EIGRP Passive interface

EIGRP Passive Interface

EIGRP does not send any hello messages on the passive interface. EIGRP router ignores any EIGRP messages received on the interface which means no neighbors relation but EIGRP still advertises about the connected subnet only if matched with an EIGRP network command.

let's understand it more clearly

In our diagram, we have 4 Routers and a switch, Router_1 and Router_2. Down side there’s the network 10.1.1.1/8 with a switch connected to Router_1 and come computers. We need Router_1 to advertise the network 10.1.1.1/8 to our Autonomous system and of course router_2 but we have one problem there is no router on the 10.1.1.1/8 network. There is no reason to send EIGRP hello packets on the interface FastEthernet 0/0

There is also one big problem it’s a security risk if someone connects his computer to our switch with an application that advertises fake routes and becomes EIGRP neighbors with our router1 this is a nightmare.

We do not allow this to be happening in our autonomous system. To prevent this from happening we need a passive-interface command and this feature ensures that the network is advertised in EIGRP but this feature disable  hello packets on the interface ( in our case fatethernet 0/0 )

There is option redistribution with the help of this command we can prevent our nightmare. 

let see the configuration:

Topology:-

TASK:

• configure the topology as per the diagram 
• assign IP addresses to their interfaces 
• configure EIGRP 61234 and advertise all the interfaces 
• configure serial 3/1 and 3/3 passive interfaces 

R1(config)#Interface fastethernet 0/0
R1(config-if)#Ip address 10.1.1.1   255.0.0.0
R1(config-if)#No shutdown
R1(config-if)#No keepalive
R1(config-if)#Exit

R1(config)#Interface serial 3/0
R1(config-if)#Ip address  1.1.1.1 255.0.0.0
R1(config-if)#No shutdown
R1(config-if)#Exit

R2(config)#Interface fastethernet 0/0

R2(config-if)#Ip address 20.1.1.1   255.0.0.0

R2(config-if)#No shutdown

R2(config-if)#No keepalive

R2(config-if)#Exit

R2(config)#Interface serial 3/1

R2(config-if)#Ip address  2.2.2.1  255.0.0.0

R2(config-if)#No shutdown

R2(config-if)#Exit

R3(config)#Interface fastethernet 0/0

R3(config-if)#Ip address 30.1.1.1   255.0.0.0

R3(config-if)#No shutdown

R3(config-if)#No keepalive

R3(config-if)#Exit

R3(config)#Interface serial 3/1

R3(config-if)#Ip address  3.3.3.1  255.0.0.0

R3(config-if)#No shutdown

R3(config-if)#Exit

R4(config)#Interface fastethernet 0/0

R4(config-if)#Ip address 40.1.1.1   255.0.0.0

R4(config-if)#No shutdown

R4(config-if)#No keepalive

R4(config-if)#Exit

R4(config)#Interface serial 3/1

R4(config-if)#Ip address  4.4.4.1  255.0.0.0

R4(config-if)#No shutdown

R4(config-if)#Exit

R5(config)#Interface fastethernet 0/0

R5(config-if)#Ip address 50.1.1.1   255.0.0.0

R5(config-if)#No shutdown

R5(config-if)#No keepalive

R5(config-if)#Exit

R5(config)#Interface serial 3/0

R5(config-if)#Ip address  1.1.1.2  255.0.0.0

R5(config-if)#No shutdown

R5(config-if)#Exit

R5(config)#Interface serial 3/1

R5(config-if)#Ip address  2.2.2.2 255.0.0.0

R5(config-if)#No shutdown

R5(config-if)#Exit

R5(config)#Interface serial 3/2

R5(config-if)#Ip address  3.3.3.2 255.0.0.0

R5(config-if)#No shutdown

R5(config-if)#Exit

R5(config)#Interface serial 3/3

R5(config-if)#Ip address  4.4.4.2 255.0.0.0

R5(config-if)#No shutdown

R5(config-if)#Exit

R1#show ip interface brief
Interface              IP-Address      OK? Method Status                Protocol
FastEthernet0/0        10.1.1.1        YES manual up                    up
Serial3/0                    1.1.1.1         YES manual up                    up

R2#show ip interface brief

Interface              IP-Address      OK? Method Status                Protocol

FastEthernet0/0        20.1.1.1        YES manual up                    up

Serial3/1                    2.2.2.1         YES manual up                    up

R3#show ip interface brief
Interface              IP-Address      OK? Method Status                Protocol
FastEthernet0/0        30.1.1.1        YES manual up                    up
Serial3/2                    3.3.3.1         YES manual up                    up

R4#show ip interface brief

Interface              IP-Address      OK? Method Status                Protocol

FastEthernet0/0        40.1.1.1        YES manual up                    up

Serial3/3                    4.4.4.1         YES manual up                    up

R5#show ip interface brief

Interface              IP-Address      OK? Method Status                Protocol

FastEthernet0/0        50.1.1.1        YES manual up                    up

Serial3/0                    1.1.1.2         YES manual up                    up

Serial3/1                    2.2.2.2         YES manual up                    up

Serial3/2                    3.3.3.2         YES manual up                    up

Serial3/3                   4.4.4.2         YES manual up                    up

R1(config)#router eigrp 61234

R1(config-router)#network 1.0.0.0

R1(config-router)#network 10.0.0.0

R1(config-router)#no auto-summary

R1(config-router)#end

R2(config)#router eigrp 61234

R2(config-router)#network 2.0.0.0

R2(config-router)#network 20.0.0.0

R2(config-router)#no auto-summary

R2(config-router)#end

R3(config)#router eigrp 61234

R3(config-router)#network 3.0.0.0

R3(config-router)#network 30.0.0.0

R3(config-router)#no auto-summary

R3(config-router)#end

R4(config)#router eigrp 61234

R4(config-router)#network 4.0.0.0

R4(config-router)#network 40.0.0.0

R4(config-router)#no auto-summary

R4(config-router)#end

R5(config)#router eigrp 61234
R5(config-router)#network 50.0.0.0
R5(config-router)#network 4.0.0.0
R5(config-router)#network 3.0.0.0
R5(config-router)#network 3.0.0.0
R5(config-router)#network 2.0.0.0
R5(config-router)#network 1.0.0.0
R5(config-router)#no auto-summary
R5(config-router)#end

*Dec  4 23:22:09.967: %DUAL-5-NBRCHANGE: EIGRP-IPv4 61234: Neighbor 4.4.4.1 (Serial3/3) is up: new adjacency

*Dec  4 23:22:14.343: %DUAL-5-NBRCHANGE: EIGRP-IPv4 61234: Neighbor 3.3.3.1 (Serial3/2) is up: new adjacency

*Dec  4 23:22:19.375: %DUAL-5-NBRCHANGE: EIGRP-IPv4 61234: Neighbor 2.2.2.1 (Serial3/1) is up: new adjacency

*Dec  4 23:22:25.239: %DUAL-5-NBRCHANGE: EIGRP-IPv4 61234: Neighbor 1.1.1.1 (Serial3/0) is up: new adjacency

R5#show ip eigrp neighbors
EIGRP-IPv4 Neighbors for AS(61234)
H   Address                 Interface              Hold Uptime   SRTT   RTO  Q  Seq
                                                   (sec)         (ms)       Cnt Num
3   1.1.1.1                 Se3/0                    12 00:04:14   78   468  0  3
2   2.2.2.1                 Se3/1                    12 00:04:20   79   474  0  5
1   3.3.3.1                 Se3/2                    14 00:04:25   57   342  0  7
0   4.4.4.1                 Se3/3                    14 00:04:30   67   402  0  9

R5#show ip route eigrp
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
       + - replicated route, % - next hop override

Gateway of last resort is not set

D     10.0.0.0/8 [90/2172416] via 1.1.1.1, 00:04:21, Serial3/0
D     20.0.0.0/8 [90/2172416] via 2.2.2.1, 00:04:27, Serial3/1
D     30.0.0.0/8 [90/2172416] via 3.3.3.1, 00:04:32, Serial3/2

D     40.0.0.0/8 [90/2172416] via 4.4.4.1, 00:04:37, Serial3/3

R5(config)#router eigrp 61234
R5(config-router)#passive-interface serial 3/1
R5(config-router)#passive-interface serial 3/3
R5(config-router)#end

*Dec  4 23:29:34.627: %DUAL-5-NBRCHANGE: EIGRP-IPv4 61234: Neighbor 2.2.2.1 (Serial3/1) is down: interface passive

*Dec  4 23:29:40.855: %DUAL-5-NBRCHANGE: EIGRP-IPv4 61234: Neighbor 4.4.4.1 (Serial3/3) is down: interface passive

R5#show ip eigrp interfaces

EIGRP-IPv4 Interfaces for AS(61234)

                              Xmit Queue   PeerQ        Mean   Pacing Time   Multicast    Pending

Interface              Peers  Un/Reliable  Un/Reliable  SRTT   Un/Reliable   Flow Timer   Routes

Fa0/0                    0        0/0       0/0           0       0/0            0           0

Se3/2                    1        0/0       0/0          60       0/16         236           0

Se3/0                    1        0/0       0/0          66       0/16         272           0

R5#show ip eigrp neighbors

EIGRP-IPv4 Neighbors for AS(61234)

H   Address                 Interface              Hold Uptime   SRTT   RTO  Q  Seq

                                                   (sec)         (ms)       Cnt Num

3   1.1.1.1                 Se3/0                    10 00:08:32   66   396  0  7

1   3.3.3.1                 Se3/2                    11 00:08:43   60   360  0  11

R5#show ip route eigrp

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

       + - replicated route, % - next hop override

Gateway of last resort is not set

D     10.0.0.0/8 [90/2172416] via 1.1.1.1, 00:08:39, Serial3/0

D     30.0.0.0/8 [90/2172416] via 3.3.3.1, 00:08:50, Serial3/2