Route Filter Using Prefix List – EIGRP

In order to filter routes in EIGRP update message we need
to configure Distribute list under EIGRP process. Distribute list can be
applied to all or specific interface.

In below example we are applying distribution list on
particular interface in order to filter route for particular neighbor.

There are three ways by which we can match routes to
apply filter.

1.
Access list

2.
Prefix list

3.
Route-map

In below example, we want to advertise only 11.11.11.11/24
from R2 to R3 and suppress all other routes.

There are two routes, one is 11.11.11.11/32 (advertised
from R1) and 11.11.11.11/24 (advertised
from R2).

Prefix list will do this for us and will block all routes expect 11.11.11..0/24.

R1:-

interface Loopback1

ip address 1.1.1.1
255.255.255.255

interface Loopback11

ip address
11.11.11.11 255.255.255.255

interface Loopback111

ip address
111.111.111.111 255.255.255.255

interface FastEthernet0/0

Description “ connected to R2”

ip address
12.12.12.1 255.255.255.0

no shut

router eigrp 1

network 1.1.1.1
0.0.0.0

network
11.11.11.11 0.0.0.0

network 12.12.12.0
0.0.0.255

network
111.111.111.111 0.0.0.0

R2:-

interface Loopback0

ip address 2.2.2.2
255.255.255.255

ip ospf network
point-to-point

no shut

interface Loopback2

ip address
11.11.11.1 255.255.255.0

interface FastEthernet0/0

ip address
12.12.12.2 255.255.255.0

no shut

interface FastEthernet0/1

ip address
23.23.23.2 255.255.255.0

no shut

router eigrp 1

distribute-list prefix Route_Filter out FastEthernet0/1

network 11.11.11.1
0.0.0.0

network 12.12.12.0
0.0.0.255

network 23.23.23.2
0.0.0.0

ip prefix-list Route_Filter seq 5 permit 11.11.11.0/24 \Only permit 11.11.11.0/24 \

R3:-

interface FastEthernet0/1

ip address
23.23.23.3 255.255.255.0

no shut

router eigrp 1

network 23.23.23.3
0.0.0.0

Verification:-

R1:-

R1# 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

+ –
replicated route, % – next hop override

Gateway of last resort is not set

1.0.0.0/32 is
subnetted, 1 subnets

C 1.1.1.1 is
directly connected, Loopback1

11.0.0.0/8 is
variably subnetted, 2 subnets, 2 masks

D 11.11.11.0/24 [90/156160] via
12.12.12.2, 02:20:21, FastEthernet0/0
\Advertised from R2

C
11.11.11.11/32 is directly connected, Loopback11

12.0.0.0/8 is
variably subnetted, 2 subnets, 2 masks

C
12.12.12.0/24 is directly connected, FastEthernet0/0

L
12.12.12.1/32 is directly connected, FastEthernet0/0

23.0.0.0/24
is subnetted, 1 subnets

D 23.23.23.0
[90/30720] via 12.12.12.2, 02:22:54, FastEthernet0/0

111.0.0.0/32
is subnetted, 1 subnets

C
111.111.111.111 is directly connected, Loopback111

R2:-

R2#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

+ –
replicated route, % – next hop override

Gateway of last resort is not set

1.0.0.0/32 is
subnetted, 1 subnets

D 1.1.1.1
[90/156160] via 12.12.12.1, 04:10:00, FastEthernet0/0

2.0.0.0/32 is
subnetted, 1 subnets

C 2.2.2.2 is
directly connected, Loopback0

11.0.0.0/8 is
variably subnetted, 3 subnets, 2 masks

C
11.11.11.0/24 is directly connected, Loopback2

L
11.11.11.1/32 is directly connected, Loopback2

D
11.11.11.11/32 [90/156160] via 12.12.12.1, 04:10:00, FastEthernet0/0

12.0.0.0/8 is
variably subnetted, 2 subnets, 2 masks

C
12.12.12.0/24 is directly connected, FastEthernet0/0

L
12.12.12.2/32 is directly connected, FastEthernet0/0

23.0.0.0/8 is
variably subnetted, 2 subnets, 2 masks

C
23.23.23.0/24 is directly connected, FastEthernet0/1

L
23.23.23.2/32 is directly connected, FastEthernet0/1

111.0.0.0/32
is subnetted, 1 subnets

D
111.111.111.111 [90/156160] via 12.12.12.1, 02:53:00, FastEthernet0/0

R2#sh ip eigrp neighbors

EIGRP-IPv4 Neighbors for AS(1)

H Address Interface Hold Uptime SRTT RTO
Q Seq

(sec) (ms) Cnt Num

1 23.23.23.3 Fa0/1 12 00:03:23 14
200 0 17

0 12.12.12.1 Fa0/0 11 04:10:18 53
318 0 14

R3:-

R3#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

+ –
replicated route, % – next hop override

Gateway of last resort is not set

11.0.0.0/24
is subnetted, 1 subnets

D 11.11.11.0 [90/156160] via 23.23.23.2,
00:04:09, FastEthernet0/1 \ only allowed prefix on R2

23.0.0.0/8 is
variably subnetted, 2 subnets, 2 masks

C
23.23.23.0/24 is directly connected, FastEthernet0/1

L
23.23.23.3/32 is directly connected, FastEthernet0/1

34.0.0.0/8 is
variably subnetted, 2 subnets, 2 masks

C
34.34.34.0/24 is directly connected, FastEthernet1/0

L
34.34.34.3/32 is directly connected, FastEthernet1/0

R3#sh ip eigrp neighbors

EIGRP-IPv4 Neighbors for AS(1)

H Address Interface Hold Uptime SRTT
RTO Q Seq

(sec) (ms) Cnt Num

0 23.23.23.2 Fa0/1 12 00:03:45 42
252 0 31