router eigrp 420
 network 2.2.2.2 0.0.0.0
 network 180.1.123.2 0.0.0.0
 no auto-summary

Rack1R3#show running-config interface s1/0
Building configuration...

Current configuration : 217 bytes
!
interface Serial1/0
 ip address 180.1.123.3 255.255.255.0
 encapsulation frame-relay
 no fair-queue
 frame-relay map ip 180.1.123.1 301 broadcast
 frame-relay map ip 180.1.123.2 301
 no frame-relay inverse-arp
end

Rack1R3#show running-config | section eigrp
router eigrp 420
 network 3.3.3.3 0.0.0.0
 network 180.1.123.3 0.0.0.0
 no auto-summary

The current routing table on these routers are as following:

Rack1R1#show ip route eigrp
     2.0.0.0/24 is subnetted, 1 subnets
D       2.2.2.0 [90/2297856] via 180.1.123.2, 00:12:32, Serial0/0
     3.0.0.0/24 is subnetted, 1 subnets
D       3.3.3.0 [90/2297856] via 180.1.123.3, 00:12:27, Serial0/0

Rack1R2#show ip route eigrp

Rack1R3#show ip route eigrp

So we can see that R1 knows about the 2.2.2.0/24 and 3.3.3.0/24 networks from the spokes, but spokes are not getting each other's loopback networks due to the split horizon on the hub. One way we can solve this problem by disabling the split horizon on R1. But since we are not allowed to do so, we can configure eigrp neighbor relationship between spokes by sending unicast hellos (TTL > 1).

The final configuration is below:

Rack1R1#show running-config | section eigrp
router eigrp 420
 network 180.1.123.1 0.0.0.0
 no auto-summary
 neighbor 180.1.123.3 Serial0/0
 neighbor 180.1.123.2 Serial0/0

Rack1R2#show running-config | section eigrp
router eigrp 420
 network 2.2.2.2 0.0.0.0
 network 180.1.123.2 0.0.0.0
 no auto-summary
 neighbor 180.1.123.3 Serial0/0
 neighbor 180.1.123.1 Serial0/0

Rack1R3#show running-config | section eigrp
router eigrp 420
 network 3.3.3.3 0.0.0.0
 network 180.1.123.3 0.0.0.0
 no auto-summary
 neighbor 180.1.123.2 Serial1/0
 neighbor 180.1.123.1 Serial1/0

Rack1R1#show ip eigrp neighbors
IP-EIGRP neighbors for process 420
H   Address                 Interface       Hold Uptime   SRTT   RTO  Q  Seq
                                            (sec)         (ms)       Cnt Num
1   180.1.123.3             Se0/0            154 00:14:51   43   258  0  13
0   180.1.123.2             Se0/0            152 00:16:10   45   270  0  13

Rack1R2#show ip eigrp neighbors
IP-EIGRP neighbors for process 420
H   Address                 Interface       Hold Uptime   SRTT   RTO  Q  Seq
                                            (sec)         (ms)       Cnt Num
1   180.1.123.3             Se0/0            142 00:12:31  208  1248  0  13
0   180.1.123.1             Se0/0            166 00:16:22   73   438  0  18

Rack1R3#show ip eigrp neighbors
IP-EIGRP neighbors for process 420
H   Address                 Interface       Hold Uptime   SRTT   RTO  Q  Seq
                                            (sec)         (ms)       Cnt Num
1   180.1.123.2             Se1/0            134 00:12:37  383  2298  0  13
0   180.1.123.1             Se1/0            160 00:15:09 1041  5000  0  18

Rack1R1#show ip route eigrp
     2.0.0.0/24 is subnetted, 1 subnets
D       2.2.2.0 [90/2297856] via 180.1.123.2, 00:17:52, Serial0/0
     3.0.0.0/24 is subnetted, 1 subnets
D       3.3.3.0 [90/2297856] via 180.1.123.3, 00:16:31, Serial0/0

Rack1R2#show ip route eigrp
     3.0.0.0/24 is subnetted, 1 subnets
D       3.3.3.0 [90/2297856] via 180.1.123.3, 00:14:11, Serial0/0

Rack1R3#show ip route eigrp
     2.0.0.0/24 is subnetted, 1 subnets
D       2.2.2.0 [90/20640000] via 180.1.123.2, 00:14:17, Serial1/0

So we can see that the eigrp neighbor relationship between the spokes has been established and routes have also been exchanged between them .