OSPF Database Overflow

Definition

OSPF requires that routers in the same area have the same LSDB.

If the number of routes on a network increases, routers may fail to carry so much routing information due to limited system resources. This is known as an OSPF database overflow.

Purpose

Configuring stub areas or NSSAs partially addresses database overflows. However, stub areas or NSSAs fail to resolve the problem of an unexpected increase in dynamic routes. To dynamically limit the LSDB capacity and thereby prevent database overflows, you can set the maximum number of external LSAs allowed in the LSDB.

Implementation

Setting the maximum number of non-default external routes on a router can prevent database overflows.

All routers on an OSPF network must be configured with the same upper limit. If the number of external routes on a router reaches the upper limit, the router enters the overflow state and starts an overflow timer. The router automatically exits the overflow state after the timer expires. By default, the timer value is 5 seconds.

**Table 1** OSPF database overflow phases
Overflow Phase	OSPF Processing
Entering the overflow state	A router deletes all non-default external routes generated by itself.
Staying in the overflow state	The router does not generate non-default external routes. The router discards the newly received non-default external routes, and does not reply with LSAck packets. When the overflow timer expires, the router checks whether the number of external routes still exceeds the upper limit and performs the following operations accordingly: If the number of external routes still exceeds the upper limit, the router restarts the timer. If the number of external routes is less than the upper limit, the router exits the overflow state.
Exiting the overflow state	The router resets the overflow timer. The router generates non-default routes. The router learns the newly received non-default external routes, and replies with LSAck packets. The router prepares to enter the overflow state in case of future occurrences.