02 · lsa types
OSPF LSA Type Visualization
One topology that produces every LSA type: area 1 on point-to-point links, area 0 with the only broadcast segment and an ASBR, and area 2 as an NSSA with its own ASBR. Select an LSA type to highlight the routers that originate it, shade the area it floods to, animate the flooding itself, and read a decoded header.
- inside the flooding scope
- out of scope for this LSA
- redistribution boundary
Database entries · type 1
Link state ID, advertising router, sequence and age are the four fields to read out loud.
| Type | Link state ID | Adv router | Area | Seq | Age | Contents |
|---|---|---|---|---|---|---|
| 1 | 1.1.1.1 | 1.1.1.1 | 1 | 0x80000124 | 1311s | 2 p2p links (A2, ABR1) + stub 10.1.1.0/24 |
| 1 | 1.1.1.2 | 1.1.1.2 | 1 | 0x80000121 | 1328s | 2 p2p links (A1, ABR1) + stub 10.1.2.0/24 |
| 1 | 1.1.1.9 | 1.1.1.9 | 1 | 0x80000107 | 1447s | 2 p2p links (A1, A2) · B bit set |
| 1 | 1.1.1.9 | 1.1.1.9 | 0 | 0x80000106 | 1446s | 1 transit link to 10.0.0.0/24 · B bit set |
| 1 | 10.0.0.20 | 10.0.0.20 | 0 | 0x80000124 | 1165s | 1 transit link to 10.0.0.0/24 · DR on that segment |
| 1 | 10.0.0.9 | 10.0.0.9 | 0 | 0x80000112 | 820s | 1 transit link · E bit set (it is an ASBR) |
| 1 | 2.2.2.9 | 2.2.2.9 | 0 | 0x80000110 | 489s | 1 transit link · B bit set |
| 1 | 2.2.2.9 | 2.2.2.9 | 2 | 0x80000112 | 491s | 2 p2p links (C1, C2) · B bit set |
| 1 | 2.2.2.1 | 2.2.2.1 | 2 | 0x8000012F | 355s | 2 p2p links + stub 10.2.1.0/24 |
| 1 | 2.2.2.2 | 2.2.2.2 | 2 | 0x8000012B | 372s | 2 p2p links + stub 10.2.2.0/24 · E bit set |
Slide sideways to see the whole table.
The whole set, side by side
| Type | Name | Originator | Scope |
|---|---|---|---|
| 1 | Router LSA | Every OSPF router, once per area it has an interface in | Flooded within a single area only |
| 2 | Network LSA | The DR on each multi-access segment | Flooded within a single area only |
| 3 | Summary LSA (inter-area prefix) | Area Border Routers | Flooded into other areas, one hop of areas at a time |
| 4 | ASBR Summary LSA | Area Border Routers | Flooded into areas that do not contain the ASBR |
| 5 | AS External LSA | The ASBR | Flooded through the whole OSPF domain, except stub and NSSA areas |
| 6 | Group Membership LSA (MOSPF) | Nothing, in practice | Never used |
| 7 | NSSA External LSA | An ASBR inside a Not-So-Stubby Area | Flooded within the NSSA only, then translated to a type 5 by the NSSA ABR |
Slide sideways to see the whole table.
Three common traps
Key point
Type 3 is not topology. Nobody runs SPF on a summary LSA. That is why a misconfigured ABR can create a routing loop that pure intra-area OSPF cannot.
A type 5 without a type 4 is useless. The external LSA names the ASBR by router ID; without the ASBR summary, routers outside area 0 cannot resolve it and the route never installs.
Type 7 does not travel. It stops at the NSSA border and is reborn as a type 5 advertised by the translating ABR — which is why the advertising router changes on the way out.