OSPF Restart Signaling
A vendor-specific Hello bit that tells OSPF neighbors a router is restarting, so they keep their adjacencies.
Nguyen, L., Roy, A., and A. Zinin, "OSPF Restart Signaling", RFC 4812, DOI 10.17487/RFC4812, March 2007, <https://www.rfc-editor.org/info/rfc4812>.
@misc{rfc4812,
series = {Request for Comments},
number = 4812,
howpublished = {RFC 4812},
publisher = {RFC Editor},
doi = {10.17487/RFC4812},
url = {https://www.rfc-editor.org/info/rfc4812},
author = {L. Nguyen and A. Roy and A. Zinin},
title = {{OSPF Restart Signaling}},
pagetotal = 7,
year = 2007,
month = mar,
} RFC text
Contents
Network Working Group L. Nguyen
Request for Comments: 4812 A. Roy
Category: Informational Cisco Systems
A. Zinin
Alcatel-Lucent
March 2007
OSPF Restart Signaling
Status of This Memo
This memo provides information for the Internet community. It does
not specify an Internet standard of any kind. Distribution of this
memo is unlimited.
Copyright Notice
Copyright (C) The IETF Trust (2007).
Abstract
OSPF is a link-state intra-domain routing protocol used in IP
networks. Routers find new and detect unreachable neighbors via the
Hello subprotocol. Hello OSPF packets are also used to ensure two-
way connectivity within time. When a router restarts its OSPF
software, it may not know its neighbors. If such a router sends a
Hello packet on an interface, its neighbors are going to reset the
adjacency, which may not be desirable in certain conditions.
This memo describes a vendor-specific mechanism that allows OSPF
routers to inform their neighbors about the restart process. Note
that this mechanism requires support from neighboring routers. The
mechanism described in this document was proposed before Graceful
OSPF Restart, as described in RFC 3623, came into existence. It is
implemented/supported by at least one major vendor and is currently
deployed in the field. The purpose of this document is to capture
the details of this mechanism for public use. This mechanism is not
an IETF standard.
Table of Contents
1. Introduction ....................................................2
2. Proposed Solution ...............................................2
2.1. Sending Hello Packets with the RS-bit Set ..................3
2.2. Receiving Hello Packets with the RS-Bit Set ................3
2.3. Ensuring Topology Stability ................................4
3. Backward Compatibility ..........................................4
4. Security Considerations .........................................4
5. IANA Considerations .............................................4
6. References ......................................................5
6.1. Normative References .......................................5
6.2. Informative References .....................................5
Appendix A. Acknowledgements ......................................6
1. Introduction
While performing a graceful restart of OSPF software [RFC3623],
routers need to prevent their neighbors from resetting their
adjacencies. However, after a reload, routers may not be aware of
the neighbors they had adjacencies with in their previous
incarnations. If such a router sends a Hello packet on an interface
and this packet does not list some neighbors, those neighbors will
reset the adjacency with the restarting router.
This document describes a technique that allows restarting routers to
inform their neighbors that they may not know about some neighbors
yet and the absence of some router IDs in the Hello packets should be
ignored.
2. Proposed Solution
With this Restart Signaling Solution, a new bit, called RS (restart
signal), is introduced into the Extended Options (EO) TLV in the
Link-Local Signaling (LLS) block (see [RFC4813]). The value of this
bit is 0x00000002; see Figure 1 below.
+---+---+---+---+---+---+---+- -+---+---+---+---+---+---+---+---+
| * | * | * | * | * | * | * |...| * | * | * | * | * | * | RS| LR|
+---+---+---+---+---+---+---+- -+---+---+---+---+---+---+---+---+
Figure 1. Bits in Extended Options TLV
For a definition of the LR-bit, see [RFC4811].
2.1. Sending Hello Packets with the RS-bit Set
OSPF routers should set the RS-bit in the EO-TLV attached to a Hello
packet when it is not known that all neighbors are listed in this
packet, but the restarting router wants them to preserve their
adjacencies. The RS-bit must not be set in Hello packets longer than
RouterDeadInterval seconds.
2.2. Receiving Hello Packets with the RS-Bit Set
When an OSPF router receives a Hello packet containing the LLS block
with the EO-TLV that has the RS-bit set, the router should skip the
two-way connectivity check with the announcing neighbor (i.e., the
router should not generate a 1-WayReceived event for the neighbor if
it does not find its own router ID in the list of neighbors as
described in Section 10.5 of [RFC2328]), provided that the neighbor
Finite State Machine (FSM) for this neighbor is in the Full state.
The router should also send a unicast Hello back to the sender in
reply to a Hello packet with RS-bit set. This is to speed up
learning of previously known neighbors. When sending such a reply
packet, care must be taken to ensure that the RS-bit is clear in it.
Two additional fields are introduced in the neighbor data structure:
RestartState flag and ResyncTimeout timer. RestartState flag
indicates that a Hello packet with the RS-bit set has been received
and the local router expects its neighbor to go through the Link
State Database (LSDB) resynchronization procedure using [RFC4811].
ResyncTimeout is a single-shot timer limiting the delay between the
first seen Hello packet with the RS-bit set and initialization of the
LSDB resynchronization procedure. The length of ResyncTimeout timer
is RouterDeadInterval seconds.
When a Hello packet with the RS-bit set is received and RestartState
flag is not set for the neighbor, the router sets RestartState flag
and starts ResyncTimeout timer. If ResyncTimeout expires,
RestartState flag is cleared and a 1-WayReceived event is generated
for the neighbor. If, while ResyncTimeout timer is running, the
neighbor starts LSDB resynchronization procedure using [RFC4811],
ResyncTimeout timer is canceled. The router also clears RestartState
flag on completion of the LSDB resynchronization process.
Two or more routers on the same segment cannot have Hello packets
with the RS-bit set at the same time, as can be the case when two or
more routers restart at about the same time. In such a scenario, the
routers should clear the RestartState flag, cancel the ResyncTimeout
timer, and generate a 1-WayReceived event.
2.3. Ensuring Topology Stability
Under certain circumstances, it might be desirable to stop announcing
the restarting router as fully adjacent if this may lead to possible
routing loops. In order to provide this functionality, a
configurable option is provided on the neighboring routers that
instructs the OSPF process to follow the logics described below.
When an OSPF router schedules a routing table calculation due to a
change in the contents of its LSDB, it should also reset all
adjacencies with restarting routers (those with RestartState set to
TRUE) by clearing the RestartState neighbor flags, canceling
ResyncTimeout timers (if running), and generating the 1-WayReceived
events for the neighbor FSMs.
3. Backward Compatibility
The described technique requires cooperation from neighboring
routers. However, if neighbors do not support this technique, they
will just reset the adjacency.
4. Security Considerations
The described technique does not introduce any new security issues
into the OSPF protocol.
5. IANA Considerations
Please refer to the "IANA Considerations" section of [RFC4813] for
more information on the Extended Options bit definitions.
6. References
6.1. Normative References
[RFC2328] Moy, J., "OSPF Version 2", STD 54, RFC 2328, April 1998.
[RFC3623] Moy, J., Pillay-Esnault, P., and A. Lindem, "Graceful OSPF
Restart", RFC 3623, November 2003.
6.2. Informative References
[RFC4813] Friedman, B., Nguyen, L., Roy, A., Yeung, D., and A.
Zinin, "OSPF Link-Local Signaling", RFC 4813, March 2007.
[RFC4811] Nguyen, L., Roy, A., and A. Zinin, "OSPF Out-of-Band Link
State Database (LSDB) Resynchronization", RFC 4811, March
2007.
Appendix A. Acknowledgments
The authors would like to thank John Moy, Russ White, Don Slice, and
Alvaro Retana for their valuable comments.
Authors' Addresses
Liem Nguyen
Cisco Systems
225 West Tasman Drive
San Jose, CA 95134
USA
EMail: lhnguyen@cisco.com
Abhay Roy
Cisco Systems
225 West Tasman Drive
San Jose, CA 95134
USA
EMail: akr@cisco.com
Alex Zinin
Alcatel-Lucent
Mountain View, CA
USA
EMail: alex.zinin@alcatel-lucent.com
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AI ELI5
On this page
§1The Restart Problem
During a graceful restart of OSPF software (RFC 3623), a router must stop its neighbors from resetting their adjacencies. After a reload, though, the router may not know which neighbors it had adjacencies with before. If it sends a Hello packet that does not list some neighbors, those neighbors reset the adjacency.
This RFC describes a technique for restarting routers to tell neighbors that they may not know about some neighbors yet, so the absence of some router IDs in the Hello packets should be ignored. The abstract says the mechanism is vendor-specific, was proposed before Graceful OSPF Restart, has at least one major vendor supporting it, and is documented here for public use. It is not an IETF standard.
When a router wakes up from a reboot it forgets who its friends are. Its friends think it has left and cross it off the list, unless it tells them it is just restarting.
§2The RS Bit
The solution adds a new bit, called RS (restart signal), to the Extended Options (EO) TLV in the Link-Local Signaling (LLS) block defined in RFC 4813. Its value is 0x00000002. It sits next to the LR bit, which is defined in RFC 4811.
A router sets the RS bit in the EO-TLV attached to a Hello packet when it does not know that all neighbors are listed, but wants them to keep their adjacencies. The RS bit must not be set in Hello packets longer than RouterDeadInterval seconds.
The RS bit is a small flag on the router's greeting that means 'I am restarting, do not forget me.'
§3How Neighbors React
A router that receives a Hello with the RS bit set should skip the two-way connectivity check for that neighbor. That means it should not generate a 1-WayReceived event when it does not find its own router ID in the neighbor list, provided the neighbor state machine for this neighbor is in the Full state. It should also send a unicast Hello back with the RS bit clear, which speeds up learning of previously known neighbors.
Two new fields are added to the neighbor data structure: a RestartState flag and a ResyncTimeout timer. The timer is a single-shot timer lasting RouterDeadInterval seconds. It limits the delay between the first RS-bit Hello and the start of link state database resynchronization using RFC 4811.
- First RS-bit Hello: set the flag and start the timer.
- Timer expires: clear the flag and generate a
1-WayReceivedevent. - Neighbor starts resynchronization: cancel the timer, and clear the flag when it completes.
- Two or more routers on the same segment with RS-bit Hellos at once, as when they restart together: clear the flag, cancel the timer, and generate a
1-WayReceivedevent.
The neighbor starts a countdown when it hears 'be right back.' If the router returns and catches up in time, all is fine. If the countdown ends first, the neighbor crosses it off the list.
§4Topology Stability, Compatibility and Security
To avoid possible routing loops, a configurable option on neighbors can make them stop treating the restarting router as fully adjacent. With it, when a router schedules a routing table calculation because its link state database changed, it also resets all adjacencies with restarting routers: it clears their RestartState flags, cancels any running timers, and generates 1-WayReceived events.
The technique needs cooperation from neighbors. If a neighbor does not support it, it will just reset the adjacency. The Security Considerations section says the technique introduces no new security issues into OSPF, and the IANA section points to RFC 4813 for the Extended Options bit definitions.
If the map changes while the router is away, neighbors stop saving its seat, to be safe. A neighbor that does not know this trick just crosses the router off.