|Title||Graceful OSPF Restart
|Author||J. Moy, P. Pillay-Esnault, A. Lindem
Network Working Group J. Moy
Request for Comments: 3623 Sycamore Networks
Category: Standards Track P. Pillay-Esnault
Graceful OSPF Restart
Status of this Memo
This document specifies an Internet standards track protocol for the
Internet community, and requests discussion and suggestions for
improvements. Please refer to the current edition of the "Internet
Official Protocol Standards" (STD 1) for the standardization state
and status of this protocol. Distribution of this memo is unlimited.
Copyright (C) The Internet Society (2003). All Rights Reserved.
This memo documents an enhancement to the OSPF routing protocol,
whereby an OSPF router can stay on the forwarding path even as its
OSPF software is restarted. This is called "graceful restart" or
"non-stop forwarding". A restarting router may not be capable of
adjusting its forwarding in a timely manner when the network topology
changes. In order to avoid the possible resulting routing loops, the
procedure in this memo automatically reverts to a normal OSPF restart
when such a topology change is detected, or when one or more of the
restarting router's neighbors do not support the enhancements in this
memo. Proper network operation during a graceful restart makes
assumptions upon the operating environment of the restarting router;
these assumptions are also documented.
Table of Contents
1. Overview . . . . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Operation of Restarting Router . . . . . . . . . . . . . . . . 3
2.1. Entering Graceful Restart. . . . . . . . . . . . . . . . 4
2.2. When to Exit Graceful Restart. . . . . . . . . . . . . . 5
2.3. Actions on Exiting Graceful Restart. . . . . . . . . . . 6
3. Operation of Helper Neighbor . . . . . . . . . . . . . . . . . 7
3.1. Entering Helper Mode . . . . . . . . . . . . . . . . . . 7
3.2. Exiting Helper Mode. . . . . . . . . . . . . . . . . . . 8
4. Backward Compatibility . . . . . . . . . . . . . . . . . . . . 9
5. Unplanned Outages. . . . . . . . . . . . . . . . . . . . . . . 10
6. Interaction with Traffic Engineering . . . . . . . . . . . . . 11
7. Possible Future Work . . . . . . . . . . . . . . . . . . . . . 11
8. Intellectual Property Rights Notice. . . . . . . . . . . . . . 11
9. References . . . . . . . . . . . . . . . . . . . . . . . . . . 11
9.1. Normative References . . . . . . . . . . . . . . . . . . 11
9.2. Informative References . . . . . . . . . . . . . . . . . 11
A. Grace-LSA Format . . . . . . . . . . . . . . . . . . . . . . . 13
B. Configurable Parameters. . . . . . . . . . . . . . . . . . . . 15
Security Considerations. . . . . . . . . . . . . . . . . . . . . . 16
Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . . 16
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 17
Full Copyright Statement . . . . . . . . . . . . . . . . . . . . . 18
Today many Internet routers implement a separation of control and
forwarding functions. Certain processors are dedicated to control
and management tasks such as OSPF routing, while other processors
perform the data forwarding tasks. This separation creates the
possibility of maintaining a router's data forwarding capability
while the router's control software is restarted/reloaded. We call
such a possibility "graceful restart" or "non-stop forwarding".
The OSPF protocol presents a problem to graceful restart whereby,
under normal operation, OSPF intentionally routes around a restarting
router while it rebuilds its link-state database. OSPF avoids the
restarting router to minimize the possibility of routing loops and/or
black holes caused by lack of database synchronization. Avoidance is
accomplished by having the router's neighbors reissue their LSAs,
omitting links to the restarting router.
However, if (a) the network topology remains stable and (b) the
restarting router is able to keep its forwarding table(s) across the
restart, it would be safe to keep the restarting router on the
forwarding path. This memo documents an enhancement to OSPF that
makes such graceful restart possible, and automatically reverts back
to a standard OSPF restart for safety when network topology changes
In a nutshell, the OSPF enhancements for graceful restart are as
- The router attempting a graceful restart originates link-local
Opaque-LSAs, herein called Grace-LSAs, announcing its intention to
perform a graceful restart within a specified amount of time or
- During the grace period, its neighbors continue to announce the
restarting router in their LSAs as if it were fully adjacent
(i.e., OSPF neighbor state Full), but only if the network topology
remains static (i.e., the contents of the LSAs in the link-state
database having LS types 1-5,7 remain unchanged and periodic
refreshes are allowed).
There are two roles being played by OSPF routers during graceful
restart. First there is the router that is being restarted. The
operation of this router during graceful restart, including how the
router enters and exits graceful restart, is the subject of Section
2. Then there are the router's neighbors, which must cooperate in
order for the restart to be graceful. During graceful restart, we
say that the neighbors are running in "helper mode". Section 3
covers the responsibilities of a router running in helper mode,
including entering and exiting helper mode.
2. Operation of Restarting Router
After the router restarts/reloads, it must change its OSPF processing
somewhat until it re-establishes full adjacencies with all its former
fully-adjacent neighbors. This time period, between the
restart/reload and the reestablishment of adjacencies, is called
"graceful restart". During graceful restart:
1) The restarting router does not originate LSAs with LS types 1-
5,7. Instead, the restarting router wants the other routers in
the OSPF domain to calculate routes using the LSAs that it
originated prior to its restart. During this time, the
restarting router does not modify or flush received self-
originated LSAs, (see Section 13.4 of ). Instead they are
accepted as valid. In particular, the grace-LSAs that the
restarting router originated before the restart are left in
place. Received self-originated LSAs will be dealt with when
the router exits graceful restart (see Section 2.3).
2) The restarting router runs its OSPF routing calculations, as
specified in Section 16 of . This is necessary to return
any OSPF virtual links to operation. However, the restarting
router does *not* install OSPF routes into the system's
forwarding table(s) and relies on the forwarding entries that
it installed prior to the restart.
3) If the restarting router determines that it was the Designated
Router on a given segment prior to the restart, it elects
itself as the Designated Router again. The restarting router
knows that it was the Designated Router if, while the
associated interface is in Waiting state, a Hello packet is
received from a neighbor listing the router as the Designated
Otherwise, the restarting router operates the same as any other OSPF
router. It discovers neighbors using OSPF's Hello protocol, elects
Designated and Backup Designated Routers, performs the Database
Exchange procedure to initially synchronize link-state databases with
its neighbors, and maintains this synchronization through flooding.
The processes of entering graceful restart, and of exiting graceful
restart (either successfully or not) are covered in the following
2.1. Entering Graceful Restart
The router (call it Router X) is informed of the desire for its
graceful restart when an appropriate command is issued by the network
operator. The network operator may also specify the length of the
grace period, or the necessary grace period may be calculated by the
router's OSPF software. In order to avoid the restarting router's
LSAs from aging out, the grace period should not exceed LSRefreshTime
(1800 second) .
In preparation for the graceful restart, Router X must perform the
following actions before its software is restarted/reloaded:
(Note that common OSPF shutdown procedures are *not* performed,
since we want the other OSPF routers to act as if Router X remains
in continuous service. For example, Router X does not flush its
locally originated LSAs, since we want them to remain in other
routers' link-state databases throughout the restart period.)
1) Router X must ensure that its forwarding table(s) is/are up-
to-date and will remain in place across the restart.
2) The router may need to preserve the cryptographic sequence
numbers being used on each interface in non-volatile storage.
An alternative is to use the router's clock for cryptographic
sequence number generation and ensure that the clock is
preserved across restarts (either on the same or redundant
route processors). If neither of these can be guaranteed, it
can take up to RouterDeadInterval seconds after the restart
before adjacencies can be reestablished and this would force
the grace period to be lengthened greatly.
Router X then originates the grace-LSAs. These are link-local
Opaque-LSAs (see Appendix A). Their LS Age field is set to 0, and
the requested grace period (in seconds) is inserted into the body of
the grace-LSA. The precise contents of the grace-LSA are described
in Appendix A.
A grace-LSA is originated for each of the router's OSPF interfaces.
If Router X wants to ensure that its neighbors receive the grace-
LSAs, it should retransmit the grace-LSAs until they are acknowledged
(i.e., perform standard OSPF reliable flooding of the grace-LSAs).
If one or more fully adjacent neighbors do not receive grace-LSAs,
they will more than likely cause premature termination of the
graceful restart procedure (see Section 4).
After the grace-LSAs have been sent, the router should store the fact
that it is performing graceful restart along with the length of the
requested grace period in non-volatile storage. (Note to
implementors: It may be easiest to simply store the absolute time of
the end of the grace period). The OSPF software should then be
restarted/reloaded. When the reloaded software starts executing the
graceful restart, the protocol modifications in Section 2 are
followed. (Note that prior to the restart, the router does not know
whether its neighbors are going to cooperate as "helpers"; the mere
reception of grace-LSAs does not imply acceptance of helper
responsibilities. This memo assumes that the router would want to
restart anyway, even if the restart is not going to be graceful).
2.2. When to Exit Graceful Restart
A Router X exits graceful restart when any of the following occurs:
1) Router X has reestablished all its adjacencies. Router X can
determine this by examining the router-LSAs that it last
originated before the restart (called the "pre-restart router-
LSA"), and, on those segments where the router is the
Designated Router, the pre-restart network-LSAs. These LSAs
will have been received from the helping neighbors, and need
not have been stored in non-volatile storage across the
restart. All previous adjacencies will be listed as type-1 and
type-2 links in the router-LSA, and as neighbors in the body of
2) Router X receives an LSA that is inconsistent with its pre-
restart router-LSA. For example, X receives a router-LSA
originated by router Y that does not contain a link to X, even
though X's pre-start router-LSA did contain a link to Y. This
indicates that either a) Y does not support graceful restart,
b) Y never received the grace-LSA or c) Y has terminated its
helper mode for some reason (Section 3.2). A special case of
LSA inconsistency is when Router X establishes an adjacency
with router Y and doesn't receive an instance of its own pre-
restart router LSA.
3) The grace period expires.
2.3. Actions on Exiting Graceful Restart
Upon exiting "graceful restart", the restarting router reverts back
to completely normal OSPF operation, reoriginating LSAs based on the
router's current state and updating its forwarding table(s) based on
the current contents of the link-state database. In particular, the
following actions should be performed when exiting, either
successfully or unsuccessfully, graceful restart:
1) The router should reoriginate its router-LSAs for all attached
areas in order to make sure they have the correct contents.
2) The router should reoriginate network-LSAs on all segments
where it is the Designated Router.
3) The router reruns its OSPF routing calculations (Section 16 of
), this time installing the results into the system
forwarding table, and originating summary-LSAs, Type-7 LSAs and
AS-external-LSAs as necessary.
4) Any remnant entries in the system forwarding table that were
installed before the restart, but that are no longer valid,
should be removed.
5) Any received self-originated LSAs that are no longer valid
should be flushed.
6) Any grace-LSAs that the router originated should be flushed.
3. Operation of Helper Neighbor
The helper relationship is per network segment. As a "helper
neighbor" on a segment S for a restarting router X, router Y has
several duties. It monitors the network for topology changes, and as
long as there are none, continues to advertise its LSAs as if X had
remained in continuous OSPF operation. This means that Y's LSAs
continue to list an adjacency to X over network segment S, regardless
of the adjacency's current synchronization state. This logic affects
the contents of both router-LSAs and network-LSAs, and also depends
on the type of network segment S (see Sections 126.96.36.199 through
188.8.131.52 and Section 12.4.2 of ). When helping over a virtual
link, the helper must also continue to set bit V in its router-LSA
for the virtual link's transit area (Section 12.4.1 of ).
Also, if X was the Designated Router on network segment S when the
helping relationship began, Y maintains X as the Designated Router
until the helping relationship is terminated.
3.1. Entering Helper Mode
When a router Y receives a grace-LSA from router X, it enters helper
mode for X on the associated network segment, as long as all the
following checks pass:
1) Y currently has a full adjacency with X (neighbor state Full)
over the associated network segment. On broadcast, NBMA and
Point-to-MultiPoint segments, the neighbor relationship with X
is identified by the IP interface address in the body of the
grace-LSA (see Appendix A). On all other segment types, X is
identified by the grace-LSA's Advertising Router field.
2) There have been no changes in content to the link-state
database (LS types 1-5,7) since router X restarted. This is
determined as follows:
- Router Y examines the link-state retransmission list for X
over the associated network segment.
- If there are any LSAs with LS types 1-5,7 on the list,
then they all must be periodic refreshes.
- If there are instead LSAs on the list whose contents have
changed (see Section 3.3 of ), Y must refuse to enter
Router Y may optionally disallow graceful restart with
Router X on other network segments. Determining whether
changed LSAs have been successfully flooded to router Y on
other network segments is feasible but beyond the scope of
3) The grace period has not yet expired. This means that the LS
age of the grace-LSA is less than the grace period specified in
the body of the grace-LSA (Appendix A).
4) Local policy allows Y to act as the helper for X. Examples of
configured policies might be a) never act as helper, b) never
allow the grace period to exceed a Time T, c) only help on
software reloads/upgrades, or d) never act as a helper for
specific routers (specified by OSPF Router ID).
5) Router Y is not in the process of graceful restart.
There is one exception to the above requirements. If Y was already
helping X on the associated network segment, the new grace-LSA should
be accepted and the grace period should be updated accordingly.
Note that Router Y may be helping X on some network segments, and not
on others. However, that circumstance will probably lead to the
premature termination of X's graceful restart, as Y will not continue
to advertise adjacencies on the segments where it is not helping (see
Alternately, Router Y may choose to enter helper mode when a grace-
LSA is received and the above checks pass for all adjacencies with
Router X. This implementation alternative of aggregating the
adjacencies with respect to helper mode is compatible with
implementations considering each adjacency independently.
A single router is allowed to simultaneously serve as a helper for
multiple restarting neighbors.
3.2. Exiting Helper Mode
Router Y ceases to perform the helper function for its neighbor
Router X on a given segment when one of the following events occurs:
1) The grace-LSA originated by X on the segment is flushed. This
indicates the successful termination of graceful restart.
2) The grace-LSA's grace period expires.
3) A change in link-state database contents indicates a network
topology change, which forces termination of a graceful
restart. Specifically, if router Y installs a new LSA in its
database with LS types 1-5,7 and having the following two
properties, it should cease helping X. The two properties of
the LSA are:
a) the contents of the LSA have changed; this includes LSAs
with no previous link-state database instance and the
flushing of LSAs from the database, but excludes periodic
LSA refreshes (see Section 3.3 of ), and
b) the LSA would have been flooded to X, had Y and X been fully
adjacent. As an example of the second property, if Y
installs a changed AS-external-LSA, it should not terminate
a helping relationship with a neighbor belonging to a stub
area, as that neighbor would not see the AS-external-LSA in
any case. An implementation MAY provide a configuration
option to disable link-state database options from
terminating graceful restart. Such an option will, however,
increase the risk of transient routing loops and black
When Router Y exits helper mode for X on a given network segment, it
reoriginates its LSAs based on the current state of its adjacency to
Router X over the segment. In detail, Y takes the following actions:
a) Y recalculates the Designated Router for the segment,
b) Y reoriginates its router-LSA for the segment's OSPF area,
c) if Y is Designated Router for the segment, it reoriginates the
network-LSA for the segment and
d) if the segment was a virtual link, Y reoriginates its router-
LSA for the virtual link's transit area.
If Router Y aggregated adjacencies with Router X when entering helper
mode (as described in section 3.1), it must also exit helper mode for
all adjacencies with Router X when any one of the exit events occurs
for an adjacency with Router X.
4. Backward Compatibility
Backward-compatibility with unmodified OSPF routers is an automatic
consequence of the functionality documented above. If one or more
neighbors of a router requesting graceful restart are unmodified, or
if they do not receive the grace-LSA, the graceful restart reverts to
a normal OSPF restart.
The unmodified routers will start routing around the restarted router
X as it performs initial database synchronization by reissuing their
LSAs with links to X omitted. These LSAs will be interpreted by
helper neighbors as a topology change, and by X as an LSA
inconsistency, in either case, reverting to normal OSPF operation.
5. Unplanned Outages
The graceful restart mechanisms in this memo can be used for
unplanned outages. (Examples of unplanned outages include the crash
of a router's control software, an unexpected switchover to a
redundant control processor, etc). However, implementors and network
operators should note that attempting graceful restart from an
unplanned outage may not be a good idea, owing to the router's
inability to properly prepare for the restart (see Section 2.1). In
particular, it seems unlikely that a router could guarantee the
sanity of its forwarding table(s) across an unplanned restart. In
any event, implementors providing the option to recover gracefully
from unplanned outages must allow a network operator to turn the
In contrast to the procedure for planned restart/reloads that was
described in Section 2.1, a router attempting graceful restart after
an unplanned outage must originate grace-LSAs *after* its control
software resumes operation. The following points must be observed
during this grace-LSA origination.
o The grace-LSAs must be originated and be sent *before* the
restarted router sends any OSPF Hello Packets. On broadcast
networks, this LSA must be flooded to the AllSPFRouters multicast
address (184.108.40.206) since the restarting router is not aware of
its previous DR state.
o The grace-LSAs are encapsulated in Link State Update Packets and
sent out to all interfaces, even though the restarted router has
no adjacencies and no knowledge of previous adjacencies.
o To improve the probability that grace-LSAs will be delivered, an
implementation may send them multiple times (see for example the
Robustness Variable in ).
o The restart reason in the grace-LSAs must be set to 0 (unknown) or
3 (switch to redundant control processor). This enables the
neighbors to decide whether they want to help the router through
an unplanned restart.
6. Interaction with Traffic Engineering
The operation of the Traffic Engineering Extensions to OSPF 
during OSPF Graceful Restart is specified in .
7. Possible Future Work
Devise a less conservative algorithm for graceful restart helper
termination that provides a comparable level of black hole and
routing loop avoidance.
8. Intellectual Property Rights Notice
The IETF takes no position regarding the validity or scope of any
intellectual property or other rights that might be claimed to
pertain to the implementation or use of the technology described in
this document or the extent to which any license under such rights
might or might not be available; neither does it represent that it
has made any effort to identify any such rights. Information on the
IETF's procedures with respect to rights in standards-track and
standards-related documentation can be found in BCP-11. Copies of
claims of rights made available for publication and any assurances of
licenses to be made available, or the result of an attempt made to
obtain a general license or permission for the use of such
proprietary rights by implementors or users of this specification can
be obtained from the IETF Secretariat.
The IETF invites any interested party to bring to its attention any
copyrights, patents or patent applications, or other proprietary
rights which may cover technology that may be required to practice
this standard. Please address the information to the IETF Executive
9.1. Normative References
 Moy, J., "OSPF Version 2", STD 54, RFC 2328, April 1998.
 Coltun, R., "The OSPF Opaque LSA Option", RFC 2370, July 1998.
9.2. Informative References
 Murphy, S., Badger, M. and B. Wellington, "OSPF with Digital
Signatures", RFC 2154, June 1997.
 Katz, D., Kompella, K. and D. Yeung, "Traffic Engineering (TE)
Extensions to OSPF Version 2", RFC 3630, September 2003.
 Murphy, P., "The OSPF Not-So-Stubby Area (NSSA) Option", RFC
3101, January 2003.
 Kompella, K., et al., "Routing Extensions in Support of
Generalized MPLS", Work in Progress.
 Moy, J., "Extending OSPF to Support Demand Circuits", RFC 1793,
 Cain, B., Deering, S., Kouvelas, I., Fenner, B. and A.
Thyagarajan, "Internet Group Management Protocol, Version 3", RFC
3376, October 2002.
A. Grace-LSA Format
The grace-LSA is a link-local scoped Opaque-LSA , having an Opaque
Type of 3 and an Opaque ID equal to 0. Grace-LSAs are originated by
a router that wishes to execute a graceful restart of its OSPF
software. A grace-LSA requests that the router's neighbors aid in
its graceful restart by continuing to advertise the router as fully
adjacent during a specified grace period.
Each grace-LSA has an LS age field set to 0 when the LSA is first
originated; the current value of the LS age then indicates how long
ago the restarting router made its request. The body of the LSA is
TLV-encoded. The TLV-encoded information includes the length of the
grace period, the reason for the graceful restart and, when the
grace-LSA is associated with a broadcast, NBMA or Point-to-MultiPoint
network segment, the IP interface address of the restarting router.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
| LS age | Options | 9 |
| 3 | 0 |
| Advertising Router |
| LS sequence number |
| LS checksum | length |
+- TLVs -+
| ... |
The format of the TLVs within the body of a grace-LSA is the same as
the format used by the Traffic Engineering Extensions to OSPF .
The LSA payload consists of one or more nested Type/Length/Value
(TLV) triplets. The format of each TLV is:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
| Type | Length |
| Value... |
The Length field defines the length of the value portion in octets
(thus a TLV with no value portion would have a length of zero). The
TLV is padded to four-octet alignment; padding is not included in the
length field (so a three octet value would have a length of three,
but the total size of the TLV would be eight octets). Nested TLVs
are also 32-bit aligned. For example, a one byte value would have
the length field set to 1, and three bytes of padding would be added
to the end of the value portion of the TLV. Unrecognized types are
The following is the list of TLVs that can appear in the body of a
o Grace Period (Type=1, length=4). The number of seconds that the
router's neighbors should continue to advertise the router as
fully adjacent, regardless of the state of database
synchronization between the router and its neighbors. Since this
time period began when grace-LSA's LS age was equal to 0, the
grace period terminates when either:
a) the LS age of the grace-LSA exceeds the value of a Grace Period
b) the grace-LSA is flushed. See Section 3.2 for other conditions
that terminate graceful restart.
This TLV must always appear in a grace-LSA.
o Graceful restart reason (Type=2, length=1). Encodes the reason
for the router restart as one of the following: 0 (unknown), 1
(software restart), 2 (software reload/upgrade) or 3 (switch to
redundant control processor). This TLV must always appear in a
o IP interface address (Type=3, length=4). The router's IP
interface address on the subnet associated with the grace-LSA.
Required on broadcast, NBMA and Point-to-MultiPoint segments,
where the helper uses the IP interface address to identify the
restarting router (see Section 3.1).
DoNotAge is never set in a grace-LSA, even if the grace-LSA is
flooded over a demand circuit . This is because the grace-LSA's
LS age field is used to calculate the duration of the grace period.
Grace-LSAs have link-local scope because they only need to be seen by
the router's direct neighbors.
Additional Grace-LSA TLVs must be described in an Internet Draft and
will be subject to the expert review of the OSPF Working Group.
B. Configurable Parameters
OSPF graceful restart parameters are suggested below. Section B.1
contains a minimum subset of parameters that should be supported.
B.2 includes some additional configuration parameters that an
implementation may choose to support.
B.1. Global Parameters (Minimum subset)
The router's level of support for OSPF graceful restart.
Allowable values are none, planned restart only, and
The graceful restart interval in seconds. The range is from 1 to
1800 seconds, with a suggested default of 120 seconds.
B.2. Global Parameters (Optional)
The router's support for acting as an OSPF restart helper.
Allowable values are none, planned restart only, and
Indicates whether or not an OSPF restart helper should terminate
graceful restart when there is a change to an LSA that would be
flooded to the restarting router or when there is a changed LSA on
the restarting router's retransmission list when graceful restart
is initiated. The suggested default is enabled.
One of the ways to attack a link-state protocol such as OSPF is to
inject false LSAs into, or corrupt existing LSAs in, the link-state
database. Injecting a false grace-LSA would allow an attacker to
spoof a router that, in reality, has been withdrawn from service.
The standard way to prevent such corruption of the link-state
database is to secure OSPF protocol exchanges using the cryptographic
authentication specified in . An even stronger way of securing
link-state database contents has been proposed in .
When cryptographic authentication  is used on the restarting
router the preservation of received sequence numbers in non-volatile
storage is not mandatory. There is a risk that a replayed Hello
packet could cause neighbor state for a deceased neighbor to be
created. However, the risk is no greater than during normal
The authors wish to thank John Drake, Vishwas Manral, Kent Wong, and
Don Goodspeed for their helpful comments. We also wish to thank Alex
Zinin and Bill Fenner for their thorough review.
Sycamore Networks, Inc.
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Phone: (978) 367-2505
Fax: (978) 256-4203
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