EIGRP: Route Filtering

EIGRP: Route Filtering

 

·         We can filter the routes in EIGRP using distribute-list

 

·         Let me take a small topology to configure route filtering in EIGRP

 

R1------S1/0-------------s1/0----R2----s1/1--------------s1/1------R3

 

10.1.12.1--------------10.1.12.2-----11.1.23.2--------------11.1.23.3

 

 

R1(config-router)#do show run | sec router

router eigrp 100

 network 1.1.1.1 0.0.0.0

 network 10.1.12.1 0.0.0.0

 network 11.11.11.11 0.0.0.0

 network 111.111.111.111 0.0.0.0

 no auto-summary

R1(config-router)#

 

R2(config-router)#do show run | sec router

router eigrp 100

 network 10.1.12.2 0.0.0.0

 network 11.1.23.2 0.0.0.0

 no auto-summary

R2(config-router)#

 

R3(config-router)#do show run | sec router

router eigrp 100

 network 11.1.23.3 0.0.0.0

 no auto-summary

R3(config-router)#

 

R1(config-router)#do show ip route

Codes: 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

 

Gateway of last resort is not set

 

     1.0.0.0/32 is subnetted, 1 subnets

C       1.1.1.1 is directly connected, Loopback0

     111.0.0.0/32 is subnetted, 1 subnets

C       111.111.111.111 is directly connected, Loopback2

     10.0.0.0/24 is subnetted, 1 subnets

C       10.1.12.0 is directly connected, Serial1/0

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

C       11.11.11.11/32 is directly connected, Loopback1

D       11.1.23.0/24 [90/2681856] via 10.1.12.2, 00:01:43, Serial1/0

R1(config-router)#

 

R2(config-router)#do show ip route

Codes: 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

 

Gateway of last resort is not set

 

     1.0.0.0/32 is subnetted, 1 subnets

D       1.1.1.1 [90/2297856] via 10.1.12.1, 00:05:54, Serial1/0

     2.0.0.0/32 is subnetted, 1 subnets

C       2.2.2.2 is directly connected, Loopback0

     111.0.0.0/32 is subnetted, 1 subnets

D       111.111.111.111 [90/2297856] via 10.1.12.1, 00:05:43, Serial1/0

     10.0.0.0/24 is subnetted, 1 subnets

C       10.1.12.0 is directly connected, Serial1/0

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

D       11.11.11.11/32 [90/2297856] via 10.1.12.1, 00:05:49, Serial1/0

C       11.1.23.0/24 is directly connected, Serial1/1

R2(config-router)#

 

R3(config-router)#do show ip  route

Codes: 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

 

Gateway of last resort is not set

 

     1.0.0.0/32 is subnetted, 1 subnets

D       1.1.1.1 [90/2809856] via 11.1.23.2, 00:01:56, Serial1/1

     111.0.0.0/32 is subnetted, 1 subnets

D       111.111.111.111 [90/2809856] via 11.1.23.2, 00:01:56, Serial1/1

     10.0.0.0/24 is subnetted, 1 subnets

D       10.1.12.0 [90/2681856] via 11.1.23.2, 00:01:56, Serial1/1

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

D       11.11.11.11/32 [90/2809856] via 11.1.23.2, 00:01:56, Serial1/1

C       11.1.23.0/24 is directly connected, Serial1/1

R3(config-router)#

 

Observation:

·         EIGRP is configured between R1 &R2 and R2 & R3

·         R1 has advertised 1.1.1.1/32, 11.11.11.11/32 and 111.111.111.111/32 into EIGRP

 

·         If R2 wants only 11.11.11.11/32, then we can filter the other routes using distribute-list

·         Let me configure this in R2:

 

R2(config-router)#ip prefix-list 11 seq 5 permit 11.11.11.11/32

R2(config-router)#ip router eigrp 100

R2(config-router)#distribute-list prefix 11 in serial 1/0

 

R2(config)#do show ip route

Codes: 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

 

Gateway of last resort is not set

 

     2.0.0.0/32 is subnetted, 1 subnets

C       2.2.2.2 is directly connected, Loopback0

     10.0.0.0/24 is subnetted, 1 subnets

C       10.1.12.0 is directly connected, Serial1/0

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

D       11.11.11.11/32 [90/2297856] via 10.1.12.1, 00:10:25, Serial1/0

C       11.1.23.0/24 is directly connected, Serial1/1

R2(config)#

 

R3(config-router)#do show ip  route

Codes: 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

 

Gateway of last resort is not set

 

     10.0.0.0/24 is subnetted, 1 subnets

D       10.1.12.0 [90/2681856] via 11.1.23.2, 00:08:00, Serial1/1

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

D       11.11.11.11/32 [90/2809856] via 11.1.23.2, 00:08:00, Serial1/1

C       11.1.23.0/24 is directly connected, Serial1/1

R3(config-router)#

 

Observation:

·         Prefix-list is matched only 11.11.11.11/32

·         R2 now has only 11.11.11.11/32

·         Even R3 has only 11.11.11.11/32

·         If I have to configure this in R1, then I have to apply it OUT

 

·         This is the major difference between router filtering in OSPF and other protocols.

·         In OSPF, we can’t do route-filtering OUT, we can do only IN.

·         Even if we do filtering IN, they will still be presenting in the OSPF database, they just don’t make it to OSPF routing table.