Internet Engineering Task Force (IETF) D. Katz
Request for Comments: 5882 D. Ward
Category: Standards Track Juniper Networks
ISSN: 2070-1721 June 2010
Generic Application of Bidirectional Forwarding Detection (BFD)
Abstract
This document describes the generic application of the Bidirectional
Forwarding Detection (BFD) protocol.
Status of This Memo
This is an Internet Standards Track document.
This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Further information on
Internet Standards is available in Section 2 of RFC 5741.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
http://www.rfc-editor.org/info/rfc5882.
Copyright Notice
Copyright (c) 2010 IETF Trust and the persons identified as the
document authors. All rights reserved.
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the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
Katz & Ward Standards Track [Page 1]RFC 5882 Generic Application of BFD June 2010
Table of Contents
1. Introduction ....................................................3
1.1. Conventions Used in This Document ..........................3
2. Overview ........................................................3
3. Basic Interaction between BFD Sessions and Clients ..............4
3.1. Session State Hysteresis ...................................4
3.2. AdminDown State ............................................5
3.3. Hitless Establishment/Reestablishment of BFD State .........5
4. Control Protocol Interactions ...................................6
4.1. Adjacency Establishment ....................................6
4.2. Reaction to BFD Session State Changes ......................7
4.2.1. Control Protocols with a Single Data Protocol .......7
4.2.2. Control Protocols with Multiple Data Protocols ......8
4.3. Interactions with Graceful Restart Mechanisms ..............8
4.3.1. BFD Fate Independent of the Control Plane ...........9
4.3.2. BFD Shares Fate with the Control Plane ..............9
4.4. Interactions with Multiple Control Protocols ..............10
5. Interactions with Non-Protocol Functions .......................10
6. Data Protocols and Demultiplexing ..............................11
7. Multiple Link Subnetworks ......................................11
7.1. Complete Decoupling .......................................12
7.2. Layer N-1 Hints ...........................................12
7.3. Aggregating BFD Sessions ..................................12
7.4. Combinations of Scenarios .................................12
8. Other Application Issues .......................................13
9. Interoperability Issues ........................................13
10. Specific Protocol Interactions (Non-Normative) ................13
10.1. BFD Interactions with OSPFv2, OSPFv3, and IS-IS ..........14
10.1.1. Session Establishment .............................14
10.1.2. Reaction to BFD State Changes .....................14
10.1.3. OSPF Virtual Links ................................15
10.2. Interactions with BGP ....................................15
10.3. Interactions with RIP ....................................15
11. Security Considerations .......................................16
12. References ....................................................16
12.1. Normative References .....................................16
12.2. Informative References ...................................16
Katz & Ward Standards Track [Page 2]RFC 5882 Generic Application of BFD June 2010
1. Introduction
The Bidirectional Forwarding Detection [BFD] protocol provides a
liveness detection mechanism that can be utilized by other network
components for which their integral liveness mechanisms are either
too slow, inappropriate, or nonexistent. Other documents have
detailed the use of BFD with specific encapsulations ([BFD-1HOP]
[BFD-MULTI] [BFD-MPLS]). As the utility of BFD has become
understood, there have been calls to specify BFD interactions with a
growing list of network functions. Rather than producing a long
series of short documents on the application of BFD, it seemed
worthwhile to describe the interactions between BFD and other network
functions ("BFD clients") in a broad way.
This document describes the generic application of BFD. Specific
protocol applications are provided for illustrative purposes.
1.1. Conventions Used in This Document
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC 2119 [KEYWORDS].
2. Overview
The Bidirectional Forwarding Detection (BFD) specification defines a
protocol with simple and specific semantics. Its sole purpose is to
verify connectivity between a pair of systems, for a particular data
protocol across a path (which may be of any technology, length, or
OSI layer). The promptness of the detection of a path failure can be
controlled by trading off protocol overhead and system load with
detection times.
BFD is *not* intended to directly provide control protocol liveness
information; those protocols have their own means and vagaries.
Rather, control protocols can use the services provided by BFD to
inform their operation. BFD can be viewed as a service provided by
the layer in which it is running.
The service interface with BFD is straightforward. The application
supplies session parameters (neighbor address, time parameters,
protocol options), and BFD provides the session state, of which the
most interesting transitions are to and from the Up state. The
application is expected to bootstrap the BFD session, as BFD has no
discovery mechanism.
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An implementation SHOULD establish only a single BFD session per data
protocol path, regardless of the number of applications that wish to
utilize it. There is no additional value in having multiple BFD
sessions to the same endpoints. If multiple applications request
different session parameters, it is a local issue as to how to
resolve the parameter conflicts. BFD in turn will notify all
applications bound to a session when a session state change occurs.
BFD should be viewed as having an advisory role to the protocol or
protocols or other network functions with which it is interacting,
which will then use their own mechanisms to effect any state
transitions. The interaction is very much at arm's length, which
keeps things simple and decoupled. In particular, BFD explicitly
does not carry application-specific information, partly for
architectural reasons and partly because BFD may have curious and
unpredictable latency characteristics and as such makes a poor
transport mechanism.
It is important to remember that the interaction between BFD and its
client applications has essentially no interoperability issues,
because BFD is acting in an advisory nature (similar to hardware
signaling the loss of light on a fiber optic circuit, for example)
and existing mechanisms in the client applications are used in
reaction to BFD events. In fact, BFD may interact with only one of a
pair of systems for a particular client application without any ill
effect.
3. Basic Interaction between BFD Sessions and Clients
The interaction between a BFD session and its associated client
applications is, for the most part, an implementation issue that is
outside the scope of this specification. However, it is useful to
describe some mechanisms that implementors may use in order to
promote full-featured implementations. One way of modeling this
interaction is to create an adaptation layer between the BFD state
machine and the client applications. The adaptation layer is
cognizant of both the internals of the BFD implementation and the
requirements of the clients.
3.1. Session State Hysteresis
A BFD session can be tightly coupled to its client applications; for
example, any transition out of the Up state could cause signaling to
the clients to take failure action. However, in some cases, this may
not always be the best course of action.
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RFC 5882 Generic Application of BFD June 2010
Implementors may choose to hide rapid Up/Down/Up transitions of the
BFD session from its clients. This is useful in order to support
process restarts without necessitating complex protocol mechanisms,
for example.
As such, a system MAY choose not to notify clients if a BFD session
transitions from Up to Down state, and returns to Up state, if it
does so within a reasonable period of time (the length of which is
outside the scope of this specification). If the BFD session does
not return to Up state within that time frame, the clients SHOULD be
notified that a session failure has occurred.
3.2. AdminDown State
The AdminDown mechanism in BFD is intended to signal that the BFD
session is being taken down for administrative purposes, and the
session state is not indicative of the liveness of the data path.
Therefore, a system SHOULD NOT indicate a connectivity failure to a
client if either the local session state or the remote session state
(if known) transitions to AdminDown, so long as that client has
independent means of liveness detection (typically, control
protocols).
If a client does not have any independent means of liveness
detection, a system SHOULD indicate a connectivity failure to a
client, and assume the semantics of Down state, if either the local
or remote session state transitions to AdminDown. Otherwise, the
client will not be able to determine whether the path is viable, and
unfortunate results may occur.
3.3. Hitless Establishment/Reestablishment of BFD State
It is useful to be able to configure a BFD session between a pair of
systems without impacting the state of any clients that will be
associated with that session. Similarly, it is useful for BFD state
to be reestablished without perturbing associated clients when all
BFD state is lost (such as in process restart situations). This
interacts with the clients' ability to establish their state
independent of BFD.
The BFD state machine transitions that occur in the process of
bringing up a BFD session in such situations SHOULD NOT cause a
connectivity failure notification to the clients.
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A client that is capable of establishing its state prior to the
configuration or restarting of a BFD session MAY do so if
appropriate. The means to do so is outside of the scope of this
specification.
4. Control Protocol Interactions
Very common client applications of BFD are control protocols, such as
routing protocols. The object, when BFD interacts with a control
protocol, is to advise the control protocol of the connectivity of
the data protocol. In the case of routing protocols, for example,
this allows the connectivity failure to trigger the rerouting of
traffic around the failed path more quickly than the native detection
mechanisms.
4.1. Adjacency Establishment
If the session state on either the local or remote system (if known)
is AdminDown, BFD has been administratively disabled, and the
establishment of a control protocol adjacency MUST be allowed.
BFD sessions are typically bootstrapped by the control protocol,
using the mechanism (discovery, configuration) used by the control
protocol to find neighbors. Note that it is possible in some failure
scenarios for the network to be in a state such that the control
protocol is capable of coming up, but the BFD session cannot be
established, and, more particularly, data cannot be forwarded. To
avoid this situation, it would be beneficial not to allow the control
protocol to establish a neighbor adjacency. However, this would
preclude the operation of the control protocol in an environment in
which not all systems support BFD.
Therefore, the establishment of control protocol adjacencies SHOULD
be blocked if both systems are willing to establish a BFD session but
a BFD session cannot be established. One method for determining that
both systems are willing to establish a BFD session is if the control
protocol carries explicit signaling of this fact. If there is no
explicit signaling, the willingness to establish a BFD session may be
determined by means outside the scope of this specification.
If it is believed that the neighboring system does not support BFD,
the establishment of a control protocol adjacency SHOULD NOT be
blocked.
The setting of BFD's various timing parameters and modes are not
subject to standardization. Note that all protocols sharing a
session will operate using the same parameters. The mechanism for
choosing the parameters among those desired by the various protocols
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RFC 5882 Generic Application of BFD June 2010
is outside the scope of this specification. It is generally useful
to choose the parameters resulting in the shortest Detection Time; a
particular client application can always apply hysteresis to the
notifications from BFD if it desires longer Detection Times.
Note that many control protocols assume full connectivity between all
systems on multiaccess media such as LANs. If BFD is running on only
a subset of systems on such a network, and adjacency establishment is
blocked by the absence of a BFD session, the assumptions of the
control protocol may be violated, with unpredictable results.
4.2. Reaction to BFD Session State Changes
If a BFD session transitions from Up state to AdminDown, or the
session transitions from Up to Down because the remote system is
indicating that the session is in state AdminDown, clients SHOULD NOT
take any control protocol action.
For other transitions from Up to Down state, the mechanism by which
the control protocol reacts to a path failure signaled by BFD depends
on the capabilities of the protocol, as specified in the following
subsections.
4.2.1. Control Protocols with a Single Data Protocol
A control protocol that is tightly bound to a single failing data
protocol SHOULD take action to ensure that data traffic is no longer
directed to the failing path. Note that this should not be
interpreted as BFD replacing the control protocol liveness mechanism,
if any, as the control protocol may rely on mechanisms not verified
by BFD (multicast, for instance) so BFD most likely cannot detect all
failures that would impact the control protocol. However, a control
protocol MAY choose to use BFD session state information to more
rapidly detect an impending control protocol failure, particularly if
the control protocol operates in-band (over the data protocol).
Therefore, when a BFD session transitions from Up to Down, action
SHOULD be taken in the control protocol to signal the lack of
connectivity for the path over which BFD is running. If the control
protocol has an explicit mechanism for announcing path state, a
system SHOULD use that mechanism rather than impacting the
connectivity of the control protocol, particularly if the control
protocol operates out-of-band from the failed data protocol.
However, if such a mechanism is not available, a control protocol
timeout SHOULD be emulated for the associated neighbor.
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4.2.2. Control Protocols with Multiple Data Protocols
Slightly different mechanisms are used if the control protocol
supports the routing of multiple data protocols, depending on whether
the control protocol supports separate topologies for each data
protocol.
4.2.2.1. Shared Topologies
With a shared topology, if one of the data protocols fails (as
signaled by the associated BFD session), it is necessary to consider
the path to have failed for all data protocols. Otherwise, there is
no way for the control protocol to turn away traffic for the failed
data protocol (and such traffic would be black-holed indefinitely).
Therefore, when a BFD session transitions from Up to Down, action
SHOULD be taken in the control protocol to signal the lack of
connectivity for the path in the topology corresponding to the BFD
session. If this cannot be signaled otherwise, a control protocol
timeout SHOULD be emulated for the associated neighbor.
4.2.2.2. Independent Topologies
With individual routing topologies for each data protocol, only the
failed data protocol needs to be rerouted around the failed path.
Therefore, when a BFD session transitions from Up to Down, action
SHOULD be taken in the control protocol to signal the lack of
connectivity for the path in the topology over which BFD is running.
Generally, this can be done without impacting the connectivity of
other topologies (since otherwise it is very difficult to support
separate topologies for multiple data protocols).
4.3. Interactions with Graceful Restart Mechanisms
A number of control protocols support Graceful Restart mechanisms,
including IS-IS [ISIS-GRACE], OSPF [OSPF-GRACE], and BGP [BGP-GRACE].
These mechanisms are designed to allow a control protocol to restart
without perturbing network connectivity state (lest it appear that
the system and/or all of its links had failed). They are predicated
on the existence of a separate forwarding plane that does not
necessarily share fate with the control plane in which the routing
protocols operate. In particular, the assumption is that the
forwarding plane can continue to function while the protocols restart
and sort things out.
BFD implementations announce via the Control Plane Independent "C"
bit whether or not BFD shares fate with the control plane. This
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information is used to determine the actions to be taken in
conjunction with Graceful Restart. If BFD does not share its fate
with the control plane on either system, it can be used to determine
whether Graceful Restart in a control protocol is NOT viable (the
forwarding plane is not operating).
If the control protocol has a Graceful Restart mechanism, BFD may be
used in conjunction with this mechanism. The interaction between BFD
and the control protocol depends on the capabilities of the control
protocol and whether or not BFD shares fate with the control plane.
In particular, it may be desirable for a BFD session failure to abort
the Graceful Restart process and allow the failure to be visible to
the network.
4.3.1. BFD Fate Independent of the Control Plane
If BFD is implemented in the forwarding plane and does not share fate
with the control plane on either system (the "C" bit is set in the
BFD Control packets in both directions), control protocol restarts
should not affect the BFD session. In this case, a BFD session
failure implies that data can no longer be forwarded, so any Graceful
Restart in progress at the time of the BFD session failure SHOULD be
aborted in order to avoid black holes, and a topology change SHOULD
be signaled in the control protocol.
4.3.2. BFD Shares Fate with the Control Plane
If BFD shares fate with the control plane on either system (the "C"
bit is clear in either direction), a BFD session failure cannot be
disentangled from other events taking place in the control plane. In
many cases, the BFD session will fail as a side effect of the restart
taking place. As such, it would be best to avoid aborting any
Graceful Restart taking place, if possible (since otherwise BFD and
Graceful Restart cannot coexist).
There is some risk in doing so, since a simultaneous failure or
restart of the forwarding plane will not be detected, but this is
always an issue when BFD shares fate with the control plane.
4.3.2.1. Control Protocols with Planned Restart Signaling
Some control protocols can signal a planned restart prior to the
restart taking place. In this case, if a BFD session failure occurs
during the restart, such a planned restart SHOULD NOT be aborted and
the session failure SHOULD NOT result in a topology change being
signaled in the control protocol.
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RFC 5882 Generic Application of BFD June 2010
4.3.2.2. Control Protocols without Planned Restart Signaling
Control protocols that cannot signal a planned restart depend on the
recently restarted system to signal the Graceful Restart prior to the
control protocol adjacency timeout. In most cases, whether the
restart is planned or unplanned, it is likely that the BFD session
will time out prior to the onset of Graceful Restart, in which case a
topology change SHOULD be signaled in the control protocol as
specified in Section 3.2.
However, if the restart is in fact planned, an implementation MAY
adjust the BFD session timing parameters prior to restarting in such
a way that the Detection Time in each direction is longer than the
restart period of the control protocol, providing the restarting
system the same opportunity to enter Graceful Restart as it would
have without BFD. The restarting system SHOULD NOT send any BFD
Control packets until there is a high likelihood that its neighbors
know a Graceful Restart is taking place, as the first BFD Control
packet will cause the BFD session to fail.
4.4. Interactions with Multiple Control Protocols
If multiple control protocols wish to establish BFD sessions with the
same remote system for the same data protocol, all MUST share a
single BFD session.
If hierarchical or dependent layers of control protocols are in use
(say, OSPF and Internal BGP (IBGP)), it may not be useful for more
than one of them to interact with BFD. In this example, because IBGP
is dependent on OSPF for its routing information, the faster failure
detection relayed to IBGP may actually be detrimental. The cost of a
peer state transition is high in BGP, and OSPF will naturally heal
the path through the network if it were to receive the failure
detection.
In general, it is best for the protocol at the lowest point in the
hierarchy to interact with BFD, and then to use existing interactions
between the control protocols to effect changes as necessary. This
will provide the fastest possible failure detection and recovery in a
network.
5. Interactions with Non-Protocol Functions
BFD session status may be used to affect other system functions that
are not protocol based (for example, static routes). If the path to
a remote system fails, it may be desirable to avoid passing traffic
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RFC 5882 Generic Application of BFD June 2010
to that remote system, so the local system may wish to take internal
measures to accomplish this (such as withdrawing a static route and
withdrawing that route from routing protocols).
If it is known, or presumed, that the remote system is BFD capable
and the BFD session is not in Up state, appropriate action SHOULD be
taken (such as withdrawing a static route).
If it is known, or presumed, that the remote system does not support
BFD, action such as withdrawing a static route SHOULD NOT be taken.
Bootstrapping of the BFD session in the non-protocol case is likely
to be derived from configuration information.
There is no need to exchange endpoints or discriminator values via
any mechanism other than configuration (via Operational Support
Systems or any other means) as the endpoints must be known and
configured by the same means.
6. Data Protocols and Demultiplexing
BFD is intended to protect a single "data protocol" and is
encapsulated within that protocol. A pair of systems may have
multiple BFD sessions over the same topology if they support (and are
encapsulated by) different protocols. For example, if two systems
have IPv4 and IPv6 running across the same link between them, these
are considered two separate paths and require two separate BFD
sessions.
This same technique is used for more fine-grained paths. For
example, if multiple differentiated services [DIFFSERV] are being
operated over IPv4, an independent BFD session may be run for each
service level. The BFD Control packets must be marked in the same
way as the data packets, partly to ensure as much fate sharing as
possible between BFD and data traffic, and also to demultiplex the
initial packet if the discriminator values have not been exchanged.
7. Multiple Link Subnetworks
A number of technologies exist for aggregating multiple parallel
links at layer N-1 and treating them as a single link at layer N.
BFD may be used in a number of ways to protect the path at layer N.
The exact mechanism used is outside the scope of this specification.
However, this section provides examples of some possible deployment
scenarios. Other scenarios are possible and are not precluded.
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RFC 5882 Generic Application of BFD June 2010
7.1. Complete Decoupling
The simplest approach is to simply run BFD over the layer N path,
with no interaction with the layer N-1 mechanisms. Doing so assumes
that the layer N-1 mechanism will deal with connectivity issues in
individual layer N-1 links. BFD will declare a failure in the layer
N path only when the session times out.
This approach will work whether or not the layer N-1 neighbor is the
same as the layer N neighbor.
7.2. Layer N-1 Hints
A slightly more intelligent approach than complete decoupling is to
have the layer N-1 mechanism inform the layer N BFD when the
aggregated link is no longer viable. In this case, the BFD session
will detect the failure more rapidly, as it need not wait for the
session to time out. This is analogous to triggering a session
failure based on the hardware-detected failure of a single link.
This approach will also work whether or not the layer N-1 neighbor is
the same as the layer N neighbor.
7.3. Aggregating BFD Sessions
Another approach would be to use BFD on each layer N-1 link and to
aggregate the state of the multiple sessions into a single indication
to the layer N clients. This approach has the advantage that it is
independent of the layer N-1 technology. However, this approach only
works if the layer N neighbor is the same as the layer N-1 neighbor
(a single hop at layer N-1).
7.4. Combinations of Scenarios
Combinations of more than one of the scenarios listed above (or
others) may be useful in some cases. For example, if the layer N
neighbor is not directly connected at layer N-1, a system might run a
BFD session across each layer N-1 link to the immediate layer N-1
neighbor and then run another BFD session to the layer N neighbor.
The aggregate state of the layer N-1 BFD sessions could be used to
trigger a layer N BFD session failure.
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8. Other Application Issues
BFD can provide liveness detection for functions related to
Operations, Administration, and Maintenance (OAM) in tunneling and
pseudowire protocols. Running BFD inside the tunnel is recommended,
as it exercises more aspects of the path. One way to accommodate
this is to address BFD packets based on the tunnel endpoints,
assuming that they are numbered.
If a planned outage is to take place on a path over which BFD is run,
it is preferable to take down the BFD session by going into AdminDown
state prior to the outage. The system asserting AdminDown SHOULD do
so for at least one Detection Time in order to ensure that the remote
system is aware of it.
Similarly, if BFD is to be deconfigured from a system, it is
desirable not to trigger any client application action. Simply
ceasing the transmission of BFD Control packets will cause the remote
system to detect a session failure. In order to avoid this, the
system on which BFD is being deconfigured SHOULD put the session into
AdminDown state and maintain this state for a Detection Time to
ensure that the remote system is aware of it.
9. Interoperability Issues
The BFD protocol itself is designed so that it will always
interoperate at a basic level; asynchronous mode is mandatory and is
always available, and other modes and functions are negotiated at run
time. Since the service provided by BFD is identical regardless of
the variants used, the particular choice of BFD options has no
bearing on interoperability.
The interaction between BFD and other protocols and control functions
is very loosely coupled. The actions taken are based on existing
mechanisms in those protocols and functions, so interoperability
problems are very unlikely unless BFD is applied in contradictory
ways (such as a BFD session failure causing one implementation to go
down and another implementation to come up). In fact, BFD may be
advising one system for a particular control function but not the
other; the only impact of this would be potentially asymmetric
control protocol failure detection.
10. Specific Protocol Interactions (Non-Normative)
As noted above, there are no interoperability concerns regarding
interactions between BFD and control protocols. However, there is
enough concern and confusion in this area so that it is worthwhile to
provide examples of interactions with specific protocols.
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RFC 5882 Generic Application of BFD June 2010
Since the interactions do not affect interoperability, they are non-
normative.
10.1. BFD Interactions with OSPFv2, OSPFv3, and IS-IS
The two versions of OSPF ([OSPFv2] and [OSPFv3]), as well as IS-IS
[ISIS], all suffer from an architectural limitation, namely that
their Hello protocols are limited in the granularity of their failure
detection times. In particular, OSPF has a minimum detection time of
two seconds, and IS-IS has a minimum detection time of one second.
BFD may be used to achieve arbitrarily small detection times for
these protocols by supplementing the Hello protocols used in each
case.
10.1.1. Session Establishment
The most obvious choice for triggering BFD session establishment with
these protocols would be to use the discovery mechanism inherent in
the Hello protocols in OSPF and IS-IS to bootstrap the establishment
of the BFD session. Any BFD sessions established to support OSPF and
IS-IS across a single IP hop must operate in accordance with
[BFD-1HOP].
10.1.2. Reaction to BFD State Changes
The basic mechanisms are covered in Section 3 above. At this time,
OSPFv2 and OSPFv3 carry routing information for a single data
protocol (IPv4 and IPv6, respectively) so when it is desired to
signal a topology change after a BFD session failure, this should be
done by tearing down the corresponding OSPF neighbor.
IS-IS may be used to support only one data protocol, or multiple data
protocols. [ISIS] specifies a common topology for multiple data
protocols, but work is under way to support multiple topologies. If
multiple topologies are used to support multiple data protocols (or
multiple classes of service of the same data protocol), the topology-
specific path associated with a failing BFD session should no longer
be advertised in IS-IS Label Switched Paths (LSPs) in order to signal
a lack of connectivity. Otherwise, a failing BFD session should be
signaled by simulating an IS-IS adjacency failure.
OSPF has a planned restart signaling mechanism, whereas IS-IS does
not. The appropriate mechanisms outlined in Section 3.3 should be
used.
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RFC 5882 Generic Application of BFD June 2010
10.1.3. OSPF Virtual Links
If it is desired to use BFD for failure detection of OSPF Virtual
Links, the mechanism described in [BFD-MULTI] MUST be used, since
OSPF Virtual Links may traverse an arbitrary number of hops. BFD
authentication SHOULD be used and is strongly encouraged.
10.2. Interactions with BGP
BFD may be useful with External Border Gateway Protocol (EBGP)
sessions [BGP] in order to more rapidly trigger topology changes in
the face of path failure. As noted in Section 4.4, it is generally
unwise for IBGP sessions to interact with BFD if the underlying IGP
is already doing so.
EBGP sessions being advised by BFD may establish either a one-hop
[BFD-1HOP] or a multihop [BFD-MULTI] session, depending on whether or
not the neighbor is immediately adjacent. The BFD session should be
established to the BGP neighbor (as opposed to any other Next Hop
advertised in BGP). BFD authentication SHOULD be used and is
strongly encouraged.
[BGP-GRACE] describes a Graceful Restart mechanism for BGP. If
Graceful Restart is not taking place on an EBGP session, and the
corresponding BFD session fails, the EBGP session should be torn down
in accordance with Section 3.2. If Graceful Restart is taking place,
the basic procedures in Section 4.3 apply. BGP Graceful Restart does
not signal planned restarts, so Section 4.3.2.2 applies. If Graceful
Restart is aborted due to the rules described in Section 4.3, the
"receiving speaker" should act as if the "restart timer" expired (as
described in [BGP-GRACE]).
10.3. Interactions with RIP
The Routing Information Protocol (RIP) [RIP] is somewhat unique in
that, at least as specified, neighbor adjacency state is not stored
per se. Rather, installed routes contain a next hop address, which
in most cases is the address of the advertising neighbor (but may not
be).
In the case of RIP, when the BFD session associated with a neighbor
fails, an expiration of the "timeout" timer for each route installed
from the neighbor (for which the neighbor is the next hop) should be
simulated.
Note that if a BFD session fails, and a route is received from that
neighbor with a next hop address that is not the address of the
neighbor itself, the route will linger until it naturally times out
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RFC 5882 Generic Application of BFD June 2010
(after 180 seconds). However, if an implementation keeps track of
all of the routes received from each neighbor, all of the routes from
the neighbor corresponding to the failed BFD session should be timed
out, regardless of the next hop specified therein, and thereby
avoiding the lingering route problem.
11. Security Considerations
This specification does not raise any additional security issues
beyond those of the specifications referred to in the list of
normative references.
12. References
12.1. Normative References
[BFD] Katz, D. and D. Ward, "Bidirectional Forwarding
Detection", RFC 5880, June 2010.
[BFD-1HOP] Katz, D. and D. Ward,"Bidirectional Forwarding Detection
(BFD) for IPv4 and IPv6 (Single Hop)", RFC 5881, June
2010.
[BFD-MPLS] Aggarwal, R., Kompella, K., Nadeau, T., and G. Swallow,
"Bidirectional Forwarding Detection (BFD) for MPLS Label
Switched Paths (LSPs)", RFC 5884, June 2010.
[BFD-MULTI] Katz, D. and D. Ward, "Bidirectional Forwarding
Detection (BFD) for Multihop Paths", RFC 5883, June
2010.
[KEYWORDS] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
12.2. Informative References
[BGP] Rekhter, Y., Ed., Li, T., Ed., and S. Hares, Ed., "A
Border Gateway Protocol 4 (BGP-4)", RFC 4271, January
2006.
[BGP-GRACE] Sangli, S., Chen, E., Fernando, R., Scudder, J., and Y.
Rekhter, "Graceful Restart Mechanism for BGP", RFC 4724,
January 2007.
[DIFFSERV] Nichols, K., Blake, S., Baker, F., and D. Black,
"Definition of the Differentiated Services Field (DS
Field) in the IPv4 and IPv6 Headers", RFC 2474, December
1998.
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[ISIS] Callon, R., "Use of OSI IS-IS for routing in TCP/IP and
dual environments", RFC 1195, December 1990.
[ISIS-GRACE] Shand, M. and L. Ginsberg, "Restart Signaling for
IS-IS", RFC 5306, October 2008.
[OSPFv2] Moy, J., "OSPF Version 2", STD 54, RFC 2328, April 1998.
[OSPFv3] Coltun, R., Ferguson, D., Moy, J., and A. Lindem, "OSPF
for IPv6", RFC 5340, July 2008.
[OSPF-GRACE] Moy, J., Pillay-Esnault, P., and A. Lindem, "Graceful
OSPF Restart", RFC 3623, November 2003.
[RIP] Malkin, G., "RIP Version 2", STD 56, RFC 2453, November
1998.
Authors' Addresses
Dave Katz
Juniper Networks
1194 N. Mathilda Ave.
Sunnyvale, CA 94089-1206
USA
Phone: +1-408-745-2000
EMail: dkatz@juniper.net
Dave Ward
Juniper Networks
1194 N. Mathilda Ave.
Sunnyvale, CA 94089-1206
USA
Phone: +1-408-745-2000
EMail: dward@juniper.net
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