RFC1765 - OSPF Database Overflow

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Network Working Group J. Moy
Request for Comments: 1765 Cascade
Category: Experimental March 1995

OSPF Database Overflow

Status of this Memo

This memo defines an Experimental Protocol for the Internet

community. This memo does not specify an Internet standard of any
kind. Discussion and suggestions for improvement are requested.
Distribution of this memo is unlimited.

Abstract

Proper operation of the OSPF protocol requires that all OSPF routers
maintain an identical copy of the OSPF link-state database. However,
when the size of the link-state database becomes very large, some
routers may be unable to keep the entire database due to resource
shortages; we term this "database overflow". When database overflow
is anticipated, the routers with limited resources can be
accommodated by configuring OSPF stub areas and NSSAs. This memo
details a way of gracefully handling unanticipated database
overflows.

This memo is a product of the OSPF Working Group. Please send
comments to ospf@gated.cornell.edu.

Table of Contents

1. Overview ............................................... 2
2. Implementation details ................................. 3
2.1 Configuration .......................................... 3
2.2 Entering OverflowState ................................. 4
2.3 Operation while in OverflowState ....................... 5
2.3.1 Modifications to Flooding .............................. 5
2.3.2 Originating AS-external-LSAs ........................... 6
2.3.3 Receiving self-originated LSAs ......................... 6
2.4 Leaving OverflowState .................................. 6
3. An example ............................................. 6
4. Administrative response to database overflow ........... 7
5. Operational experience ................................. 8
6. Possible enhancements .................................. 8
A. Related MIB parameters ................................ 8
References ............................................ 9
Security Considerations ............................... 9
Author's Address ...................................... 9

1. Overview

OSPF requires that all OSPF routers within a single area maintain an
identical copy of the OSPF link-state database. However, when the
size of the link-state database becomes very large, some routers may
be unable to keep the entire database due to resource shortages; we
term this "database overflow". For example, a regional network may
have a very large OSPF database because it is importing a large
number of external routes into OSPF. Unless database overflow is
handled correctly, routers will end up with inconsistent views of the
network, possibly leading to incorrect routing.

One way of handling database overflow is to encase routers having
limited resources within OSPF stub areas (see Section 3.6 of [1]) or
NSSAs ([2]). AS-external-LSAs are omitted from these areas' link-
state databases, thereby controlling database size.

However, unexpected database overflows cannot be handled in the above
manner. This memo describes a way of dynamically limiting database
size under overflow conditions. The basic mechanism is as follows:

(1) A parameter, ospfExtLsdbLimit, is configured in each router
indicating the maximum number of AS-external-LSAs (excluding
those describing the default route) that are allowed in the
link-state database. This parameter must be the same in all
routers in the routing domain (see Section 2.1); synchronization
of the parameter is achieved via network management.

(2) In any router's database, the number of AS-external-LSAs
(excluding default) is never allowed to exceed ospfExtLsdbLimit.
If a router receives a non-default AS-external-LSA that would
cause the limit of ospfExtLsdbLimit to be exceeded, it drops the
LSA and does NOT acknowledge it.

(3) If the number of non-default AS-external-LSAs in a router's
database hits ospfExtLsdbLimit, the router a) flushes all non-
default AS-external-LSAs that it has itself originated (see
Section 2.2) and b) goes into "OverflowState".

(4) While in OverflowState, the router refuses to originate any
non-default AS-external-LSAs (see Section 2.3.2).

(5) Optionally, the router can attempt to leave OverflowState after
the configurable parameter ospfExitOverflowInterval has elapsed
since entering OverflowState (see Section 2.4). Only at this
point can the router resume originating non-default AS-
external-LSAs.

The reason for limiting non-default AS-external-LSAs, but not other
LSA types, is twofold. First of all, the non-default AS-external LSAs
are the most likely to dominate database size in those networks with
huge databases (e.g., regional networks; see [5]). Second, the non-
default AS-external-LSAs can be viewed as "optional" in the following
sense: the router can probably be monitored/reconfigured without
them. (However, using similar strategies, other LSA types can also be
limited; see Section 5.)

The method of dealing with database overflow described herein has the
following desirable properties:

o After a short period of convergence, all routers will have
identical link-state databases. This database will contain less
than ospfExtLsdbLimit non-default AS-external-LSAs.

o At all times, routing WITHIN the OSPF Autonomous System will
remain intact. Among other things, this means that the routers
will continue to be manageable.

o Default routing to external destinations will also remain
intact. This hopefully will mean that a large amount of external
connectivity will be preserved, although possibly taking less
efficient routes.

o If parameter ospfExitOverflowInterval is configured, the OSPF
system will recover fully and automatically (i.e., without
network management intervention) from transient database
overflow conditions (see Section 2.4).

2. Implementation details

This section describes the mechanism for dealing with database
overflow in more detail. The section is organized around the concept
OverflowState, describing how routers enter the OverflowState, the
operation of the router while in OverflowState, and when the router
leaves OverflowState.

2.1. Configuration

The following configuration parameters are added to support the
database overflow functionality. These parameters are set by
network management.

ospfExtLsdbLimit
When the number of non-default AS-external-LSAs in a
router's link-state database reaches ospfExtLsdbLimit, the
router enters OverflowState. The router never holds more

than ospfExtLsdbLimit non-default AS-external-LSAs in its
database.

ospfExtLsdbLimit MUST be set identically in all routers
attached to the OSPF backbone and/or any "regular" OSPF
area. (This memo does not pertain to routers contained
within OSPF stub areas nor NSSAs, since such routers do not
receive AS-external-LSAs.) If ospfExtLsdbLimit is not set
identically in all routers, then when the database
overflows: 1) the routers will NOT converge on a common
link-state database, 2) incorrect routing, possibly
including routing loops, will result and 3) constant
retransmission of AS-external-LSAs will occur. Identical
setting of ospfExtLsdbLimit is achieved/ensured by network
management.

When ospfExtLsdbLimit is set in a router, the router must
have some way to guarantee that it can hold that many non-
default AS-external-LSAs in its link-state database. One way
of doing this is to preallocate resources (e.g., memory) for
the configured number of LSAs.

ospfExitOverflowInterval
The number of seconds that, after entering OverflowState, a
router will attempt to leave OverflowState. This allows the
router to again originate non-default AS-external-LSAs. When
set to 0, the router will not leave OverflowState until
restarted. The default setting for ospfExitOverflowInterval
is 0.

It is not necessary for ospfExitOverflowInterval to be
configured the same in all routers. A smaller value may be
configured in those routers that originate the "more
important" AS-external-LSAs. In fact, setting
ospfExitOverflowInterval the same may cause problems, as
multiple routers attempt to leave OverflowState
simultaneously. For this reason, the value of
ospfExitOverflowInterval must be "jittered" by randomly
varying its value within the range of plus or minus 10
percent before using.

2.2. Entering OverflowState

The router enters OverflowState when the number of non-default
AS-external-LSAs in the database hits ospfExtLsdbLimit. There are
two cases when this can occur. First, when receiving an LSA during
flooding. In this case, an LSA which does not already have a
database instance is added in Step 5 of Section 13 of [1]. The

second case is when the router originates a non-default AS-
external-LSA itself.

Whenever the router enters OverflowState it flushes all non-
default AS-external-LSAs that it itself had originated. Flushing
is accomplished through the premature aging scheme described in
Section 14.1 of [1]. Only self-originated LSAs are flushed; those
originated by other routers are kept in the link-state database.

2.3. Operation while in OverflowState

While in OverflowState, the flooding and origination of non-
default AS-external-LSAs are modified in the following fashion.

2.3.1. Modifications to Flooding

Flooding while in OverflowState is modified as follows. If in
Step 5 of Section 13 of [1], a non-default AS-external-LSA has
been received that a) has no current database instance and b)
would cause the count of non-default AS-external-LSAs to exceed
ospfExtLsdbLimit, then that LSA is discarded. Such an LSA is
not installed in the link-state database, nor is it
acknowledged.

When all routers have identical values for ospfExtLsdbLimit (as
required), the above flooding modification will only be invoked
during a short period of convergence. During convergence, there
will be retransmissions of LSAs. However, after convergence the
retransmissions will cease, as the routers settle on a database
having less than ospfExtLsdbLimit non-default As-external-LSAs.

In OverflowState, non-default AS-external-LSAs ARE still
accepted in the following conditions:

(1) If the LSA updates an LSA that currently exists in the
router's link-state database.

(2) LSAs having LS age of MaxAge are always accepted. The
processing of these LSAs follows the procedures
described in Sections 13 and 14 of [1].

(3) If adding the LSA to the router's database would keep
the number of non-default AS-external-LSAs less than or
equal to ospfExtLsdbLimit, the LSA is accepted.

2.3.2. Originating AS-external-LSAs

Originating AS-external-LSAs is described in Section 12.4.5 of
[1]. When a router is in OverflowState, it does not originate
non-default AS-external-LSAs. In other words, the only AS-
external-LSAs originated by a router in OverflowState have Link
State ID 0.0.0.0.

2.3.3. Receiving self-originated LSAs

Receiving self-originated LSAs is described in Section 13.4 of
[1]. When in OverflowState, a router receiving a self-
originated non-default AS-external-LSA responds by flushing it
from the routing domain using the premature aging scheme
described in Section 14.1 of [1].

2.4. Leaving OverflowState

If ospfExitOverflowInterval is non-zero, then as soon as a router
enters OverflowState, it sets a timer equal to the value of
ospfExitOverflowInterval (plus or minus a random value in the
range of 10 percent). When this timer fires, the router leaves
OverflowState and begins originating non-default AS-external-LSAs
again.

This allows a router to automatically recover from transient
overflow conditions. For example, an AS boundary router that
imports a great many AS-external-LSAs may crash. Other routers may
then start importing the routes, but until the crashed AS boundary
router is either a) restarted or b) its AS-external-LSAs age out,
there will be a much larger database than usual. Since such an
overflow is guaranteed to go away in MaxAge seconds (1 hour),
automatic recovery may be appropriate (and fast enough) if the
overflow happens off-hours.

As soon as the router leaves OverflowState, it is again eligible
to reenter OverflowState according to the text of Section 2.2.

3. An example

As an example, suppose that a router implements the database overflow
logic, and that its ospfExtLsdbLimit is 10,000 and its
ospfExitOverflowInterval is set to 600 seconds. Suppose further that
the router itself is originating 400 non-default AS-external-LSAs,
and that the current number of non-default AS-external-LSAs in the
router's database is equal to 9,997.

Next, it receives a Link State Update packet from a neighbor,
containing 6 non-default AS-external-LSAs, none of which have current
database copies. The first two LSAs are then installed in the
database. The third LSA is also installed in the database, but causes
the router to go into OverflowState. Going into OverflowState causes
the router to flush (via premature aging) its 400 self-originated
non-default LSAs. However, these 400 LSAs are still considered to be
part of the link-state database until their re-flooding (with age set
to MaxAge) is acknowledged (see Section 14 of [1]); for this reason,
the last three LSAs in the received update are discarded without
being acknowledged.

After some small period of time all routers will converge on a common
database, having less than 10,000 non-default AS-external-LSAs.
During this convergence period there may be some link-state
retransmissions; for example, the sender of the above Link State
Update packet may retransmit the three LSAs that were discarded. If
this retransmission happens after the flushing of the 400 self-
originated LSAs is acknowledged, the 3 LSAs will then be accepted.

Going into OverflowState also causes the router to set a timer that
will fire some time between 540 and 660 seconds later. When this
timer fires, the router will leave OverflowState and re-originate its
400 non-default AS-external-LSAs, provided that the current database
has less than 9600 (10,000 - 400) non-default AS-external-LSAs. If
there are more than 9600, the timer is simply restarted.

4. Administrative response to database overflow

Once the link-state database has overflowed, it may take intervention
by network management before all routing is restored. (If the
overflow condition is transient, routing may be restored
automatically; see Section 2.4 for details.) An overflow condition is
indicated by SNMP traps (see Appendix B). Possible responses by a
network manager may include:

o Increasing the value of ospfExtLsdbLimit. Perhaps it had been
set too conservatively, and the routers are able to support
larger databases than they are currently configured for.

o Isolating routers having limited resources within OSPF stub
areas or NSSAs. This would allow increasing the value of
ospfExtLsdbLimit in the remaining routers.

o Reevaluating the need to import certain external routes. If
ospfExtLsdbLimit cannot be increased, the network manager will
want to make sure that the more important routes continue to be
imported; this is accomplished by turning off the importing of

less important routes.

5. Operational experience

The database overflow scheme described in this memo has been
implemented in the Proteon router for a number of years, with the
following differences. First, the router did not leave OverflowState
until it was restarted (i.e., ospfExitOverflowInterval was always 0).
Second, default AS-external-LSAs were not separated from non-default
AS-external-LSAs. Operationally the scheme performed as expected:
during overflow conditions, the routers converged on a common
database having less than a configured number of AS-external-LSAs.

6. Possible enhancements

Possible enhancements to the overflow scheme include the following:

o Other LSA types, with the exception of the transit LSAs
(router-LSAs and network-LSAs), could be limited in a similar
fashion. For example, one could limit the number of summary-
LSAs, or group-membership-LSAs (see [6]).

o Rather than flushing all of its non-default AS-external-LSAs
when entering OverflowState, a router could flush a fixed number
whenever the database size hits ospfExtLsdbLimit. This would
allow the router to prioritize its AS-external-LSAs, flushing
the least important ones first.

A. Related MIB parameters

The following OSPF MIB variables have been defined to support the
database overflow procedure described in this memo (see [4] for more
information):

ospfExtLsdbLimit
As in Section 2.1 of this memo, the maximum number of non-
default AS-external-LSAs that can be stored within the database.
If set to -1, there is no limit.

ospfExitOverflowInterval
As in Section 2.1 of this memo, the number of seconds that,
after entering OverflowState, a router will attempt to leave
OverflowState. This allows the router to again originate non-
default AS-external-LSAs. When set to 0, the router will not
leave OverflowState until restarted.

ospfLsdbOverflow
A trap indicating that the number of non-default AS-external-
LSAs has exceeded or equaled ospfExtLsdbLimit. In other words,
this trap indicates that the router is entering OverflowState.

ospfLsdbApproachingOverflow
A trap indicating that the number of non-default AS-external-
LSAs has exceeded ninety percent of "ospfExtLsdbLimit".

References

[1] Moy, J., "OSPF Version 2", RFC1583, Proteon, Inc., March 1994.

[2] Coltun, R., and V. Fuller, "The OSPF NSSA Option", RFC1587,
RainbowBridge Communications, Stanford University, March 1994.

[3] Moy, J., Editor, "OSPF Protocol Analysis", RFC1245, Proteon,
Inc., July 1991.

[4] Baker F., and R. Coltun, "OSPF Version 2 Management Information
Base", Work in Progress.

[5] Moy, J., Editor, "Experience with the OSPF Protocol", RFC1246,
Proteon, Inc., July 1991.

[6] Moy, J., "Multicast Extensions to OSPF", RFC1584, Proteon, Inc.,
March 1994.

Security Considerations

Security issues are not discussed in this memo.

Author's Address

John Moy
Cascade Communications Corp.
5 Carlisle Road
Westford, MA 01886

Phone: 508-692-2600 Ext. 394
Fax: 508-692-9214

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