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Auto summarization determines which routes are advertised.  R1 and R3 are both advertising 172.16.0.0/16.  

Changing auto summarization on R2 will have no impact.

adding a static route to 172.16.2.0 with a more specific mask will resolve the issue




IOU2# sh ip route
Codes: L - local, C - connected, S - static, R - RIP, M - mobile, B - BGP
      D - EIGRP, EX - EIGRP external, O - OSPF, IA - OSPF inter area
      N1 - OSPF NSSA external type 1, N2 - OSPF NSSA external type 2
      E1 - OSPF external type 1, E2 - OSPF external type 2
      i - IS-IS, su - IS-IS summary, L1 - IS-IS level-1, L2 - IS-IS level-2
      ia - IS-IS inter area, * - candidate default, U - per-user static route
      o - ODR, P - periodic downloaded static route, H - NHRP, l - LISP
      a - application route
      + - replicated route, % - next hop override, p - overrides from PfR

Gateway of last resort is not set

      10.0.0.0/8 is variably subnetted, 4 subnets, 2 masks
C        10.1.1.0/24 is directly connected, Ethernet1/0
L        10.1.1.2/32 is directly connected, Ethernet1/0
C        10.1.2.0/24 is directly connected, Ethernet0/0
L        10.1.2.2/32 is directly connected, Ethernet0/0
D    172.16.0.0/16 [90/409600] via 10.1.2.3, 00:00:07, Ethernet0/0
                    [90/409600] via 10.1.1.1, 00:00:07, Ethernet1/0
IOU2#conf t
Enter configuration commands, one per line.  End with CNTL/Z.
IOU2(config)#router eigrp 100
IOU2(config-router)#no auto
IOU2(config-router)#

IOU2(config-router)#do sh ip route
Codes: L - local, C - connected, S - static, R - RIP, M - mobile, B - BGP
      D - EIGRP, EX - EIGRP external, O - OSPF, IA - OSPF inter area
      N1 - OSPF NSSA external type 1, N2 - OSPF NSSA external type 2
      E1 - OSPF external type 1, E2 - OSPF external type 2
      i - IS-IS, su - IS-IS summary, L1 - IS-IS level-1, L2 - IS-IS level-2
      ia - IS-IS inter area, * - candidate default, U - per-user static route
      o - ODR, P - periodic downloaded static route, H - NHRP, l - LISP
      a - application route
      + - replicated route, % - next hop override, p - overrides from PfR

Gateway of last resort is not set

      10.0.0.0/8 is variably subnetted, 4 subnets, 2 masks
C        10.1.1.0/24 is directly connected, Ethernet1/0
L        10.1.1.2/32 is directly connected, Ethernet1/0
C        10.1.2.0/24 is directly connected, Ethernet0/0
L        10.1.2.2/32 is directly connected, Ethernet0/0
D    172.16.0.0/16 [90/409600] via 10.1.2.3, 00:00:31, Ethernet0/0
                    [90/409600] via 10.1.1.1, 00:00:31, Ethernet1/0
IOU2(config-router)#exit
IOU2(config)#ip route 172.17.2.0 255.255.255.0 10.1.2.3 254
IOU2(config)#do sh ip route
Codes: L - local, C - connected, S - static, R - RIP, M - mobile, B - BGP
      D - EIGRP, EX - EIGRP external, O - OSPF, IA - OSPF inter area
      N1 - OSPF NSSA external type 1, N2 - OSPF NSSA external type 2
      E1 - OSPF external type 1, E2 - OSPF external type 2
      i - IS-IS, su - IS-IS summary, L1 - IS-IS level-1, L2 - IS-IS level-2
      ia - IS-IS inter area, * - candidate default, U - per-user static route
      o - ODR, P - periodic downloaded static route, H - NHRP, l - LISP
      a - application route
      + - replicated route, % - next hop override, p - overrides from PfR

Gateway of last resort is not set

      10.0.0.0/8 is variably subnetted, 4 subnets, 2 masks
C        10.1.1.0/24 is directly connected, Ethernet1/0
L        10.1.1.2/32 is directly connected, Ethernet1/0
C        10.1.2.0/24 is directly connected, Ethernet0/0
L        10.1.2.2/32 is directly connected, Ethernet0/0
D    172.16.0.0/16 [90/409600] via 10.1.2.3, 00:01:23, Ethernet0/0
                    [90/409600] via 10.1.1.1, 00:01:23, Ethernet1/0
      172.17.0.0/24 is subnetted, 1 subnets
S        172.17.2.0 [254/0] via 10.1.2.3
IOU2(config)#
IOU2#
*Feb 15 17:02:03.432: %SYS-5-CONFIG_I: Configured from console by console
IOU2#