R1 Configuration :

R1#sh run
output omitted
!
key chain pembajak
 key 1
  key-string elektro
 key 2
  key-string listrik
!
interface Serial0/0
 ip address 172.16.10.1 255.255.255.252
 ip authentication mode eigrp 100 md5
 ip authentication key-chain eigrp 100 pembajak
 clock rate 2000000
!
interface Serial0/1
 ip address 172.16.10.5 255.255.255.252
 ip authentication mode eigrp 100 md5
 ip authentication key-chain eigrp 100 pembajak
 clock rate 2000000
!
router eigrp 100
 network 172.16.10.0 0.0.0.3
 network 172.16.10.4 0.0.0.3
 network 192.168.1.1 0.0.0.0
 no auto-summary
 eigrp router-id 1.1.1.1
!
R1#

R2 Configuration :

R2#sh run
output omitted
!
interface Serial0/0
 ip address 172.16.10.2 255.255.255.252
 clock rate 2000000
!
interface Serial0/1
 ip address 172.16.10.9 255.255.255.252
 clock rate 2000000
!
router eigrp 100
 network 172.16.10.0 0.0.0.3
 network 172.16.10.8 0.0.0.3
 network 192.168.2.1 0.0.0.0
 no auto-summary
 eigrp router-id 2.2.2.2
!
R2#

R3 Configuration :

R3#sh run
!
key chain pembajak
 key 1
  key-string elektro
 key 2
  key-string listrik
!
interface Serial0/0
 ip address 172.16.10.6 255.255.255.252
 ip authentication mode eigrp 100 md5
 ip authentication key-chain eigrp 100 pembajak
 clock rate 2000000
!
interface Serial0/1
 ip address 172.16.10.10 255.255.255.252
 clock rate 2000000
!
router eigrp 100
 network 172.16.10.4 0.0.0.3
 network 172.16.10.8 0.0.0.3
 network 192.168.3.1 0.0.0.0
 no auto-summary
 eigrp router-id 3.3.3.3
R3#

Focuses on R2 configuration, there is no key chain config. With configuration above, eigrp on R1 just establish neighborship with R3 only. But R3 establish neighborship with R2 too because in interface s0/1 there is no ip authencation config. For verification, we can use :

  • show ip route eigrp
  • show ip eigrp neighbors
  • show ip eigrp interface detail
  • show key chain