2 · OSPF¶
OSPF is the IGP most enterprises run, and the one every later phase in this curriculum quietly depends on. This page covers what actually matters in practice — plus where OSPFv3 diverges for IPv6.
The command output below was captured from a live three-router cEOS fabric, not written from memory.
Areas: the organising idea¶
OSPF splits a network into areas, and every area must touch area 0 — the backbone. Traffic between two non-backbone areas always transits area 0.
That constraint feels arbitrary until you see what it prevents: without a single mandatory hub, inter-area routing would need loop prevention between areas, and OSPF deliberately doesn't have one. The rigid hub-and-spoke is the loop prevention.
Routers get named by where they sit:
| Role | Position |
|---|---|
| Internal | all interfaces in one area |
| ABR (Area Border Router) | interfaces in two or more areas — one must be area 0 |
| ASBR (AS Boundary Router) | injects external routes into OSPF |
Area membership is per-interface
A router isn't "in" an area — each of its interfaces is. This is a real difference from IS-IS, where the whole router belongs to a level, and it's a common interview question.
Here's a single-area fabric — note Area 0.0.0.0 on each interface:
Interface Instance VRF Area IP Address Cost State Nbrs
Et2 1 default 0.0.0.0 10.1.2.1/24 10 P2P 1
Et1 1 default 0.0.0.0 10.1.1.1/24 10 P2P 1
Lo0 1 default 0.0.0.0 1.1.1.1/32 10 DR 0
Area types: trading visibility for size¶
Not every area needs the full picture. If an area has one way out, telling its routers about every external route in the world is wasted memory — they'd all resolve to the same exit anyway.
So OSPF lets you block LSA types at the ABR and substitute a default route. Each area type blocks a bit more:
| Area type | Blocks | Still gets | Use when |
|---|---|---|---|
| Standard | nothing | everything | the backbone, or areas needing full detail |
| Stub | type 5 (external) | types 1–4 + default | branch with one exit and no external sources |
| Totally stubby | types 3, 4, 5 | types 1–2 + default | same, but you also don't need inter-area detail |
| NSSA | type 5 | types 1–4 + type 7 | stub area that does have its own external source |
| Totally NSSA | types 3, 4, 5 | types 1–2 + type 7 + default | NSSA that doesn't need inter-area detail either |
Why NSSA exists is the part people miss. A stub area cannot carry external routes — that's the definition. But what if a stub-like area has a redistributing router of its own? You'd have to make it standard and lose the savings.
NSSA solves it with type 7: functionally an external route, but permitted inside an NSSA. The ABR translates type 7 into type 5 as it leaves. Same information, different wrapper, because the area's rules forbid the type-5 wrapper.
Two rules that catch people out
Every router in the area must agree on the area type. It's negotiated in the hello, so a mismatch means no adjacency — the symptom is a neighbour that never forms, not a routing oddity.
Area 0 can never be a stub of any kind. It carries transit for everything; blocking LSAs there would break inter-area routing by definition.
Path preference: which route actually wins¶
When OSPF learns the same prefix more than once, it doesn't compare costs first. It compares route type first, and only breaks ties within a type by cost:
intra-area (O) ← always wins, whatever the cost
inter-area (O IA)
external E1 (O E1)
external E2 (O E2) ← always loses
A high-cost intra-area route beats a low-cost inter-area one. Cost is the tiebreaker inside a class, never across classes — a genuinely common exam trap.
E1 vs E2 — the one everybody gets asked¶
Both are redistributed from outside OSPF. The difference is what happens to the metric as it travels:
- E2 — cost stays fixed at whatever the ASBR set, no matter how far you are from it. This is the default.
- E1 — cost is the external metric plus the internal cost to reach the ASBR, so it rises with distance.
So with two exits to the same external destination, E2 can't tell them apart — every router sees the same cost and picks by tiebreaker rather than by proximity. E1 can, because each router adds its own distance to the ASBR.
The rule of thumb
One exit — E2 is fine and keeps the metric simple. Multiple exits where you want routers to use the nearest — use E1. Sticking with the E2 default in a multi-exit design is a classic cause of traffic crossing the network to reach a further-away exit.
Router ID: how it's chosen¶
OSPF needs a unique 32-bit ID. It picks, in strict order:
- Manually configured
router-id— always wins - Highest IP on an up loopback interface
- Highest IP on any up physical interface
Always configure it explicitly. Relying on the automatic choice means the ID can change when an interface goes down or a new one appears — and since the router ID is baked into every LSA that router originated, a change forces the entire area to re-flood and re-run SPF.
A changed router ID also does not take effect until the OSPF process restarts, which surprises people who set it and see nothing happen.
Timers and cost¶
Hello and dead intervals must match between neighbours or the adjacency never forms. Defaults depend on network type:
| Network type | Hello | Dead |
|---|---|---|
| Broadcast, point-to-point | 10s | 40s |
| NBMA, point-to-multipoint | 30s | 120s |
Dead is conventionally 4× hello. Lowering both speeds up failure detection, but for sub-second convergence use BFD instead — a lightweight dedicated hello that detects loss in milliseconds and tells OSPF to drop the neighbour, without the overhead of very aggressive OSPF timers.
Cost is reference bandwidth ÷ interface bandwidth, default reference 100 Mbps.
That default is why every link at 100 Mbps and above lands on cost 1 — a 1G and a
100G link look identical to OSPF. Raise the reference bandwidth so modern speeds
differentiate, and set it identically on every router, since mismatched
reference bandwidths mean routers disagree about cost.
Passive interface advertises a network into OSPF while refusing to send hellos on it. Correct for any interface with no OSPF neighbour — user-facing subnets, loopbacks — since it removes needless adjacency attempts and a small attack surface.
LSA types: who advertises what¶
The LSDB isn't one kind of record. Different situations produce different LSA types, and knowing the common ones turns database output from noise into information.
| Type | Name | Says |
|---|---|---|
| 1 | Router | "here are my links and their costs" — every router, within its area |
| 2 | Network | "here's who's on this multi-access segment" — from the DR |
| 3 | Summary | "this prefix exists in another area" — from an ABR |
| 4 | ASBR Summary | "here's how to reach the ASBR" — from an ABR |
| 5 | External | "here's a route from outside OSPF" — from an ASBR |
| 7 | NSSA External | like type 5, but inside a not-so-stubby area |
Types 1 and 2 stay within an area. That's the containment that makes areas worth having — topology detail never leaves.
Real Router LSAs from the fabric, one per router, each with a sequence number and age:
Router Link States (Area 0.0.0.0)
Link ID ADV Router Age Seq# Checksum Link count
2.2.2.2 2.2.2.2 120 0x80000006 0x6556 3
3.3.3.3 3.3.3.3 1811 0x80000005 0xbfee 3
1.1.1.1 1.1.1.1 169 0x80000008 0xbaae 5
Read it as: three routers, each describing itself. Link count is how many links
that router is advertising. Seq# rises on every change — a rapidly climbing
sequence number means something is flapping.
Adjacency: the state machine¶
Two OSPF routers walk through defined states before exchanging routes. Knowing where it stalls tells you the cause immediately.
graph LR
D["Down"] --> I["Init"] --> T["2-Way"] --> E["ExStart"] --> X["Exchange"] --> L["Loading"] --> F["Full"]
classDef progress fill:#1565c0,stroke:#90caf9,color:#ffffff,stroke-width:2px,font-size:15px;
classDef working fill:#2e7d32,stroke:#a5d6a7,color:#ffffff,stroke-width:2px,font-size:15px;
class D,I,T,E,X,L progress; class F working;- Init — I heard a hello, but didn't see myself in it yet.
- 2-Way — we can both see each other. On a LAN, routers that aren't the DR or BDR stop here on purpose, and that's healthy.
- ExStart / Exchange — deciding who leads, then describing databases.
- Loading — requesting the details each side is missing.
- Full — databases synchronised. This is the only state that means working.
Where it sticks, and why:
| Stuck at | Cause |
|---|---|
| Init | one-way hellos — filtering, or an ACL dropping multicast |
| 2-Way (non-LAN) | DR election problem, or both routers have priority 0 |
| ExStart / Exchange | MTU mismatch — the classic. Each side sends a database packet the other can't accept |
| Flapping in and out of Full | mismatched dead intervals, or an unstable link |
A healthy pair looks like this:
Neighbor ID Instance VRF Pri State Dead Time Address Interface
3.3.3.3 1 default 0 FULL 00:00:38 10.1.2.2 Ethernet2
2.2.2.2 1 default 0 FULL 00:00:38 10.1.1.2 Ethernet1
Dead Time counts down and resets on every hello. If you watch it approach zero,
hellos aren't arriving.
Network types, DR, and why point-to-point is better¶
On a shared segment with n routers, forming a full mesh of adjacencies means
n(n-1)/2 relationships — wasteful and noisy. OSPF elects a Designated Router
that everyone adjoins instead, plus a BDR standing by.
Election is by priority, highest wins, router-ID breaking ties. Priority 0 means
"never elect me."
But most modern links are point-to-point. Two routers on a routed link have nothing to elect, and DR election just adds delay. Hence:
interface Ethernet1
no switchport
ip address 10.1.1.1 255.255.255.0
ip ospf area 0.0.0.0
ip ospf network point-to-point
Notice State P2P and Nbrs 1 in the interface output earlier — no DR involved.
Set point-to-point on routed links between two routers
It skips DR election, forms adjacency faster, and produces a simpler database (no type-2 LSA). On a spine-leaf fabric where every link is a routed point-to-point, this should be your default.
The loopback still shows DR — that's normal and harmless. A loopback has no
neighbours to elect anything with.
Summarisation, and where it's allowed¶
OSPF only summarises at borders — never arbitrarily, because routers inside an area must hold identical databases and summarising mid-area would break that.
| Where | Command | Summarises |
|---|---|---|
| ABR | area <id> range <prefix> |
inter-area (type 3) |
| ASBR | summary-address <prefix> |
external (type 5 / 7) |
Mixing these up is a common interview stumble: area range on an ASBR does nothing
to external routes, and summary-address on an ABR does nothing to inter-area ones.
Summarisation is the main defence against a large OSPF domain becoming unstable: a link flapping behind a summary doesn't change the summary, so the flap stays local instead of forcing SPF runs network-wide. The cost is lost visibility, and potentially attracting traffic for addresses inside the range that aren't actually reachable.
Virtual links: the escape hatch¶
Every area must touch area 0 — but occasionally one doesn't, usually after a merger or a backbone that got physically split. A virtual link tunnels through an intervening area to restore the connection:
interface Ethernet1
...
router ospf 1
area 0.0.0.1 virtual-link 4.4.4.4
The transit area must be standard (never stub, which can't carry the necessary LSAs) and cannot be area 0 itself.
Treat a virtual link as a repair, not a design
It works, and it's the right tool in an emergency. But it's fragile — it depends on the transit area staying healthy — and it hides a structural problem in the address plan. If a virtual link is load-bearing in your network, the real fix is re-homing that area to the backbone.
OSPFv3: what changes for IPv6¶
OSPFv3 is the same algorithm with meaningfully different plumbing.
Per-link, not per-subnet. OSPFv2 forms adjacencies with routers in the same IP subnet. OSPFv3 works per link — addressing doesn't gate neighbour formation. Multiple prefixes on one link stop being a problem.
Neighbours are link-local. Adjacencies use fe80:: addresses. Peers appear as
link-local even though they carry global prefixes — that's correct, not a
misconfiguration.
Router IDs are still 32-bit. Written in dotted-quad form, exactly like IPv4. In an IPv6-only network there's no address to derive one from, so you must set it manually — a genuinely common cause of OSPFv3 refusing to start.
Addressing moved out of the topology LSAs. Router and network LSAs describe topology only; prefixes travel in separate LSAs (Link LSAs and Intra-Area-Prefix LSAs). This is what makes OSPFv3 able to carry address families other than the one it was designed around.
Authentication was removed from the protocol and delegated to IPsec — a cleaner design in theory, more operationally awkward in practice.
Arista cEOS OSPFv3 Configuration¶
Configuring OSPFv3 on Arista cEOS requires enabling IPv6 routing, assigning a manual 32-bit router-ID, and binding interfaces to the OSPFv3 process ID:
! Enable IPv6 unicast routing globally
ipv6 unicast-routing
! Define the OSPFv3 process (Process ID 100) and explicit Router ID
router ospf3 100
router-id 1.1.1.1
exit
! Loopback interface configuration
interface Loopback0
ipv6 address 2001:db8:1::1/128
ipv6 ospf 100 area 0.0.0.0
! Point-to-point inter-router link
interface Ethernet1
no switchport
ipv6 address 2001:db8:12::1/64
ipv6 ospf 100 area 0.0.0.0
ipv6 ospf network point-to-point
Process ID requirement in interface syntax
Unlike OSPFv2 (ip ospf area 0.0.0.0), OSPFv3 interface configuration requires the process ID explicitly: ipv6 ospf <process-id> area <area-id>.
OSPFv3 Operational Verification¶
Verify adjacency, link states, and IPv6 routes using dedicated ipv6 CLI commands:
# Check OSPFv3 adjacencies (note neighbour IP is link-local fe80::)
docker exec clab-ceos-r1 Cli -p 15 -c "show ipv6 ospf neighbor"
# Verify IPv6 routes installed by OSPFv3
docker exec clab-ceos-r1 Cli -p 15 -c "show ipv6 route ospf"
Sample output of show ipv6 ospf neighbor:
Neighbor ID Instance VRF Pri State Dead Time Address Interface
2.2.2.2 100 default 1 FULL/P2P 00:00:34 fe80::2 Ethernet1
Sample output of show ipv6 route ospf:
Codes: C - connected, S - static, K - kernel,
O - OSPF, IA - OSPF inter area, E1 - OSPF external type 1,
E2 - OSPF external type 2
O 2001:db8:2::2/128 [110/20] via fe80::2, Ethernet1
v2 and v3 are separate protocols, not one protocol with two modes
Running dual-stack means running two independent OSPF instances with two databases and two SPF computations. They can disagree: IPv4 taking one path and IPv6 another across the same physical topology is entirely possible, and debugging it means checking both.
IS-IS handles this differently — see choosing.
Interview questions¶
Why must every OSPF area connect to area 0?
OSPF has no loop-prevention mechanism between areas — inter-area routes are distance-vector-like summaries, and a router can't see the topology behind them. Forcing all inter-area traffic through a single backbone makes loops structurally impossible instead of algorithmically prevented.
Two routers are stuck in ExStart/Exchange. What do you check first?
MTU. During database exchange each side sends packets sized to its own MTU; if the other can't receive them, the exchange never completes. The interfaces are up and hellos work fine, which is what makes it confusing — small packets pass, large ones don't.
A router on a LAN is stuck at 2-Way. Is it broken?
Probably not. On a multi-access segment only the DR and BDR go to Full — everyone else deliberately stays at 2-Way with each other. It's a problem only if the router should have become DR or BDR and didn't.
What's the practical difference between OSPFv2 and OSPFv3 areas?
Conceptually none — the area model is identical. The differences are in transport and encoding: v3 is per-link rather than per-subnet, peers over link-local addresses, separates addressing from topology LSAs, and drops built-in authentication in favour of IPsec.
Why prefer point-to-point over broadcast on a routed link?
There's nothing to elect with only two routers, so DR election adds startup delay for no benefit. Point-to-point forms adjacency faster and keeps the database simpler by omitting the type-2 network LSA.
Next: IS-IS → — the same job, a very different design, and the reason providers keep choosing it.