OSPFv2 (Open Shortest Path First version 2 is a link-state routing protocol designed for IPv4 networks. It is defined in RFC 2328 and is widely used in enterprise and service provider networks due to its efficiency and scalability.
Key Features
of OSPFv2:
- Maintains a complete topological map of the network (LSDB - Link-State Database).
- Uses the Dijkstra (SPF) algorithm to compute the shortest path to all destinations.
- Divides networks into areas to reduce routing overhead.
- Backbone Area (Area 0) is mandatory; all other areas must connect to it.
Types of
areas:
- Normal Areas (Standard areas)
- Stub Areas (Block external routes)
- Totally Stubby Areas (Block external and inter-area routes)
- Not-So-Stubby Areas (NSSA) (Allows limited external routes)
Metric
(Cost):
- Calculated based on interface bandwidth (Cost = Reference BW / Interface BW, default Reference BW = 100 Mbps).
Packet Types:
- Hello (Discover/maintain neighbors)
- DBD (Database Description) (Exchange LSDB info)
- LSR (Link-State Request) (Request specific LSA)
- LSU (Link-State Update) (Send LSA updates)
- LSAck (Link-State Acknowledgment) (Acknowledge LSUs)
LSA Types
(Link-State Advertisements):
- Type 1 (Router LSA) – Intra-area router info.
- Type 2 (Network LSA) – Multi-access network info.
- Type 3 (Summary LSA) – Inter-area routes (ABR-generated).
- Type 4 (ASBR Summary LSA) – Location of ASBR.
- Type 5 (External LSA) – Routes from other AS (redistributed).
- Type 7 (NSSA External LSA) – External routes in NSSA.
Neighbor
States:
- Down → Init → 2-Way → ExStart → Exchange → Loading → Full
Authentication:
Supports plain-text,
MD5, and SHA authentication for security.
Let's see the configuration of the OSPFv2 subcommand.
Topology:-
- Configure the topology as per the diagram
- Configure the IP addresses as per the topology
- Configure OSPFv2 with the subcommand
- advertise directly connected routes and loopbacks
- Make sure all the routes are exchanged
- Verify with ping and show commands
router-1(config)#interface serial 5/0
router-1(config-if)#ip address 12.1.1.1 255.0.0.0
router-1(config-if)#no shutdown
router-1(config-if)#exit
router-1(config)#interface fastethernet 0/0
router-1(config-if)#ip address 13.1.1.1 255.0.0.0
router-1(config-if)#no shutdown
router-1(config-if)#exit
router-1(config)#interface gig 3/0
router-1(config-if)#ip address 14.1.1.1 255.0.0.0
router-1(config-if)#no shutdown
router-1(config-if)#exit
R2(config)#interface serial 5/0
R2(config-if)#ip address 12.1.1.2 255.0.0.0
R2(config-if)#no shutdown
R2(config-if)#exit
R2(config)#interface loopback 1
R2(config-if)#ip address 192.168.20.1 255.255.255.0
R2(config-if)#exit
R3(config)#interface fastethernet 0/0
R3(config-if)#ip address 13.1.1.2 255.0.0.0
R3(config-if)#no shutdown
R3(config-if)#exit
R3(config)#interface loopback 1
R3(config-if)#ip address 192.168.30.1 255.255.255.0
R3(config-if)#exit
router-1(config)#interface serial 5/0
router-1(config-if)#ip ospf 10 area 0
router-1(config-if)#exit
router-1(config)#interface fastethernet 0/0
router-1(config-if)#ip ospf 10 area 0
router-1(config-if)#exit
router-1(config)#interface gig 3/0
router-1(config-if)#ip ospf 10 area 0
router-1(config-if)#exit
*May 30 16:23:05.531: %OSPF-5-ADJCHG: Process 10, Nbr 192.168.20.1 on Serial5/0 from LOADING to FULL, Loading Done
*May 30 16:24:03.999: %OSPF-5-ADJCHG: Process 10, Nbr 192.168.30.1 on FastEthernet0/0 from LOADING to FULL, Loading Done
*May 30 16:25:00.667: %OSPF-5-ADJCHG: Process 10, Nbr 192.168.40.1 on GigabitEthernet3/0 from LOADING to FULL, Loading Done
R2(config)#interface serial 5/0
R2(config-if)#ip ospf 10 area 0
R2(config-if)#exit
*May 30 16:23:05.015: %OSPF-5-ADJCHG: Process 10, Nbr 14.1.1.1 on Serial5/0 from LOADING to FULL, Loading Done
R2(config)#interface loopback 1
R2(config-if)#ip ospf 10 area 1
R2(config-if)#exit
R3(config)#interface fastethernet 0/0
R3(config-if)#ip ospf 10 area 0
R3(config-if)#exit
*May 30 16:24:03.255: %OSPF-5-ADJCHG: Process 10, Nbr 14.1.1.1 on FastEthernet0/0 from LOADING to FULL, Loading Done
R3(config)#interface loopback 1
R3(config-if)#ip ospf 10 area 2
R3(config-if)#exit
R4(config)#interface gig 3/0
R4(config-if)#ip ospf 10 area 0
R4(config-if)#exit
*May 30 16:25:00.579: %OSPF-5-ADJCHG: Process 10, Nbr 14.1.1.1 on GigabitEthernet3/0 from LOADING to FULL, Loading Done
R4(config)#interface loopback 1
R4(config-if)#ip ospf 10 area 3
R4(config-if)#exit
router-1#show ip protocols
Routing Protocol is "ospf 10"
Outgoing update filter list for all interfaces is not set
Incoming update filter list for all interfaces is not set
Router ID 14.1.1.1
Number of areas in this router is 1. 1 normal 0 stub 0 nssa
Maximum path: 4
Routing for Networks:
Routing on Interfaces Configured Explicitly (Area 0):
GigabitEthernet3/0
FastEthernet0/0
Serial5/0
Reference bandwidth unit is 100 mbps
Routing Information Sources:
Gateway Distance Last Update
192.168.40.1 110 00:06:36
192.168.30.1 110 00:06:39
192.168.20.1 110 00:06:39
Distance: (default is 110)
router-1#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
192.168.30.0/32 is subnetted, 1 subnets
O IA 192.168.30.1 [110/2] via 13.1.1.2, 00:07:05, FastEthernet0/0
192.168.40.0/32 is subnetted, 1 subnets
O IA 192.168.40.1 [110/2] via 14.1.1.2, 00:07:02, GigabitEthernet3/0
192.168.20.0/32 is subnetted, 1 subnets
O IA 192.168.20.1 [110/65] via 12.1.1.2, 00:07:05, Serial5/0
C 12.0.0.0/8 is directly connected, Serial5/0
C 13.0.0.0/8 is directly connected, FastEthernet0/0
C 14.0.0.0/8 is directly connected, GigabitEthernet3/0
router-1#show ip ospf interface gigabitEthernet 3/0
GigabitEthernet3/0 is up, line protocol is up
Internet Address 14.1.1.1/8, Area 0
Process ID 10, Router ID 14.1.1.1, Network Type BROADCAST, Cost: 1
Enabled by interface config, including secondary ip addresses
Transmit Delay is 1 sec, State DR, Priority 1
Designated Router (ID) 14.1.1.1, Interface address 14.1.1.1
Backup Designated router (ID) 192.168.40.1, Interface address 14.1.1.2
Timer intervals configured, Hello 10, Dead 40, Wait 40, Retransmit 5
oob-resync timeout 40
Hello due in 00:00:03
Supports Link-local Signaling (LLS)
Index 3/3, flood queue length 0
Next 0x0(0)/0x0(0)
Last flood scan length is 0, maximum is 1
Last flood scan time is 0 msec, maximum is 4 msec
Neighbor Count is 1, Adjacent neighbor count is 1
Adjacent with neighbor 192.168.40.1 (Backup Designated Router)
Suppress hello for 0 neighbor(s)
router-1#ping 192.168.20.1
Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to 192.168.20.1, timeout is 2 seconds:
!!!!!
Success rate is 100 percent (5/5), round-trip min/avg/max = 20/34/40 ms
router-1#
router-1#ping 192.168.30.1
Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to 192.168.30.1, timeout is 2 seconds:
!!!!!
Success rate is 100 percent (5/5), round-trip min/avg/max = 28/39/60 ms
router-1#
router-1#ping 192.168.40.1
Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to 192.168.40.1, timeout is 2 seconds:
!!!!!
Success rate is 100 percent (5/5), round-trip min/avg/max = 32/40/48 ms
router-1#show ip ospf database
OSPF Router with ID (14.1.1.1) (Process ID 10)
Router Link States (Area 0)
Link ID ADV Router Age Seq# Checksum Link count
14.1.1.1 14.1.1.1 1097 0x80000006 0x002C55 4
192.168.20.1 192.168.20.1 1192 0x80000002 0x002C4C 2
192.168.30.1 192.168.30.1 1130 0x80000003 0x003AC3 1
192.168.40.1 192.168.40.1 1075 0x80000003 0x009750 1
Net Link States (Area 0)
Link ID ADV Router Age Seq# Checksum
13.1.1.1 14.1.1.1 1153 0x80000001 0x0084FA
14.1.1.1 14.1.1.1 1097 0x80000001 0x00F97A
Summary Net Link States (Area 0)
Link ID ADV Router Age Seq# Checksum
192.168.20.1 192.168.20.1 1187 0x80000001 0x005CE2
192.168.30.1 192.168.30.1 1126 0x80000001 0x00A783
192.168.40.1 192.168.40.1 1071 0x80000001 0x00F224
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