OSPF Mechanisms
Session Establishment · LSA Types · Areas · Route Redistribution
🔗 Session Establishment
📦 LSA Types
🗺️ Areas
🔄 Redistribution
OSPF Neighbor State Machine — 7 states
Down
Init
2-Way
ExStart
Exchange
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Full
Router A RID 1.1.1.1 Down Router B RID 2.2.2.2 Down — — Hello multicast 224.0.0.5 · contains RID, Area, Hello/Dead intervals, neighbors seen Hello (A's RID in neighbor list) DBD (I=1,M=1,MS=1) — Master Election A has higher RID → becomes Master DBD (MS=0) — Slave ACK + own DBD LSR → ← LSR LSU (full LSAs) → ← LSU (full LSAs) LSAck → ← LSAck ✓ OSPF Adjacency FULL LSDB synchronized · SPF runs · routes installed
📡 Hello (Type 1)
Multicast224.0.0.5
Interval10s (p2p/bcast)
Dead interval40s (4×Hello)
ContainsRID, Area, Auth
📋 DBD — Database Description (Type 2)
PurposeLSDB summary
FlagsI M MS
I=Initfirst in exchange
MS=1Master role
📥 LSR/LSU/LSAck (3/4/5)
LSRrequest missing
LSUfull LSA data
LSAckconfirm receipt
RetransmitRxmtInterval 5s
OSPF Neighbor FSM
OSPF builds an adjacency through 7 states. Press Next step to walk through it step by step. These parameters must match on both neighbors: Area ID, Hello/Dead intervals, Authentication, Stub flag, MTU.
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OSPF LSA Types 1–7
Area 0 (Backbone) Area 1 (Regular) Area 2 (Stub) Area 3 (NSSA) ABR 1.1.1.1 ABR 2.2.2.2 ASBR 3.3.3.3 IR 4.4.4.4 IR 5.5.5.5 IR 6.6.6.6 External BGP/EIGRP IR 7.7.7.7 ASBR 8.8.8.8 Ext RIP/BGP
T1
Router LSA
OriginEvery router
ScopeArea-local
FloodingWithin area
T2
Network LSA
OriginDR only
ScopeArea-local
WhenMulti-access
T3
Summary LSA
OriginABR
ScopeArea → Area
ContentInter-area routes
T4
ASBR Summary
OriginABR
ScopeArea → Area
ContentASBR location
T5
AS External LSA
OriginASBR
ScopeAS-wide
E1/E2metric type
T6
Group Membership
OriginDR (MOSPF)
ScopeArea-local
NoteRare
T7
NSSA External LSA
OriginASBR in NSSA
ScopeNSSA only
→T5at ABR
T8/T9
Opaque LSA
RFC5250
UsageTE, MPLS, GRACE
Scopelink/area/AS
Select an LSA type
Click an LSA card above to see the details and the highlight on the topology. Each LSA type has its own flooding scope and is originated by a specific type of router.
OSPF Area Types — comparison
LSA TypeRegularStubTotally StubNSSATotally NSSA
T1/T2 ✓✓ ✓✓✓
T3 inter-area ✓✓ ✗ (default only)✓✗
T4 ASBR Sum. ✓✗ ✗✗✗
T5 AS External ✓✗ ✗✗✗
T7 NSSA Ext. ✗✗ ✗✓✓
Default route optionalT3 auto T3 autoT7 manualT3 auto
External routes T5none noneT7 (local ASBR)T7 only
Area 0 ABR 1.1.1.1 ABR 2.2.2.2 Area 1 Regular IR ASBR Area 2 Stub IR Area 3 NSSA ASBR BGP external RIP external
Area Types
Click an area type above to see the comparison. OSPF areas let you limit LSA flooding and reduce the load on SPF. Each type defines which LSAs are allowed inside the area.
OSPF Route Redistribution
OSPF Domain ASBR redistribute — IR-1 IR-2 IR-3 IR-4 BGP AS 65100 EIGRP AS 100 Static + Connected — — — metric E1 E2 +=IGP fixed ext metric
StepCommandSeed metricLSA typeE typeResult
Press a step →
Route Redistribution
Redistribution lets OSPF import routes from other protocols. It happens on the ASBR (Autonomous System Boundary Router). This generates LSA Type 5 (or LSA Type 7 in an NSSA). You must set a seed metric — OSPF does not know the "cost" of external routes.
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