OSPF Overview – Open Shortest Path First Explained

1. What is OSPF?

OSPF (Open Shortest Path First) is an open-standard, link-state Interior Gateway Protocol (IGP) defined in RFC 2328 (OSPFv2 for IPv4) and RFC 5340 (OSPFv3 for IPv6). It is the most widely deployed IGP in enterprise and service-provider networks and is a core topic in the CCNA exam.

Unlike distance-vector protocols such as RIP, OSPF routers build a complete map of the network topology — called the Link-State Database (LSDB) — and run Dijkstra's Shortest Path First algorithm on that map to independently calculate the best loop-free path to every destination.

OSPF Property	Value
Protocol type	Link-state IGP
Standard	Open standard — RFC 2328 (v2 for IPv4), RFC 5340 (v3 for IPv6)
Algorithm	Dijkstra's SPF (Shortest Path First)
Metric	Cost (based on interface bandwidth)
Administrative Distance (AD)	110
Transport	IP protocol number 89 — not TCP or UDP
Multicast addresses	224.0.0.5 (all OSPF routers), 224.0.0.6 (DR/BDR only)
Supports VLSM / CIDR	Yes — classless routing protocol
Supports IPv6	Yes — via OSPFv3
Supports hierarchical design	Yes — via multi-area OSPF with mandatory backbone Area 0

2. Link-State vs Distance-Vector

Understanding what makes OSPF a link-state protocol — and how that differs from distance-vector — is fundamental to grasping why OSPF behaves the way it does.

Feature	Link-State (OSPF)	Distance-Vector (RIP)	Advanced Distance-Vector (EIGRP)
What each router knows	Complete topology map (LSDB)	Only its neighbour's routing table	Neighbour's metrics + topology table
Algorithm	Dijkstra's SPF	Bellman-Ford	DUAL (Diffusing Update Algorithm)
Metric	Cost (bandwidth-based)	Hop count (max 15)	Composite (bandwidth + delay + load + reliability)
Updates	Triggered; full LSA refresh every 30 min	Periodic (every 30 s); full table	Triggered; partial updates only
Convergence speed	Fast	Slow (counting to infinity problem)	Very fast (DUAL guarantees loop-free backup)
Scalability	High — scales with multi-area design	Low — 15-hop maximum limit	High — supports large networks
Loop prevention	SPF on identical LSDB guarantees loop-free paths	Split horizon, route poisoning	DUAL feasibility condition
Open standard	Yes	Yes	Partially (Cisco advanced features)

Related pages: RIP Concepts | EIGRP Overview | BGP Overview

3. How OSPF Works — Step by Step

OSPF brings up a routing domain through a well-defined sequence of steps. Every OSPF router goes through these phases — understanding them is critical for both configuration and troubleshooting.

  Step 1: Discover Neighbours
  ┌──────────────────────────────────────────────────────────────────┐
  │  Routers send Hello packets every 10 s (broadcast) / 30 s (NBMA)│
  │  Hello contains: Router ID, area ID, hello/dead timers, network  │
  │  mask, DR/BDR info, and neighbour list                           │
  └──────────────────────────────────────────────────────────────────┘
             ↓
  Step 2: Form Adjacencies
  ┌──────────────────────────────────────────────────────────────────┐
  │  Two neighbours exchange DBD (Database Description) packets      │
  │  to compare their LSDBs, then request missing LSAs via LSR       │
  │  (Link-State Request). The neighbour replies with LSU            │
  │  (Link-State Update). LSAck confirms receipt.                    │
  └──────────────────────────────────────────────────────────────────┘
             ↓
  Step 3: Flood LSAs — Build the LSDB
  ┌──────────────────────────────────────────────────────────────────┐
  │  Each router generates Type 1 (Router) LSAs describing its own   │
  │  links. The DR generates Type 2 (Network) LSAs on broadcast      │
  │  segments. All LSAs flood within the area until every router     │
  │  has an identical LSDB.                                          │
  └──────────────────────────────────────────────────────────────────┘
             ↓
  Step 4: Run SPF — Calculate Best Paths
  ┌──────────────────────────────────────────────────────────────────┐
  │  Each router independently runs Dijkstra's SPF algorithm on its  │
  │  LSDB to build a shortest-path tree (SPT) rooted at itself.      │
  │  The best path to each destination is installed in the routing   │
  │  table as an OSPF route (O or O IA).                             │
  └──────────────────────────────────────────────────────────────────┘
             ↓
  Step 5: Maintain — Hello keepalives + LSA refreshes
  ┌──────────────────────────────────────────────────────────────────┐
  │  Hello packets maintain neighbour relationships (dead timer =    │
  │  4 x hello interval). LSAs are re-flooded every 30 minutes.     │
  │  Any topology change triggers a new LSA flood + SPF recalc.     │
  └──────────────────────────────────────────────────────────────────┘

Related pages: OSPF Neighbor States | OSPF DR/BDR | OSPF Areas & LSAs

4. OSPF Router ID (RID)

Every OSPF router must have a unique Router ID (RID) — a 32-bit value written in dotted-decimal notation (like an IP address) that identifies the router within the OSPF domain. Duplicate Router IDs cause adjacency and routing failures.

Router ID Selection Order (highest wins)

Priority	Source	Notes
1 (highest)	Manually configured with `router-id` command	Best practice — always configure this explicitly
2	Highest IP address on a loopback interface	Loopbacks never go down — more stable than physical interfaces
3 (fallback)	Highest IP address on any active physical interface at OSPF start-up	Risky — can change if the interface goes down and OSPF restarts

! Best practice — always manually set the Router ID
router ospf 1
 router-id 1.1.1.1

! Verify the Router ID
Router# show ip ospf
! Look for: "Router ID 1.1.1.1"

! After changing router-id, clear OSPF process to apply the new RID
Router# clear ip ospf process
! WARNING: This drops all OSPF adjacencies temporarily

Exam tip: The Router ID is chosen at OSPF process start-up. If you change it, you must use clear ip ospf process for the new RID to take effect. The RID does not have to be reachable or even match an actual interface — it is simply a unique identifier for the router within the OSPF domain.

5. OSPF Cost — The Metric

OSPF uses cost as its metric — a dimensionless value inversely proportional to interface bandwidth. Lower cost = preferred path. The total cost of a route is the cumulative cost of all outgoing interfaces along the entire path.

Cost Formula

  Cost = Reference Bandwidth / Interface Bandwidth

  Default reference bandwidth = 100 Mbps (100,000,000 bps)

  Interface costs with default reference bandwidth:
  ┌──────────────────────────┬────────────────────────────────────────┐
  │ Interface Type            │ Cost                                   │
  ├──────────────────────────┼────────────────────────────────────────┤
  │ 10 Mbps Ethernet          │ 100 / 10    = 10                       │
  │ 100 Mbps FastEthernet     │ 100 / 100   = 1                        │
  │ 1 Gbps GigabitEthernet    │ 100 / 1000  = 0.1  → rounds up to 1   │
  │ 10 Gbps TenGigabitEth.    │ 100 / 10000 = 0.01 → rounds up to 1   │
  └──────────────────────────┴────────────────────────────────────────┘

  Problem: GigE, 10GigE, and 100GigE all get cost = 1 with default reference.
  OSPF cannot differentiate them — it may choose a slower path!
  Fix: change reference-bandwidth to 10000 (10 Gbps) or 100000 (100 Gbps).

! Fix: increase reference bandwidth so GigE and 10GigE get distinct costs
router ospf 1
 auto-cost reference-bandwidth 10000    ! Units are Mbps — this sets 10 Gbps
! Apply this on ALL routers in the OSPF domain — must be consistent

! Or manually override cost on a specific interface
interface GigabitEthernet0/1
 ip ospf cost 50

! Verify interface cost
Router# show ip ospf interface GigabitEthernet0/1
! Look for: "Cost: 50"

Related pages: Routing Metrics | show ip protocols | show interfaces

6. OSPF Packet Types

OSPF uses five packet types — all carried directly in IP (protocol 89) without TCP or UDP. Understanding each packet is essential for reading debug output and diagnosing adjacency problems.

Type	Name	Purpose	When Sent
1	Hello	Discover and maintain neighbours; elect DR/BDR on broadcast segments	Every hello interval (10 s on broadcast/P2P, 30 s on NBMA). Sent to 224.0.0.5.
2	DBD (Database Description)	Exchange LSDB summaries during adjacency formation — lists LSA headers without full content	During ExStart and Exchange states of adjacency formation
3	LSR (Link-State Request)	Request specific LSAs that are missing or outdated after comparing DBDs	After DBD exchange, when a router discovers it needs specific LSAs
4	LSU (Link-State Update)	Deliver the actual LSA content in response to an LSR, or to flood a topology change	In response to LSR; also triggered by any topology change
5	LSAck (Link-State Acknowledgement)	Acknowledge received LSUs — ensures reliable flooding	After receiving an LSU

Hello Packet — Fields That Must Match for Adjacency

The Hello packet contains several fields that must match between neighbours for an adjacency to form. A mismatch silently prevents adjacency — the routers see each other's Hellos but never advance beyond Init or 2-Way.

Hello Field	Must Match?	Notes
Area ID	✅ Yes	Both interfaces must be in the same OSPF area
Hello interval	✅ Yes	Default 10 s (broadcast/P2P), 30 s (NBMA)
Dead interval	✅ Yes	Default 4 × hello interval (40 s on broadcast)
Subnet mask	✅ Yes (broadcast)	Not checked on point-to-point links
Area type (stub flag)	✅ Yes	Both must agree on stub/NSSA; mismatch blocks adjacency
Authentication	✅ Yes	Type and key must match if configured
Router ID	Must be unique	Duplicate RIDs cause routing problems throughout the domain
MTU	⚠️ Should match	MTU mismatch causes DBD exchange failure (stuck at ExStart)

Related pages: OSPF Neighbor States | OSPF DR/BDR | Debug Commands

7. OSPF Neighbour States — Summary

OSPF adjacency formation progresses through eight states. A healthy, fully converged adjacency ends at the Full state. Each state represents a distinct phase of the relationship between two routers.

  Down → Init → 2-Way → ExStart → Exchange → Loading → Full

  Down     — No Hello received from this neighbour yet
  Init     — Hello received but our Router ID is NOT yet in their neighbour list
  2-Way    — Bidirectional communication confirmed (our RID is in their Hello)
             ★ DR/BDR election takes place here on broadcast segments
  ExStart  — Master/slave relationship negotiated for DBD exchange
  Exchange — DBD packets exchanged (LSDB header summaries compared)
  Loading  — LSRs sent; missing LSAs received via LSUs
  Full     — LSDBs are identical ✅ — fully adjacent, routing is active

Important: On broadcast networks (Ethernet), only DR and BDR reach Full state with each other and with all DROther routers. DROther routers stay at 2-Way with each other — this is completely normal and expected, not a problem or misconfiguration.

For the complete neighbour state machine with causes and troubleshooting steps: OSPF Neighbor States — Full Guide

8. DR and BDR Election

On multi-access broadcast networks (Ethernet), OSPF elects a Designated Router (DR) and a Backup Designated Router (BDR) to reduce adjacency overhead. Without DR/BDR, every pair of routers on the segment would form a full adjacency — creating an n(n-1)/2 adjacency explosion on large segments.

Role	Function	Adjacency Behaviour
DR	Collects and re-floods LSAs for the entire segment. Generates the Type 2 Network LSA.	Full adjacency with ALL routers on the segment
BDR	Listens passively to all exchanges; takes over instantly if the DR fails.	Full adjacency with ALL routers on the segment
DROther	All other routers — send/receive LSAs only through the DR and BDR.	2-Way with other DROthers; Full only with DR and BDR

DR/BDR Election Rules

Priority	Criterion	Notes
1 (highest)	Highest OSPF interface priority (0–255)	Default priority = 1. Setting priority to 0 prevents a router from ever becoming DR or BDR.
2 (tiebreaker)	Highest Router ID	Used only when priorities are equal

! Set interface priority to influence DR election (higher = preferred DR)
interface GigabitEthernet0/0
 ip ospf priority 100       ! This router wins DR election on this segment

! Prevent a router from ever becoming DR or BDR
interface GigabitEthernet0/0
 ip ospf priority 0

! Verify current DR/BDR on a segment
Router# show ip ospf interface GigabitEthernet0/0
! Look for: "Designated Router (ID)...", "Backup Designated Router (ID)..."

DR election is non-preemptive. If a new router with a higher priority joins the segment after DR election, it does not take over from the existing DR. The current DR stays in place until it fails or the OSPF process is cleared. Always plan your DR assignments before deployment.

Full guide: OSPF DR/BDR Election — Complete Explanation

9. OSPF Areas — Why They Matter

In a single-area OSPF deployment, every router floods LSAs to every other router and every topology change triggers a full SPF recalculation on every router. In large networks this becomes unsustainable. OSPF areas solve this by confining LSA flooding within each area boundary — a link failure in Area 2 only triggers SPF recalculation on Area 2 routers.

     Area 1                Area 0 (Backbone)              Area 2
  ┌──────────┐         ┌─────────────────────┐         ┌──────────┐
  │ R1 ── R2 │──[ABR]──│  R5 ──── R6         │──[ABR]──│ R9 ──R10 │
  │          │         │  |                  │         │          │
  │  R3      │         │  R7 ── R8 [ASBR]   │         │  R11     │
  └──────────┘         └──────────┬──────────┘         └──────────┘
                                  │ Type 5 External LSAs
                            External / Internet

Area Concept	Key Rule
Area 0 (Backbone)	Mandatory — all inter-area traffic must transit it. Every area must attach to Area 0 directly or via a virtual link.
ABR (Area Border Router)	Router with interfaces in 2+ areas including Area 0. Maintains a separate LSDB per area. Generates Type 3 Summary LSAs to share routes between areas.
ASBR (AS Boundary Router)	Redistributes routes from outside OSPF (other IGPs, BGP, static routes) into OSPF as Type 5 External LSAs.
Intra-area routes	Routes within the same area — shown as `O` in the routing table
Inter-area routes	Routes between areas via ABR — shown as `O IA`
External routes	Routes redistributed from outside OSPF — shown as `O E1` or `O E2`

For the full deep-dive on area types (Stub, Totally Stubby, NSSA, Totally NSSA), LSA Types 1–7, ABR/ASBR configuration, and virtual links: OSPF Areas & LSAs — Detailed Guide

10. Basic OSPF Configuration

The minimum OSPF configuration requires three things: enabling OSPF with a process ID, assigning a Router ID, and advertising networks into OSPF using the network command with a wildcard mask and area assignment.

! ── Minimal OSPF single-area configuration ──────────────────────────────
router ospf 1                                   ! Process ID 1 — local significance only
 router-id 1.1.1.1                              ! Manually set Router ID (best practice)
 network 192.168.1.0 0.0.0.255 area 0           ! Advertise this network into Area 0
 network 10.0.0.0 0.0.0.3 area 0                ! Point-to-point link in Area 0

! ── Passive interface — suppress Hellos on user-facing interfaces ────────
router ospf 1
 passive-interface GigabitEthernet0/1           ! No OSPF Hellos on this interface
 ! The network is still advertised — Hellos are just suppressed

! ── Alternatively: set all interfaces passive, then enable selectively ──
router ospf 1
 passive-interface default
 no passive-interface GigabitEthernet0/0        ! Only this interface sends Hellos

! ── Essential verification commands ─────────────────────────────────────

! Show OSPF neighbours and their current state (target: Full or 2-Way)
Router# show ip ospf neighbor

! Show OSPF-learned routes (O = intra-area, O IA = inter-area, O E1/E2 = external)
Router# show ip route ospf

! Show OSPF process details: Router ID, area count, SPF statistics
Router# show ip ospf

! Show per-interface OSPF settings: cost, timers, DR/BDR, network type
Router# show ip ospf interface

! Show the Link-State Database (all LSA types)
Router# show ip ospf database

! Show which networks OSPF is advertising and other protocol parameters
Router# show ip protocols

Step-by-step labs: OSPF Single-Area Lab | OSPF Multi-Area Lab | Default Route Redistribution into OSPF | Troubleshooting OSPF Adjacency Lab

11. OSPF vs EIGRP vs RIP — Quick Comparison

Feature	OSPF	EIGRP	RIP v2
Protocol type	Link-state	Advanced distance-vector	Distance-vector
Standard	Open — RFC 2328	Cisco (RFC 7868 partial)	Open — RFC 2453
Algorithm	Dijkstra SPF	DUAL	Bellman-Ford
Metric	Cost (bandwidth)	Composite (BW + delay + …)	Hop count
AD	110	90 internal / 170 external	120
Max hop count	None (area-based scalability)	100 default / 255 max	15 hops (16 = unreachable)
Supports VLSM	Yes	Yes	Yes (v2 only)
Supports IPv6	Yes — OSPFv3	Yes — EIGRPv6	Yes — RIPng
Convergence	Fast	Very fast	Slow
Scalability	High — multi-area design	High	Low
Transport	IP protocol 89 (multicast)	IP protocol 88 (multicast/unicast)	UDP port 520
Best used when	Multi-vendor enterprise or SP networks	Cisco-only enterprise networks	Very small or lab networks only

12. Common OSPF Problems and Fixes

Symptom	Likely Cause	Fix / Diagnostic Command
Routers stay in Init state	ACL blocking multicast 224.0.0.5; wrong area ID; duplex/speed mismatch	`show ip ospf neighbor` — verify area ID; check for ACL dropping Hello packets
Routers stay at 2-Way	Expected between DROthers on a broadcast segment	Normal — only DR and BDR reach Full state; 2-Way between DROthers is correct
Stuck at ExStart / Exchange	MTU mismatch between neighbours	`show ip ospf interface` — compare MTU values; use `ip ospf mtu-ignore` to confirm cause; fix MTU
No adjacency — Hello mismatch	Mismatched hello/dead timers, area type, subnet mask, or authentication	`debug ip ospf hello` — compare all hello fields on both sides
Routes missing from routing table	Network not advertised; wrong area in `network` statement; passive-interface blocking Hellos	`show ip route ospf`; `show ip protocols`; verify `network` commands
Routes present but suboptimal path chosen	GigE and FastEthernet both have cost 1 (default reference bandwidth too low)	Set `auto-cost reference-bandwidth 10000` on all routers
Adjacency flapping	Unstable link; duplicate Router IDs; dead timer expiry on unreliable link	`show ip ospf` — check for duplicate RIDs; `debug ip ospf events` to trace flap cause
External routes missing	Redistribution not configured on ASBR; area is stub (blocks Type 5 LSAs)	`show ip ospf database external` — verify Type 5 LSAs exist; check ASBR `redistribute` command

13. Key Points Summary

Topic	Key Facts
What is OSPF	Open-standard link-state IGP (RFC 2328 v2, RFC 5340 v3); uses IP protocol 89; classless
Algorithm	Dijkstra's SPF runs on an identical LSDB — guarantees loop-free paths
Metric	Cost = Reference BW / Interface BW; lower = preferred; increase reference BW for GigE+ networks
AD	110 — preferred over RIP (120); less trusted than EIGRP internal (90) or static routes (1)
Router ID	Manual > Highest Loopback IP > Highest Physical IP; must be unique; change requires `clear ip ospf process`
Hello interval	10 s on broadcast/P2P; 30 s on NBMA — must match on both sides for adjacency
Dead interval	4 × hello interval (40 s default on broadcast) — must match on both sides
5 Packet types	Hello (discover), DBD (compare), LSR (request), LSU (deliver), LSAck (acknowledge)
DR/BDR	Elected on broadcast segments; highest priority then highest RID; non-preemptive; DROthers stay 2-Way with each other
Multicast	224.0.0.5 = all OSPF routers; 224.0.0.6 = DR and BDR only
Areas	Area 0 is mandatory backbone; ABRs connect areas; LSA flooding confined per area; reduces SPF scope
Route codes	O = intra-area; O IA = inter-area; O E1/E2 = external (redistributed from outside OSPF)
LSA refresh	Every 30 minutes (LSRefreshTime); MaxAge = 60 minutes; sequence numbers prevent stale data

OSPF Overview Quiz

1. What type of routing protocol is OSPF, and what does that mean for how routers learn the topology?

A. Distance-vector — each router only knows its neighbour's routing table and advertised metrics B. Path-vector — each router tracks the full AS-path to every destination to prevent loops C. Link-state — each router builds a complete topology map (LSDB) and independently runs Dijkstra's SPF to calculate the best paths D. Hybrid — it combines link-state flooding with a distance-vector metric calculation like EIGRP

Correct answer is C. OSPF is a link-state protocol. Every router floods LSAs (Link-State Advertisements) to all other routers in its area so they all build an identical LSDB — a complete map of the topology. Each router then independently runs Dijkstra's SPF algorithm on that map to calculate the loop-free shortest path to every destination. This is fundamentally different from distance-vector protocols like RIP, where each router only knows what its direct neighbours tell it.

2. What is the OSPF metric, and why is the default reference bandwidth a problem in modern networks?

A. Hop count — the default gives the same weight to all interface speeds, which is the same problem as RIP B. Cost (= reference bandwidth / interface bandwidth) — the default reference of 100 Mbps means FastEthernet, GigabitEthernet, and 10GigE all get cost 1 and cannot be differentiated C. Composite of bandwidth + delay — the default reference is too high for modern networks D. Cost equals the number of routers in the path — higher router count means higher total cost

Correct answer is B. OSPF cost = reference bandwidth / interface bandwidth. The default reference bandwidth is 100 Mbps. A 100 Mbps FastEthernet interface gets cost 1 (100/100 = 1). A 1 Gbps GigabitEthernet interface also gets cost 1 (100/1000 = 0.1 → rounds up). Even a 10 GigE interface gets cost 1. OSPF cannot distinguish between them and may choose a slower path. Fix this with auto-cost reference-bandwidth 10000 (for 10 Gbps reference) on ALL routers in the domain.

3. How is the OSPF Router ID determined if it is not manually configured?

A. The lowest IP address on any active interface is selected automatically B. A random 32-bit value is generated at OSPF process start-up C. The Router ID is derived from the first neighbour's IP address at process start D. The highest loopback IP address is used first; if no loopback exists, the highest IP on any active physical interface at OSPF start-up is used

Correct answer is D. OSPF Router ID selection follows this priority order: (1) manually configured router-id command — always wins and is best practice; (2) highest IP address among all configured loopback interfaces — loopbacks are always up, making this stable; (3) highest IP address on any active physical interface when OSPF starts — risky because if that interface goes down and OSPF restarts, the RID can change. Always manually set the Router ID to a loopback address for predictable and stable operation.

4. Which OSPF packet maintains neighbour relationships, and what happens if it stops being received?

A. Hello packet — if not received within the dead interval (default 40 s on broadcast), the neighbour is declared down and its routes are removed from the routing table B. LSU packet — if not received, the LSDB is considered incomplete and SPF is suspended C. DBD packet — missing DBDs cause the adjacency to reset to Down state immediately D. LSAck packet — if missing, the router retransmits LSUs indefinitely until acknowledged

Correct answer is A. OSPF Hello packets (Type 1) are sent at the hello interval — every 10 seconds on broadcast and point-to-point links, every 30 seconds on NBMA networks. They are sent to multicast 224.0.0.5 (all OSPF routers). Each router tracks when it last received a Hello from each neighbour. If no Hello arrives within the dead interval (default = 4 × hello = 40 s on broadcast), that neighbour is declared down, the adjacency goes to Down state, all routes learned from that neighbour are removed, and SPF recalculates.

5. Why are DR and BDR elected on broadcast Ethernet segments, and which state do DROther routers reach with each other?

A. To speed up SPF calculation — DROthers reach Full state with each other but only exchange LSAs via the DR B. To elect a primary router for the segment — all routers including DROthers reach Full adjacency with each other C. To reduce adjacency count and flooding overhead — DROthers only reach Full adjacency with the DR and BDR; DROthers stay at 2-Way with each other, which is normal D. To provide path redundancy — DROthers reach Loading state with each other until the DR sends them a full LSDB copy

Correct answer is C. Without DR/BDR on a broadcast segment with n routers, you would need n(n-1)/2 full adjacencies. With DR/BDR, only the DR and BDR form Full adjacency with every other router — DROther routers send all their LSUs to 224.0.0.6 (AllDRouters) so only DR and BDR receive them, then the DR re-floods to 224.0.0.5. DROther routers stay at 2-Way state with each other — this is completely correct behaviour, not a problem or misconfiguration to fix.

6. An OSPF adjacency between two routers is stuck at ExStart. What is the most likely cause?

A. The hello or dead timers do not match — this prevents DBD exchange from starting B. MTU mismatch — DBD packets are dropped by the interface with the smaller MTU, so the exchange never completes C. Area type mismatch — one router has stub configured, the other does not D. The Router IDs are identical on both routers

Correct answer is B. ExStart/Exchange is the phase where routers exchange DBD (Database Description) packets. If there is an MTU mismatch, the larger DBD packets cannot pass through the interface with the smaller MTU and are dropped — the exchange can never complete, leaving the adjacency stuck at ExStart. Timer mismatches (A) and area type mismatches (C) prevent adjacency earlier, at the Hello/Init phase. Duplicate Router IDs (D) cause routing table problems but the symptom differs. Diagnose with show ip ospf interface and fix with either ip ospf mtu-ignore (temporary) or by matching the actual MTU.

7. What is the Administrative Distance of OSPF, and what happens when both OSPF and EIGRP learn the same prefix?

A. AD 90 — same as EIGRP, so both routes are installed simultaneously as equal-cost paths B. AD 120 — same as RIP, meaning OSPF routes are equally trusted as RIP routes C. AD 1 — OSPF is the most trusted dynamic routing source after directly connected routes D. AD 110 — if EIGRP (AD 90) and OSPF (AD 110) both learn the same prefix, the router installs only the EIGRP route because lower AD wins

Correct answer is D. OSPF has an Administrative Distance of 110. AD is the router's measure of trustworthiness of a routing source — lower AD is more trusted. If two routing protocols advertise a route to the same destination, the router installs only the route from the lower-AD source. EIGRP internal (AD 90) beats OSPF (AD 110) — only the EIGRP route appears in the routing table. OSPF beats RIP (AD 120). Static routes have AD 1. Directly connected routes have AD 0.

8. What are the two OSPF multicast addresses used on broadcast networks, and what is the difference between them?

A. 224.0.0.5 is used by all OSPF routers (for Hello packets and DR-to-all floods); 224.0.0.6 is used by DROther routers to send LSUs to the DR and BDR only B. 224.0.0.5 is for OSPF; 224.0.0.6 is for EIGRP — they use different multicast groups C. 224.0.0.5 is for IPv4 OSPFv2; 224.0.0.6 is exclusively for IPv6 OSPFv3 D. OSPF only uses unicast on Ethernet segments — no multicast addresses are used

Correct answer is A. OSPF uses two multicast addresses on broadcast networks. 224.0.0.5 (AllSPFRouters) — all OSPF routers listen on this address; used for Hello packets and for the DR to re-flood LSUs to every OSPF router on the segment. 224.0.0.6 (AllDRouters) — only the DR and BDR listen on this address; DROther routers send their LSUs to 224.0.0.6 so that only the DR and BDR receive them. The DR then re-floods the LSU content to 224.0.0.5 so all routers on the segment get the update.

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