OSPF LSA Types Made Simple — Learn Them by the 4 Authors

Most people try to memorize OSPF LSA types as six unrelated numbers and forget them by Friday. Here's a faster system: stop learning the numbers, learn the four authors. Every LSA is written by exactly one of four roles — a Router, a DR, an ABR, or an ASBR — and the type number just tells you who wrote it and what they're describing. Learn the authors once and the whole table becomes obvious.

01The one idea: LSAs are map pieces

OSPF is a link-state protocol: every router in an area builds the same map (the link-state database, LSDB) and then runs SPF on it to find shortest paths. LSAs — Link-State Advertisements — are the pieces of that map. Different pieces describe different things (a router's own links, a shared segment, a network in another area, an external route), and each kind is authored by whichever router is in a position to know it.

LSA type = who wrote it (author) + what it describes (scope)

02The system: four authors

Memorize these four roles and the "who writes what" falls out for free:

Every Router describes itself → Type 1.
The DR (designated router) describes the shared segment → Type 2.
The ABR (area border router) is the translator between areas → Type 3 (a network in another area) and Type 4 (how to reach the ASBR).
The ASBR (autonomous-system border router) brings in the outside world → Type 5 (external routes) — or Type 7 when it lives inside an NSSA.

A mnemonic for the order 1-2-3-4-5-7, if you want one:

R N S A E N = Really Nice Sysadmins Always Escalate Nicely → Router · Network · Summary · ASBR-summary · External · NSSA

03The six types, one line each

Type 1 — Router	Author: every router. "Here are my own links and their costs." Stays inside the area.
Type 2 — Network	Author: the DR. "Here's who is attached to this multi-access segment." Stays inside the area.
Type 3 — Summary	Author: the ABR. "A network exists over in another area." Crosses area borders (inter-area routes).
Type 4 — ASBR summary	Author: the ABR. "Here's how to reach the ASBR." A signpost so other areas can use Type 5s.
Type 5 — AS external	Author: the ASBR. "A route from outside OSPF (redistributed)." Floods the whole domain — except stub/NSSA areas.
Type 7 — NSSA external	Author: an ASBR inside an NSSA. Same job as Type 5, but Type 5 isn't allowed in an NSSA — so the ABR converts 7 → 5 at the border.

Two questions unlock any exam item: who could have written this? (which of the four roles) and how far does it travel? (area-local for 1 & 2, inter-area for 3 & 4, domain-wide for 5).

04Follow a route across areas

The types only truly stick once you watch them hand off. Picture a network in Area 1 that must be reachable from Area 2, plus an external route redistributed by an ASBR:

Inside Area 1, routers flood Type 1 (their links) and, on any LAN segment, the DR floods a Type 2. Everyone in Area 1 now has an identical map of Area 1.
The ABR between Area 1 and Area 0 doesn't leak Area 1's Type 1/2 across — it summarizes each Area 1 prefix as a Type 3 into Area 0, and the Area 0/2 ABR re-originates Type 3 onward into Area 2. That's how inter-area routing works: a chain of Type 3 re-advertisements, not raw flooding.
The ASBR redistributes an external route as a Type 5, which floods everywhere. But routers in other areas need to know where the ASBR is to use it — so the ABR emits a Type 4 pointing at the ASBR. Type 5 = the news; Type 4 = the address of the reporter.

LSDB(area) = Type1(every router) + Type2(each DR) + Type3/4(from ABRs) + Type5(from ASBRs)

05Stub & NSSA: who gets blocked

Stub area types are just "which authors are muted to keep the database small." Same four-authors lens:

Stub	Blocks Type 5 (& 4). The ABR injects a default route (as a Type 3) to reach everything external.
Totally stubby	Blocks Type 3, 4, 5. Only a single default Type 3 gets in. (Cisco `no-summary`.)
NSSA	Blocks Type 5 but allows its own external routes as Type 7, translated to Type 5 at the ABR.
Totally NSSA	Blocks Type 3, 4, 5; allows Type 7; injects a default.

That single fact — "a stub area can't carry Type 5, but a site there still needs to redistribute something" — is the entire reason Type 7 exists. It's the NSSA's workaround.

06See them for real

Nothing cements it like watching your own database sort itself into these buckets:

# Cisco IOS
show ip ospf database                 # the whole LSDB, grouped by type
show ip ospf database router          # Type 1
show ip ospf database network         # Type 2
show ip ospf database summary         # Type 3
show ip ospf database asbr-summary    # Type 4
show ip ospf database external        # Type 5
show ip ospf database nssa-external   # Type 7

# FortiGate equivalent
get router info ospf database          # brief LSDB, same type groupings

            OSPF Router with ID (10.0.0.1) (Process ID 1)

                Router Link States (Area 0)          <- Type 1
Link ID         ADV Router      Age   Seq#       Link count
10.0.0.1        10.0.0.1        112   0x80000005  3
10.0.0.2        10.0.0.2         98   0x80000004  2

                Net Link States (Area 0)             <- Type 2 (originated by the DR)
10.1.1.2        10.0.0.2         98   0x80000001

                Summary Net Link States (Area 0)     <- Type 3 (from the ABR)
10.2.2.0        10.0.0.3         75   0x80000002

The section headers are the LSA types. "ADV Router" is the author's Router ID — tie it back to the four roles and you can read a live database at a glance.

07Drill it in 60 seconds

Cover the answers and run these both ways (type→author and author→type). Three passes and it's yours:

Who writes Type 1?	Every router — its own links. (Area-local.)
Who writes Type 2?	The DR — the shared segment. (Area-local.)
Which types does the ABR write?	3 (inter-area network) and 4 (path to the ASBR).
Which types does the ASBR write?	5 (external) — or 7 inside an NSSA.
Why does Type 4 exist?	So other areas know how to reach the ASBR whose Type 5s they're receiving.
Why does Type 7 exist?	NSSAs forbid Type 5; Type 7 carries their externals until the ABR converts 7→5.
What does a stub area block?	Type 5 (external) — replaced by a default route.

08Where to go next

Once the six types are reflex, the natural follow-ons are DR/BDR election (why Type 2 exists at all), E1 vs E2 external metrics on Type 5, and area design (where to place ABRs and stubs). For the IPv6 world, OSPFv3 keeps this model and adds Type 8 (Link LSA) and Type 9 (Intra-Area-Prefix LSA), while opaque LSAs (9/10/11) carry MPLS-TE and Segment Routing — but the four-authors instinct still gets you most of the way. Back to the Knowledge Base, or browse more Networking guides.