OSPF and Secondary Addresses
OSPF and Secondary Addresses
There are two rules of OSPF when it comes to Secondary IP addresses on an interface-
- OSPF will advertise a secondary subnetwork only if it also running on the primary subnetwork.
- Secondary networks are considered STUB network and hence no Hellos are sent on them. Hence, no adjacencies can be established on secondary networks.
Network topology:
Configuration:
R1 Configuration
interface Serial 1/0
ip address 192.168.10.1 255.255.255.240
!
interface Serial 1/1
ip address 192.168.20.1 255.255.255.240
!
router ospf 1
router-id 1.1.1.1
network 192.168.20.0 0.0.0.15 area 0
network 192.168.10.0 0.0.0.15 area 1
!
The Fastethernet 0/0 interface of R2 router is configured with a secondary IP address from subnet 172.19.35.0/24.
R2 Configuration
interface Serial 1/0
ip address 192.168.10.2 255.255.255.240
!
interface Fastethernet 0/0
ip address 192.168.10.33 255.255.255.240
ip address 172.19.35.2 255.255.255.0 secondary
!
router ospf 1
router-id 2.2.2.2
network 192.168.10.32 0.0.0.15 area 1
network 172.19.35.0 0.0.0.255 area 1
!
OSPF is not configured on R3 router. However, a static route is configured to reach supernet 192.16.0.0/16
R3 Configuration
interface Fastethernet 0/0
ip address 172.19.35.1 255.255.255.0
!
ip route 192.168.0.0 255.255.0.0 172.19.35.2
!
OSPF Neighbor Adjacency:
R1 router forms adjacency with R2 router. While R2 router forms adjacency with only R1 router. Even if R3 router was configured with OSPF, it will not form adjacency with R2 router since R2 will not send Hellos to secondary networks. R2 router does not act as an ASBR; it is only connected to R3 router via a secondary network.
OSPF Neighbor Adjacency
R1# show ip ospf neighbor
Neighbor ID Pri State Dead Time Address Interface
2.2.2.2 0 FULL/ - 00:00:34 192.168.10.2 Serial1/0
R2# show ip ospf neighbor
Neighbor ID Pri State Dead Time Address Interface
1.1.1.1 0 FULL/ - 00:00:38 192.168.10.1 Serial1/0
R2# show ip ospf
Routing Process "ospf 1" with ID 2.2.2.2
Start time: 01:33:47.132, Time elapsed: 00:00:10.536
Supports only single TOS(TOS0) routes
Supports opaque LSA
Supports Link-local Signaling (LLS)
Supports area transit capability
Router is not originating router-LSAs with maximum metric
Initial SPF schedule delay 5000 msecs
Minimum hold time between two consecutive SPFs 10000 msecs
Maximum wait time between two consecutive SPFs 10000 msecs
Incremental-SPF disabled
Minimum LSA interval 5 secs
Minimum LSA arrival 1000 msecs
LSA group pacing timer 240 secs
Interface flood pacing timer 33 msecs
Retransmission pacing timer 66 msecs
Number of external LSA 0. Checksum Sum 0x000000
Number of opaque AS LSA 0. Checksum Sum 0x000000
Number of DCbitless external and opaque AS LSA 0
Number of DoNotAge external and opaque AS LSA 0
Number of areas in this router is 1. 1 normal 0 stub 0 nssa
Number of areas transit capable is 0
External flood list length 0
Area 1
Number of interfaces in this area is 2
Area has no authentication
SPF algorithm last executed 00:00:01.460 ago
SPF algorithm executed 2 times
Area ranges are
Number of LSA 3. Checksum Sum 0x00CF9C
Number of opaque link LSA 0. Checksum Sum 0x000000
Number of DCbitless LSA 0
Number of indication LSA 0
Number of DoNotAge LSA 0
Flood list length 0
R2 router still advertises 172.19.35.0/24 network to R1 router and the connected interface (172.19.35.1) of R3 router is reachable from R1 router.
172.19.35.0/24 reachable from R1
R1# show ip route | begin Gateway
Gateway of last resort is not set
192.168.10.0/28 is subnetted, 2 subnets
O 192.168.10.32 [110/65] via 192.168.10.2, 00:12:16, Serial1/0
C 192.168.10.0 is directly connected, Serial1/0
172.19.0.0/24 is subnetted, 1 subnets
O 172.19.35.0 [110/65] via 192.168.10.2, 00:12:16, Serial1/0
192.168.20.0/28 is subnetted, 1 subnets
C 192.168.20.0 is directly connected, Serial1/1
R1#ping 172.19.35.1
Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to 172.19.35.1, timeout is 2 seconds:
!!!!!
Success rate is 100 percent (5/5), round-trip min/avg/max = 140/189/236 ms
To prove Rule#1, if the primary network (192.168.10.32/28) was removed from OSPF on R2 router, it will not advertise 172.19.35.0/24 to R1 router.
Rule#1 confirmation
R2 router:
router ospf 1
no network 192.168.10.32 0.0.0.15 area 1
!
R1# show ip route | begin Gateway
Gateway of last resort is not set
192.168.10.0/28 is subnetted, 1 subnets
C 192.168.10.0 is directly connected, Serial1/0
192.168.20.0/28 is subnetted, 1 subnets
C 192.168.20.0 is directly connected, Serial1/1
RIP between R2 and R3:
R2 router will now act as an ASBR since it is redistributing routes between OSPF and RIP.
RIP has no problems with secondary addresses and is run between R2 and R3 on secondary network. Redistribution is done on R2 router.
R2 RIP Configuration
router rip
version 2
no auto-summary
network 172.19.0.0
redistribute ospf 1 metric 10 match internal external
!
router ospf 1
....
redistribute rip metric 10 subnets metric-type 2
!
The following output shows routes learnt by R1 router via OSPF.
R1 routes
R1# show ip route | begin Gateway
Gateway of last resort is not set
3.0.0.0/32 is subnetted, 1 subnets
O E2 3.3.3.3 [110/10] via 192.168.10.2, 00:00:09, Serial1/0
4.0.0.0/32 is subnetted, 1 subnets
O E2 4.4.4.4 [110/10] via 192.168.10.2, 00:00:09, Serial1/0
5.0.0.0/32 is subnetted, 1 subnets
O E2 5.5.5.5 [110/10] via 192.168.10.2, 00:00:09, Serial1/0
192.168.10.0/28 is subnetted, 2 subnets
O 192.168.10.32 [110/65] via 192.168.10.2, 00:00:09, Serial1/0
C 192.168.10.0 is directly connected, Serial1/0
172.19.0.0/24 is subnetted, 1 subnets
O 172.19.35.0 [110/65] via 192.168.10.2, 00:00:09, Serial1/0
192.168.20.0/28 is subnetted, 1 subnets
C 192.168.20.0 is directly connected, Serial1/1
The following output shows routes learnt by R2 router.
R2 routes
R2# show ip route | begin Gateway
Gateway of last resort is not set
3.0.0.0/32 is subnetted, 1 subnets
R 3.3.3.3 [120/1] via 172.19.35.1, 00:00:22, FastEthernet0/0
4.0.0.0/32 is subnetted, 1 subnets
R 4.4.4.4 [120/1] via 172.19.35.1, 00:00:22, FastEthernet0/0
5.0.0.0/32 is subnetted, 1 subnets
R 5.5.5.5 [120/1] via 172.19.35.1, 00:00:22, FastEthernet0/0
192.168.10.0/28 is subnetted, 2 subnets
C 192.168.10.32 is directly connected, FastEthernet0/0
C 192.168.10.0 is directly connected, Serial1/0
172.19.0.0/24 is subnetted, 1 subnets
C 172.19.35.0 is directly connected, FastEthernet0/0
192.168.20.0/28 is subnetted, 1 subnets
O IA 192.168.20.0 [110/128] via 192.168.10.1, 00:00:43, Serial1/0
The following output shows routes on R3 router.
R3 routes
R3# show ip route | begin Gateway
Gateway of last resort is not set
3.0.0.0/32 is subnetted, 1 subnets
C 3.3.3.3 is directly connected, Loopback0
4.0.0.0/32 is subnetted, 1 subnets
C 4.4.4.4 is directly connected, Loopback1
5.0.0.0/32 is subnetted, 1 subnets
C 5.5.5.5 is directly connected, Loopback2
192.168.10.0/28 is subnetted, 1 subnets
R 192.168.10.0 [120/10] via 172.19.35.2, 00:00:19, FastEthernet0/0
172.19.0.0/24 is subnetted, 1 subnets
C 172.19.35.0 is directly connected, FastEthernet0/0
192.168.20.0/28 is subnetted, 1 subnets
R 192.168.20.0 [120/10] via 172.19.35.2, 00:00:19, FastEthernet0/0
A ping from R1 router to R3 router is confirmed below.
Ping output
R1# ping 3.3.3.3
Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to 3.3.3.3, timeout is 2 seconds:
!!!!!
Success rate is 100 percent (5/5), round-trip min/avg/max = 196/230/264 ms
R1# ping 172.19.35.1
Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to 172.19.35.1, timeout is 2 seconds:
!!!!!
Success rate is 100 percent (5/5), round-trip min/avg/max = 192/211/220 ms