OSPF Network Types

✅ OSPF Network Types – Complete Technical Notes

🔧 What Are OSPF Network Types?

OSPF (Open Shortest Path First) network types define how routers communicate over different media. They control:

1. DR/BDR Election – Whether Designated Router/Backup DR is needed
2. Neighbor Discovery – Automatic or static neighbor configuration
3. Timers – Default Hello and Dead intervals
4. LSA Flooding – How link-state updates are shared

⚙️ OSPF supports multiple network types based on the underlying media or topology, and Cisco supports 5 primary types (plus loopback):

📋 Summary Table: OSPF Network Types

Network Type	DR/BDR Election	Neighbor Discovery	Hello	Dead	Typical Media
Broadcast	Yes	Automatic	10s	40s	Ethernet LAN
Nonbroadcast (NBMA)	Yes	Static	30s	120s	Frame Relay
Point-to-Point (P2P)	No	Automatic	10s	40s	Serial, GRE
Point-to-Multipoint (P2MP)	No	Automatic	30s	120s	Hub-and-Spoke
P2MP Nonbroadcast	No	Static	30s	120s	NBMA without multicast
Loopback	No	N/A	N/A	N/A	Loopback Interface

🧠 Deep Dive: Network Types Explained

1. 🔵 Broadcast

   • Media: Ethernet LAN
   • Characteristics: Supports multicast and automatic discovery
   • DR/BDR: Required to reduce adjacency complexity
   • LSA Updates:
     • DROTHERs → DR/BDR via 224.0.0.6
     • DR floods updates to 224.0.0.5
   • Timers: Hello: 10s, Dead: 40s
   • Command:ip ospf network broadcast

2. 🟠 Nonbroadcast (NBMA)

   • Media: Frame Relay, ATM, X.25 (No multicast support)
   • Characteristics: Multi-access, but no dynamic discovery
   • Neighbor Configuration: Manual using neighbor <IP>
   • DR/BDR: Required
   • Timers: Hello: 30s, Dead: 120s
   • Command:ip ospf network non-broadcast

3. 🟢 Point-to-Point (P2P)

   • Media: Serial (HDLC/PPP), GRE tunnels
   • Characteristics: Direct link between 2 routers
   • DR/BDR: Not used
   • Adjacency: Simplified and fast
   • Timers: Hello: 10s, Dead: 40s
   • Command:ip ospf network point-to-point

4. 🟡 Point-to-Multipoint (P2MP)

   • Media: Frame Relay Hub-and-Spoke
   • Characteristics: All destinations treated as P2P links
   • DR/BDR: Not used
   • Discovery: Automatic
   • Routing Behavior:
     • Advertises interfaces as /32
     • Next-hop = local interface IP
   • Timers: Hello: 30s, Dead: 120s
   • Command:ip ospf network point-to-multipoint

5. 🔴 Point-to-Multipoint Nonbroadcast

   • Media: NBMA topologies without multicast (Frame Relay)
   • Characteristics: Same as P2MP but no multicast
   • Discovery: Manual (static neighbors)
   • DR/BDR: Not used
   • Timers: Hello: 30s, Dead: 120s
   • Command:ip ospf network point-to-multipoint non-broadcast

6. ⚪ Loopback Interfaces

   • Purpose: Used for Router ID or stable routing interfaces
   • Behavior: Always advertised as /32 regardless of actual subnet mask
   • OSPF LSA Type: Type 1 Router LSA (Stub link)
   • DR/BDR: Not applicable
   • Command: Not needed (auto-recognized by OSPF)

🧰 OSPFv3 Considerations

OSPFv3 supports all the same network types as OSPFv2, but configuration is interface-based.

OSPFv3 Key Notes:

• Uses IPv6 link-local addresses for adjacency
• Configured under the interface, not with network statements
• Same commands with ospfv3:show ospfv3 interface GigabitEthernet0/1

❗ Troubleshooting: Network Type Mismatch

Issue	Cause	Result
Adjacency not forming	Different network types	Timers mismatch
DR/BDR election not expected	P2P set as Broadcast	Wasted CPU cycles
Static neighbor config required	NBMA without config	No adjacency

🔍 Tip: Always verify network types and timers match on both sides.

✅ OSPF Verification Commands

Task	Command
Show OSPFv2 interface config	show ip ospf interface [int]
Show OSPFv3 interface config	show ospfv3 interface [int]
Show neighbor relationships	show ip ospf neighbor / show ospfv3 neighbor

🔚 Final Tips for Learners & Interviewees

• Learn which media types use which network types (Ethernet = Broadcast, Serial = P2P, Frame Relay = NBMA/P2MP).
• Understand timer mismatches – common interview and real-world issue.
• Practice configuring static neighbors for NBMA and forcing P2P on Ethernet.

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