OSPF Prefix-Suppression Concept & Configurations
OSPF Prefix-Suppression Concept & Configurations
OSPF broadcasts all LSAs, including the transit link LSAs, by default. Prefixes on transit links cause a lot of capacity to be consumed in the routing tables and LSDB of large OSPF networks.
No need to advertise the IP prefixes of transit-only networks which reduces less number of prefixes in the database (DB) of OSPF routers. OSPF prefix-suppression function reduces advertised Type 1 (router) and Type 2 (network) LSAs.
Lets take a scenario, where we have two users USER1 and USER2 which are connected via a network which has 4 routers, R1, R2, R3, and R4.
Ensuring that end-to-end reachability should be there, you can enable OSPF prefix-suppression on backbone links of R1, R2, R3, and R4 which will help in reduction of a number of LSAs.
⭐Related : Introduction to OSPF LSA Types
⭐Related : Introduction to OSPF Area Types
Fig 1.1- OSPF Prefix-Suppression Concept & Configurations
There are two ways to configure OSPF prefix-suppression
- Global mode
- Interface mode
Before OSPF prefix-suppression, let see what we get on Router R1 which is also connected to the USER 1 as below in red :
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, l - LISP
+ - replicated route, % - next hop override
Gateway of last resort is not set
10.0.0.0/8 is variably subnetted, 9 subnets, 2 masks
C 10.10.10.0/24 is directly connected, GigEthernet0/0
L 10.10.10.254/32 is directly connected, GigEthernet0/0
C 10.10.40.0/24 is directly connected, GigEthernet0/1
L 10.10.40.1/32 is directly connected, GigEthernet0/1
C 10.10.30.0/24 is directly connected, GigEthernet0/2
L 10.10.30.1/32 is directly connected, GigEthernet0/2
O 10.10.60.0/24 [110/20] via 10.10.40.2, 00:01:19, GigEthernet0/1
O 10.10.50.0/24 [110/20] via 10.10.30.2, 00:01:12, GigEthernet0/2
O 10.10.20.0/24 [110/30] via 10.10.60.2, 00:02:21, GigEthernet0/2
[110/30] via 10.10.40.2, 00:03:12, GigEthernet0/1
R1#
Let's configure first in Global mode R1,R2,R3 & R4
R1> enable
R1#conf t
Enter configuration commands, one per line. End with CNTL/Z.
R1(config)#prefix-suppression
R1(config-if)#end
R1#
R2> enable
R2#conf t
Enter configuration commands, one per line. End with CNTL/Z.
R2(config)#prefix-suppression
R2(config-if)#end
R2#
R3> enable
R3#conf t
Enter configuration commands, one per line. End with CNTL/Z.
R3(config)#prefix-suppression
R3(config-if)#end
R3#
R4> enable
R4#conf t
Enter configuration commands, one per line. End with CNTL/Z.
R4(config)#prefix-suppression
R4(config-if)#end
R4#
Now's lets see how to configure OSPF prefix-suppression on Interface level
Router R1:
Now Lets apply the OSPF prefix-suppression on the router R1 as shown below
Now Lets apply the OSPF prefix-suppression on the router R1 as shown below
R1> enable
R1#conf t
Enter configuration commands, one per line. End with CNTL/Z.
R1(config)#int Gi0/1
R1(config-if)#ip ospf prefix-suppression
R1(config-if)#int Gi0/2
R1(config-if)#ip ospf prefix-suppression
R1(config-if)#end
R1#
Router R2:
Lets apply the OSPF prefix-suppression on the router R2 as shown below
Lets apply the OSPF prefix-suppression on the router R2 as shown below
R2> enable
R2#conf t
Enter configuration commands, one per line. End with CNTL/Z.
R2(config)#int Gi0/0
R2(config-if)#ip ospf prefix-suppression
R2(config-if)#int Gi0/1
R2(config-if)#ip ospf prefix-suppression
R2(config-if)#end
R2#
Router R3:
Lets apply the OSPF prefix-suppression on the router R3 as shown below
Lets apply the OSPF prefix-suppression on the router R3 as shown below
R3> enable
R3#conf t
Enter configuration commands, one per line. End with CNTL/Z.
R3(config-if)#int Gi0/0
R3(config-if)# ip ospf prefix-suppression
R3(config-if)#int Gi0/1
R3(config-if)# ip ospf prefix-suppression
R3(config-if)#end
R3#
Router R4:
Lets apply the OSPF prefix-suppression on the router R4 as shown below
Lets apply the OSPF prefix-suppression on the router R4 as shown below
R4> enable
R4#conf t
Enter configuration commands, one per line. End with CNTL/Z.
R4(config)#int Gi0/1
R4(config-if)#ip ospf prefix-suppression
R4(config-if)#int Gi0/2
R4(config-if)#ip ospf prefix-suppression
R4(config-if)#end
R4#
After applying and configure OSPF prefix-suppression, Let's see what we get on Router R1 now. You will notice that the prefix's 10.10.50.0/24 & 10.10.60.0/24 is not in the routing table of router R1 as shown below:
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, l - LISP
+ - replicated route, % - next hop override
Gateway of last resort is not set
10.0.0.0/8 is variably subnetted, 9 subnets, 2 masks
C 10.10.10.0/24 is directly connected, GigEthernet0/0
L 10.10.10.254/32 is directly connected, GigEthernet0/0
C 10.10.40.0/24 is directly connected, GigEthernet0/1
L 10.10.40.1/32 is directly connected, GigEthernet0/1
C 10.10.30.0/24 is directly connected, GigEthernet0/2
L 10.10.30.1/32 is directly connected, GigEthernet0/2
O 10.10.20.0/24 [110/30] via 10.10.60.2, 00:02:21, GigEthernet0/2
[110/30] via 10.10.40.2, 00:03:12, GigEthernet0/1
R1#
Similarly, you can check the same on Router R4 where USER2 is connected, you will see that prefix's 10.10.30.0/24 & 10.10.40.0/24 will not be there in the routing table of router R4.
So your routing table is cleaned up with the use of OSPF prefix-suppression helps OSPF to converge fast.
⭐Related : Top OSPF Protocol Interview Questions and Answers
⭐Related : OSPF protocol : OSPF Packet Types
⭐Related : Top OSPF Protocol Interview Questions and Answers
⭐Related : OSPF protocol : OSPF Packet Types
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