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Adjacent Node
Networking, explained. No BS.

OSPF

What It Is

Open Shortest Path First is a link-state interior gateway protocol. Routers exchange link-state advertisements, build a shared link-state database for an area, and calculate shortest paths with Dijkstra SPF. OSPF is still common in enterprise, service provider, data center, and lab networks, but the modern design target is usually clean areas, clear summarization, authentication where supported, and fewer clever special cases.

Core Attributes

Attribute OSPFv2 OSPFv3
Primary use IPv4 routing IPv6 routing, also address families on some platforms
Transport IP protocol 89 IP protocol 89
Multicast all SPF routers 224.0.0.5 ff02::5
Multicast all DR routers 224.0.0.6 ff02::6
Administrative distance on Cisco 110 110
Metric Cost Cost
Algorithm Dijkstra SPF Dijkstra SPF

Modern note: OSPFv2 is for IPv4. OSPFv3 was built for IPv6 and has different operational details. Do not assume every OSPFv2 command maps directly to OSPFv3.

Packet Types

Type Packet Purpose
1 Hello Neighbor discovery and keepalive
2 Database Description Summary of LSDB contents during adjacency build
3 Link State Request Requests missing or newer LSAs
4 Link State Update Carries LSAs
5 Link State Acknowledgment Confirms LSA receipt

Neighbor States

State Meaning What To Check If Stuck
Down No hellos received Interface, multicast, timers
Attempt NBMA neighbor configured but not heard NBMA config
Init Hello received, but not bidirectional Neighbor sees you, masks, ACLs
2-Way Bidirectional hello Normal on DROTHER to DROTHER
ExStart Master/slave negotiation MTU mismatch, duplicate router ID
Exchange Database summaries exchanged MTU, packet loss
Loading Requesting missing LSAs LSDB mismatch, packet loss
Full Databases synchronized Healthy adjacency

Watch out: 2-Way is normal between non-DR routers on a broadcast segment. Do not troubleshoot it like a failure unless the neighbor should be DR or BDR adjacent.

LSAs And Areas

LSA Type Name Scope Purpose
1 Router Area Router links inside an area
2 Network Area Multiaccess segment represented by DR
3 Summary Inter-area Prefixes advertised by ABR
4 ASBR Summary Inter-area Path to an ASBR
5 External AS External routes redistributed into OSPF
7 NSSA External NSSA External routes inside NSSA, translated by ABR
Area Type Allows Type 5 Allows Type 7 Default Route Behavior
Standard Yes No Not automatic
Stub No No ABR injects default
Totally stubby No No ABR injects default and suppresses most summaries
NSSA No Yes Optional default depending on config
Totally NSSA No Yes Default plus reduced summaries

Design note: Area types should reduce complexity, not create surprises. If the team cannot quickly explain why an area is NSSA or totally stubby, the design may be too clever.

Network Types

Network Type DR/BDR Default Hello / Dead Common Use
Broadcast Yes 10 / 40 seconds Ethernet VLANs
Point-to-point No 10 / 40 seconds Routed links
Nonbroadcast Yes 30 / 120 seconds Legacy NBMA
Point-to-multipoint No 30 / 120 seconds Hub and spoke when supported

Modern note: On Ethernet point-to-point routed links, setting OSPF network type point-to-point can remove unnecessary DR/BDR behavior.

Metrics

OSPF cost is based on interface bandwidth by default on many platforms. Cisco historically used a 100 Mbps reference bandwidth unless changed.

Link Speed Cost With 100 Mbps Ref Cost With 100 Gbps Ref
100 Mbps 1 1000
1 Gbps 1 100
10 Gbps 1 10
100 Gbps 1 1

Watch out: If every fast link has cost 1, OSPF cannot prefer 100 Gbps over 1 Gbps by cost. Set reference bandwidth consistently across the OSPF domain.

DR And BDR Election

Rule Behavior
Highest priority wins Priority range is commonly 0 to 255
Priority 0 Cannot become DR or BDR
Tie breaker Highest router ID wins
No preemption Existing DR remains until failure or reset
Only some network types Broadcast and NBMA elect DR/BDR

Design note: In a VLAN with many routers, make the intended routers DR and BDR. In point-to-point designs, avoid DR/BDR where it is unnecessary.

Cisco IOS/IOS-XE Examples

Interface-based OSPF:

router ospf 100
 router-id 10.255.0.1
 auto-cost reference-bandwidth 100000
 passive-interface default
 no passive-interface GigabitEthernet1/0/1
!
interface GigabitEthernet1/0/1
 description Routed link to DIST-02
 ip address 10.0.12.1 255.255.255.252
 ip ospf 100 area 0
 ip ospf network point-to-point

Area summarization on an ABR:

router ospf 100
 area 10 range 10.10.0.0 255.255.252.0

Stub area:

router ospf 100
 area 20 stub

Authentication example:

interface GigabitEthernet1/0/1
 ip ospf authentication message-digest
 ip ospf message-digest-key 1 md5 ExampleKey

Notes:

  • MD5 examples exist because they are still seen in OSPFv2 brownfield networks. Prefer stronger platform-supported authentication where available.
  • Use passive-interface default so OSPF does not send hellos toward endpoints by accident.
  • Keep auto-cost reference-bandwidth consistent on every router in the OSPF domain.
  • Use interface OSPF config when possible. It is harder to accidentally match the wrong network than wildcard network statements.

Troubleshooting

Symptom Check Likely Cause
Neighbor stuck Init Hellos, ACLs, multicast, router ID visibility One-way reachability or hello mismatch
Neighbor stuck ExStart or Exchange MTU, duplicate router ID, packet loss MTU mismatch or negotiation issue
Route missing in one area LSDB, ABR, area type, filters Summary or stub behavior
External route missing ASBR, redistribution, type 5 or type 7 LSAs Redistribution or NSSA translation
Bad path chosen Interface cost, reference bandwidth Inconsistent or default costs
OSPF flaps Logs, interface errors, BFD, timers Physical instability or aggressive timers
Unexpected DR Priority and router ID Election happened before intended router joined

Commands

show ip ospf
show ip ospf interface brief
show ip ospf interface GigabitEthernet1/0/1
show ip ospf neighbor
show ip ospf database
show ip route ospf
show ip protocols

Expected clues:

  • Router ID is unique and intentional.
  • Neighbors reach Full where they should.
  • Hello and dead timers match.
  • Area ID and network type match.
  • MTU does not block adjacency formation.
  • LSDB contains expected router, network, summary, and external LSAs.
  • Route table has the expected OSPF intra-area, inter-area, or external routes.

Watch Out

  • Do not build area 0 through hope. ABRs need real backbone connectivity unless a temporary virtual link is justified.
  • Do not leave all fast links at cost 1.
  • Do not redistribute routes without tags, filters, and a clear reason.
  • Do not use virtual links as a permanent design crutch.
  • Do not ignore duplicate router IDs. They create strange adjacency and LSDB behavior.
  • Do not make every interface active in OSPF. Passive by default is safer.

References