Why does an OSPF router re-originate its LSAs every 30 minutes without any change, and how is an LSA flushed from every database?

## How LS age works The `LS age` field in an OSPF LSA header counts seconds since the LSA was originated. - It is set to **0** when the originator creates the instance. - It grows by **one per second** while the LSA sits in any router's database. - It grows by **`InfTransDelay`** (configured per interface, always above zero) each time the LSA is copied into a Link State Update. - It never passes **`MaxAge`**, 1 hour (3,600 s). - It is **excluded from the LS checksum**, so ageing never forces a checksum recomputation. - Each time the age crosses a multiple of **`CheckAge`** (5 minutes), the router re-verifies the checksum; a failure there means a memory or software fault, and RFC 2328 says the router should at least be restarted. ## The constants involved | RFC 2328 constant | Value | Role | |---|---|---| | `LSRefreshTime` | 30 minutes | age at which an originator re-originates its LSA | | `MaxAge` | 1 hour | age at which an LSA is unusable and flushed | | `MinLSInterval` | 5 seconds | minimum gap between two originations of one LSA | | `CheckAge` | 5 minutes | interval for re-verifying a stored checksum | These are **architectural constants** in RFC 2328 Appendix B, not configuration defaults: the specification fixes them for every OSPFv2 router, which is what lets routers agree on when an LSA expires. ## Why refresh at all A refresh is a new instance with an incremented sequence number and an unchanged body. RFC 2328 lists it as the first of the events that cause an origination and says it adds robustness to the link-state algorithm: - **Self-repair**: a copy that was lost or corrupted somewhere is replaced within 30 minutes, without anyone having to detect the fault. - **Garbage collection**: if a router disappears without withdrawing its LSAs, nobody refreshes them, and every copy reaches `MaxAge` within about an hour of its last refresh and is flushed. - **No recalculation**: when installing a new instance, a router compares contents; differences in sequence number and checksum alone are not differences, so a refresh does not trigger a routing table calculation. The 30-minute margin below `MaxAge` gives the refreshed instance ample time to reach every router before the old one could expire. LSAs that only describe unreachable destinations are not refreshed; they are flushed instead. ## Flushing: MaxAge and premature ageing An LSA leaves the domain the same way whether its age reached `MaxAge` naturally or was set there on purpose: 1. The router floods the `MaxAge` instance like a newly originated LSA. 2. Receivers accept it: with equal sequence number and checksum, an instance at `MaxAge` counts as more recent. 3. Every router stops using the LSA for routing and floods it on. 4. Each router deletes it once it is on no neighbour's retransmission list and no neighbour is in the Exchange or Loading state of the database exchange. Setting the age to `MaxAge` deliberately is **premature ageing**. RFC 2328 uses it: - when a sequence number reaches `0x7fffffff` and must start again at `0x80000001`; - to withdraw an AS-external route that is no longer reachable — preferred over re-advertising it with the metric `LSInfinity`; - when a router is no longer the designated router and must withdraw the network-LSA it originated; - when a router receives a stale copy of its own LSA that it no longer wants to originate. ## Who may flush **A router may prematurely age only its own LSAs** — those whose `Advertising Router` is its Router ID, or network-LSAs whose `Link State ID` is one of its interface addresses. A router that finds a stale LSA from another router leaves it to age out. This keeps every LSA under the authority of the one router that describes those links first-hand. ## Putting a timeline on it Suppose R10 in a ten-router area is powered off at 10:00, having last refreshed its router-LSA at 09:50. - Once the Hello protocol declares R10 dead, its neighbours re-originate their own router-LSAs without the link to it, and the computed routes change. - R10's own router-LSA remains in every database, but routes through R10 are already gone because no live router still advertises a link to it. - At about 10:50 the stale LSA reaches `MaxAge` and is flushed; nobody had to withdraw it.