This function lets you turn off the transit link advertisement, which will make the RIB smaller.
First, we’ll take a look at the topology down below.
R1, R2, and R3 are all connected together with Ethernet cables. OSPF process 1 is running on all three routers. All of them have had the command “network 0.0.0.0 255.255.255.255 area 0” run on them.
R3#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
O 1.1.1.1 [110/21] via 100.1.23.2, 00:18:40, FastEthernet0/1
2.0.0.0/32 is subnetted, 1 subnets
O 2.2.2.2 [110/11] via 100.1.23.2, 00:18:50, FastEthernet0/1
100.0.0.0/24 is subnetted, 2 subnets
O 100.1.12.0 [110/20] via 100.1.23.2, 00:18:50, FastEthernet0/1
C 100.1.23.0 is directly connected, FastEthernet0/1
3.0.0.0/32 is subnetted, 1 subnets
C 3.3.3.3 is directly connected, Loopback0
Using OSPF, R3 discovers R1’s loopback IP (1.1.1.1/32) and the connection between R1 and R2 (100.1.12.0/24) as we can see.
Here in R3’s OSPF database, we can verify the network LSA.
R3#show ip ospf database network
OSPF Router with ID (3.3.3.3) (Process ID 1)
Net Link States (Area 0)
Routing Bit Set on this LSA
LS age: 1327
Options: (No TOS-capability, DC)
LS Type: Network Links
Link State ID: 100.1.12.2 (address of Designated Router)
Advertising Router: 2.2.2.2
LS Seq Number: 80000001
Checksum: 0x1C96
Length: 32
Network Mask: /24
Attached Router: 2.2.2.2
Attached Router: 1.1.1.1
Routing Bit Set on this LSA
LS age: 1329
Options: (No TOS-capability, DC)
LS Type: Network Links
Link State ID: 100.1.23.3 (address of Designated Router)
Advertising Router: 3.3.3.3
LS Seq Number: 80000001
Checksum: 0xCECB
Length: 32
Network Mask: /24
Attached Router: 3.3.3.3
Attached Router: 2.2.2.2
As can be seen from the output that is shown above, R1 is the DR for the segment that is located between R1 and R2. It promotes the transit network, which can be found at 100.1.12.0/24.
The following step is to execute the command “ip ospf prefix-suppression” on the f0/0 interface(facing R2) of R1.
R1(config)#int FastEthernet0/0
R1(config-if)#ip ospf prefix-suppression
The entry for the loopback IP (1.1.1.1/32) is still visible in the routing table of R3, but the prefix of the transit link (100.1.12.0/24) has been removed.
R3#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
O 1.1.1.1 [110/21] via 100.1.23.2, 00:00:35, FastEthernet0/1
2.0.0.0/32 is subnetted, 1 subnets
O 2.2.2.2 [110/11] via 100.1.23.2, 00:00:35, FastEthernet0/1
100.0.0.0/24 is subnetted, 1 subnets
C 100.1.23.0 is directly connected, FastEthernet0/1
3.0.0.0/32 is subnetted, 1 subnets
C 3.3.3.3 is directly connected, Loopback0
Examining the network LSA once more in the OSPF database on R3.
R3#show ip ospf database network
OSPF Router with ID (3.3.3.3) (Process ID 1)
Net Link States (Area 0)
LS age: 99
Options: (No TOS-capability, DC)
LS Type: Network Links
Link State ID: 100.1.12.1 (address of Designated Router)
Advertising Router: 1.1.1.1
LS Seq Number: 80000001
Checksum: 0x5463
Length: 32
Network Mask: /32
Attached Router: 1.1.1.1
Attached Router: 2.2.2.2
Following the execution of the command for prefix suppression, R1 is currently utilizing a /32 network mask to counteract its network LSAs on the network.
By doing a test of the link between R3 and R1’s Loopback, we are able to confirm that it is fully operational.
R3#ping 1.1.1.1
Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to 1.1.1.1, timeout is 2 seconds:
!!!!!
Success rate is 100 percent (5/5), round-trip min/avg/max = 16/28/36 ms
Now another question is that What happens when we use a point-to-point link for this? In a point-to-point network, DR/BDR is not an option.
We may test it by configuring the link between R2 and R3 as an ospf point-to-point network
The OSPF process advertises the Loopback on R3, making it visible in R1’s routing table.
R1#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
2.0.0.0/32 is subnetted, 1 subnets
O 2.2.2.2 [110/11] via 100.1.12.2, 00:06:19, FastEthernet0/0
100.0.0.0/24 is subnetted, 2 subnets
C 100.1.12.0 is directly connected, FastEthernet0/0
O 100.1.23.0 [110/20] via 100.1.12.2, 00:06:09, FastEthernet0/0
3.0.0.0/32 is subnetted, 1 subnets
O 3.3.3.3 [110/21] via 100.1.12.2, 00:05:59, FastEthernet0/0
The transit subnet that connects R2 and R3 (10.1.23.0/24) is also included in the routing table, as can be seen by us.
We can see that R3 advertises this subnet using Link Type-3 if we check the Router LSA on R1, which is something that we can do.
R1#show ip ospf database router 3.3.3.3
OSPF Router with ID (1.1.1.1) (Process ID 1)
Router Link States (Area 0)
LS age: 84
Options: (No TOS-capability, DC)
LS Type: Router Links
Link State ID: 3.3.3.3
Advertising Router: 3.3.3.3
LS Seq Number: 8000000C
Checksum: 0x9933
Length: 60
Number of Links: 3
Link connected to: a Stub Network
(Link ID) Network/subnet number: 3.3.3.3
(Link Data) Network Mask: 255.255.255.255
Number of TOS metrics: 0
TOS 0 Metrics: 1
Link connected to: another Router (point-to-point)
(Link ID) Neighboring Router ID: 2.2.2.2
(Link Data) Router Interface address: 100.1.23.3
Number of TOS metrics: 0
TOS 0 Metrics: 10
Link connected to: a Stub Network
(Link ID) Network/subnet number: 100.1.23.0
(Link Data) Network Mask: 255.255.255.0
Number of TOS metrics: 0
TOS 0 Metrics: 10
Applying the command “ip ospf prefix-suppression” to R3’s interface is the next step.
R3(config)#interface FastEthernet0/1
R3(config-if)#ip ospf prefix-suppression
Taking another look at R1’s OSPF database
R1#show ip ospf database router 3.3.3.3
OSPF Router with ID (1.1.1.1) (Process ID 1)
Router Link States (Area 0)
LS age: 115
Options: (No TOS-capability, DC)
LS Type: Router Links
Link State ID: 3.3.3.3
Advertising Router: 3.3.3.3
LS Seq Number: 8000000D
Checksum: 0x1C46
Length: 48
Number of Links: 2
Link connected to: a Stub Network
(Link ID) Network/subnet number: 3.3.3.3
(Link Data) Network Mask: 255.255.255.255
Number of TOS metrics: 0
TOS 0 Metrics: 1
Link connected to: another Router (point-to-point)
(Link ID) Neighboring Router ID: 2.2.2.2
(Link Data) Router Interface address: 100.1.23.3
Number of TOS metrics: 0
TOS 0 Metrics: 10
It looks like R3 has stopped selling the “Type 3” entry.
Check R1’s routing table to see if the record for the transit subnet (100.1.23.0/24) has been taken out.
