📘 CCNA 200-301 v1.1

3.4 Configure and verify single area OSPFv2

3.4.b OSPFv2 Point-to-point

1️⃣ What is OSPFv2?

OSPF (Open Shortest Path First) is a link-state routing protocol used in IPv4 networks.
It uses cost as its metric (based on interface bandwidth) and builds a map of the entire network so routers can choose the shortest path to each destination.

OSPFv2 = Version 2, used for IPv4.
(OSPFv3 is for IPv6.)

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2️⃣ OSPF Network Types

OSPF operates differently depending on the network type of the interface.
The main network types are:

Network Type	Common Example	DR/BDR Election?
Broadcast	Ethernet LAN	Yes
Non-Broadcast	Frame Relay, ATM	Yes
Point-to-Point	Serial links, dedicated connections	No

So our focus here is Point-to-Point — one of the simplest and most important types.

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3️⃣ What is a Point-to-Point Network?

A Point-to-Point (P2P) network type in OSPF is used when a router is directly connected to one other router — there are only two devices on the link.

Characteristics:

• Only two routers connected.
• No need for DR/BDR election (because there’s no multi-access network).
• OSPF Hello packets are sent directly to the multicast address 224.0.0.5.
• Each router forms one adjacency with the other router.
• Link type is identified as Point-to-Point in the OSPF database.

This makes configuration simple and efficient for WAN links or private router interconnections.

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4️⃣ Why Use Point-to-Point in OSPF?

Using the Point-to-Point type is beneficial when:

• You have only two routers connected via a serial or dedicated connection.
• You want to reduce OSPF overhead (no DR/BDR election, no unnecessary LSAs).
• You need faster convergence (since there’s only one neighbor).

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5️⃣ OSPF Hello and Dead Timers (for P2P)

Parameter	Default Value (on P2P)
Hello Interval	10 seconds
Dead Interval	40 seconds

These must match on both routers for adjacency to form.

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6️⃣ OSPF Neighbor Adjacency on P2P

When two routers are directly connected using a P2P link, the adjacency process works as follows:

1. Hello packets are exchanged between the two routers.
2. Each router learns about its neighbor.
3. OSPF states move from Down → Init → 2-Way → ExStart → Exchange → Loading → Full.
4. Once the LSDBs are synchronized, adjacency becomes Full.

You can verify this using:

R1# show ip ospf neighbor

The output will show one neighbor in Full state on that interface.

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7️⃣ OSPF Network Type Configuration

a) Default Behavior

• When OSPF is enabled on a serial interface, Cisco routers usually automatically detect it as point-to-point (if it uses HDLC or PPP encapsulation).
• For Ethernet interfaces, it defaults to broadcast, but you can manually set it to point-to-point if needed.

b) Manually Setting the Network Type

If needed, you can configure it manually:

Router(config-if)# ip ospf network point-to-point

This is useful when you want to override the default type — for example, to simplify OSPF operation on Ethernet links that only connect two routers.

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8️⃣ Example: Configure Point-to-Point OSPF

Let’s imagine we have two routers connected by a serial link.

Network Diagram (simple)

R1 ——— Serial 0/0/0 ——— R2

IP Addresses

• R1: 10.1.1.1/30
• R2: 10.1.1.2/30

Configuration Steps

R1 Configuration:

R1(config)# interface serial 0/0/0
R1(config-if)# ip address 10.1.1.1 255.255.255.252
R1(config-if)# ip ospf network point-to-point
R1(config-if)# ip ospf 1 area 0

R2 Configuration:

R2(config)# interface serial 0/0/0
R2(config-if)# ip address 10.1.1.2 255.255.255.252
R2(config-if)# ip ospf network point-to-point
R2(config-if)# ip ospf 1 area 0

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9️⃣ Verification Commands

✅ Check OSPF neighbors:

show ip ospf neighbor

• Should show one neighbor.
• State: FULL.
• No DR/BDR shown (because point-to-point doesn’t elect them).

✅ Check OSPF interface details:

show ip ospf interface serial 0/0/0

You should see:

Network Type POINT-TO-POINT
Hello 10, Dead 40, No DR/BDR election

✅ Check OSPF routes:

show ip route ospf

To confirm that OSPF routes are learned successfully.

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🔟 OSPF LSAs in Point-to-Point Links

In Point-to-Point links:

• Type 1 (Router LSA) — generated by each router for its own interfaces.
• Type 2 (Network LSA) — not generated, since there’s no multi-access network.
• This keeps the LSDB smaller and simpler.

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1️⃣1️⃣ Summary Table

Feature	Point-to-Point OSPF
Number of routers	2
DR/BDR election	No
Hello Interval	10 sec
Dead Interval	40 sec
Multicast Address	224.0.0.5
OSPF LSAs	Type 1 only
Configuration	ip ospf network point-to-point
Use Case	Serial or direct WAN links
Advantage	Fast convergence, low overhead

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🧠 Key Points to Remember for the Exam

• Point-to-point has no DR/BDR.
• Hello and Dead intervals must match between neighbors.
• Default network type on serial interfaces is often point-to-point.
• Use the command ip ospf network point-to-point to manually configure it.
• Use show ip ospf neighbor and show ip ospf interface to verify.

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