IS-IS BFD TLV: IS-IS BFD-Enabled TLV
A flag in IS-IS hello packets that tells neighbors BFD is on, so a broken forwarding path is not missed.
Hopps, C. and L. Ginsberg, "IS-IS BFD-Enabled TLV", RFC 6213, DOI 10.17487/RFC6213, April 2011, <https://www.rfc-editor.org/info/rfc6213>.
@misc{rfc6213,
series = {Request for Comments},
number = 6213,
howpublished = {RFC 6213},
publisher = {RFC Editor},
doi = {10.17487/RFC6213},
url = {https://www.rfc-editor.org/info/rfc6213},
author = {C. Hopps and L. Ginsberg},
title = {{IS-IS BFD-Enabled TLV}},
pagetotal = 7,
year = 2011,
month = apr,
} RFC text
Contents
Internet Engineering Task Force (IETF) C. Hopps
Request for Comments: 6213 L. Ginsberg
Category: Standards Track Cisco Systems
ISSN: 2070-1721 April 2011
IS-IS BFD-Enabled TLV
Abstract
This document describes a type-length-value (TLV) for use in the IS-
IS routing protocol that allows for the proper use of the
Bidirectional Forwarding Detection (BFD) protocol. There exist
certain scenarios in which IS-IS will not react appropriately to a
BFD-detected forwarding plane failure without use of either this TLV
or some other method.
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/rfc6213.
Copyright Notice
Copyright (c) 2011 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
Table of Contents
1. Introduction ....................................................2
1.1. Requirements Language ......................................2
2. The Problem .....................................................2
3. The Solution ....................................................3
3.1. State Definitions ..........................................3
3.2. Adjacency Establishment and Maintenance ....................4
3.3. Advertisement of Topology-Specific IS Neighbors ............4
4. Transition ......................................................4
5. Graceful Restart ................................................5
6. The BFD-Enabled TLV .............................................5
7. Security Considerations .........................................6
8. IANA Considerations .............................................6
9. Acknowledgements ................................................6
10. Normative References ...........................................7
1. Introduction
The Bidirectional Forwarding Detection (BFD) protocol [RFC5880] is a
protocol that allows for detection of a forwarding plane failure
between two routers. A router can use [RFC5880] to validate that a
peer router's forwarding ability is functioning.
One specific application of BFD as described in [RFC5882] is to
verify the forwarding ability of an IS-IS [RFC1195] router's
adjacencies; however, the method described in [RFC5882] does not
allow for certain failure scenarios. We will define a TLV that will
allow for proper response to the detection of all forwarding failures
where the use of BFD is employed with IS-IS.
1.1. Requirements Language
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 [RFC2119].
2. The Problem
We observe that, in order to allow for mixed use (i.e., some routers
running BFD and some not), [RFC5882] does not require a BFD session
be established prior to the establishment of an IS-IS adjacency.
Thus, if a router A has neighbors B and C, and B does not support
BFD, A would still form adjacencies with B and C, and it would only
establish a BFD session with C.
The problem with this solution is that it assumes that the
transmission and receipt of IS-IS Hellos (IIHs) shares fate with
forwarded data packets. This is not a fair assumption to make given
that the primary use of BFD is to protect IPv4 (and IPv6) forwarding,
and IS-IS does not utilize IPv4 or IPv6 for sending or receiving its
hellos.
Thus, if we consider our previous example, and if C is currently
experiencing an IPv4 forwarding failure that allows for IIHs to be
sent and received, when A first starts (or restarts), A will assume
that C simply does not support BFD, will form an adjacency with C,
and may incorrectly forward IPv4 traffic through C.
3. The Solution
A simple solution to this problem is for an IS-IS router to advertise
that it has BFD enabled on a given interface. It can do this through
the inclusion of a TLV in its IIHs as described in this document.
When sending an IIH on a BFD enabled interface, a router that
supports this extension MUST include the BFD-enabled TLV in its IIH.
The contents of the TLV MUST indicate what topologies/protocols
[RFC5120] have been enabled for BFD by including the appropriate
Multi-Topology Identifier (MTID)/ Network Layer Protocol Identifier
(NLPID) pairs.
When sending an IIH on an interface on which BFD is NOT enabled, a
router MUST NOT include the BFD-enabled TLV.
3.1. State Definitions
The following definitions apply to each IS-IS neighbor:
For each locally supported MTID/NLPID pair, an
"ISIS_TOPO_NLPID_BFD_REQUIRED" variable is assigned. If BFD is
supported by both the local system and the neighbor of the MTID/
NLPID, this variable is set to "TRUE". Otherwise, the variable is
set to "FALSE".
For each locally supported MTID, an "ISIS_TOPO_BFD_REQUIRED" variable
is set to the logical "OR" of all "ISIS_TOPO_NLPID_BFD_REQUIRED"
variables associated with that MTID.
An "ISIS_BFD_REQUIRED" variable is set to the logical "AND" of all
"ISIS_TOPO_BFD_REQUIRED" variables.
For each locally supported MTID/NLPID pair, an
"ISIS_TOPO_NLPID_STATE" variable is assigned. If
"ISIS_TOPO_NLPID_BFD_REQUIRED" is "TRUE", this variable follows the
BFD session state for that MTID/NLPID ("UP == TRUE"). Otherwise, the
variable is set to "TRUE".
For each locally supported topology (MTID), an "ISIS_TOPO_USEABLE"
variable is set to the logical "AND" of the set of
"ISIS_TOPO_NLPID_STATE" variables associated with that MTID.
An "ISIS_NEIGHBOR_USEABLE" variable is set to the logical "OR" of all
"ISIS_TOPO_USEABLE" variables.
3.2. Adjacency Establishment and Maintenance
Whenever "ISIS_BFD_REQUIRED" is "TRUE", the following extensions to
the rules for adjacency establishment and maintenance MUST apply:
o "ISIS_NEIGHBOR_USEABLE" MUST be "TRUE" before the adjacency can
transition from "INIT" to "UP" state.
o When the IS-IS adjacency is "UP" and "ISIS_NEIGHBOR_USEABLE"
becomes "FALSE", the IS-IS adjacency MUST transition to "DOWN".
o On a Point-to-Point circuit whenever "ISIS_NEIGHBOR_USEABLE" is
"FALSE", the Three-Way adjacency state MUST be set to "DOWN" in
the Point-to-Point Three-Way Adjacency TLV [RFC5303] in all
transmitted IIHs.
o On a LAN circuit whenever "ISIS_NEIGHBOR_USEABLE" is "FALSE", the
IS Neighbors TLV advertising the Media Access Control (MAC)
address of the neighbor MUST be omitted in all transmitted IIHs.
3.3. Advertisement of Topology-Specific IS Neighbors
The advertisement of a topology-specific IS neighbor (as well as the
use of the neighbor in the topology-specific decision process) is
determined by the value of "ISIS_TOPO_USEABLE" for each topology. If
"ISIS_TOPO_USEABLE" is "TRUE", then the topology-specific neighbor is
advertised. If "ISIS_TOPO_USEABLE" is "FALSE", then the topology-
specific neighbor is not advertised.
4. Transition
To allow for a non-disruptive transition to the use of BFD, some
amount of time should be allowed before bringing down an "UP"
adjacency on a BFD enabled interface when the value of
"ISIS_BFD_REQUIRED" becomes "TRUE" as a result of the introduction of
the BFD TLV or the modification (by adding a new supported MTID/
NLPID) of an existing BFD TLV in a neighbor's IIH. A simple way to
do this is to not update the adjacency hold time when receiving such
an IIH from a neighbor with whom we have an "UP" adjacency until
"ISIS_NEIGHBOR_USEABLE" becomes "TRUE".
If the value of "ISIS_BFD_REQUIRED" becomes "FALSE" as a result of
the removal the BFD TLV or the modification (by removing a supported
MTID/NLPID) of an existing BFD TLV in a neighbor's IIH, then BFD
session establishment is no longer required to maintain the adjacency
or transition the adjacency to the "UP" state.
If a BFD session is administratively shut down [RFC5880] and the BFD
session state change impacts the value of "ISIS_NEIGHBOR_USEABLE",
then IS-IS SHOULD allow time for the corresponding MTID/NLPID to be
removed from the neighbor's BFD TLV by not updating the adjacency
hold time until "ISIS_BFD_REQUIRED" becomes "FALSE". Note that while
this allows a non-disruptive transition, it still enforces
consistency between the administrative state of the BFD session and
the MTID/NLPID(s) advertised in the BFD TLV. This is necessary to
provide consistent behavior regardless of whether the BFD AdminDown
state is introduced before or after an IS-IS adjacency "UP" state has
been achieved.
5. Graceful Restart
This section describes IS-IS implementation considerations when both
IS-IS graceful restart [RFC5306] and BFD are co-deployed.
In cases where BFD shares fate with the control plane, it can be
expected that BFD session failure may occur in conjunction with the
control-plane restart. In such cases, premature abort of IS-IS
graceful restart as a result of BFD session failure is undesirable.
Therefore, some mechanism to ignore the BFD session failure for a
limited period of time would be beneficial. The issue of the
interaction between graceful restart and BFD is described at length
in RFC 5882. The implementation of this interaction is outside the
scope of this document.
6. The BFD-Enabled TLV
The BFD-enabled TLV is formatted as shown below. The TLV SHALL only
be included in an IIH and only when BFD is enabled for one or more
supported MTID/protocols on the interface over which the IIH is being
sent. The NLPIDs encoded in the TLV are defined in [ISO9577].
Type 148
Length # of octets in the value field (3 to 255)
Value 3 octets specifying the MTID/NLPID for each
topology/data protocol for which BFD support is enabled
No. of octets
+-----------------------+
|R|R|R|R| MTID | 2
+-----------------------+
| NLPID | 1
+-----------------------+
: :
: :
+-----------------------+
|R|R|R|R| MTID | 2
+-----------------------+
| NLPID | 1
+-----------------------+
7. Security Considerations
The TLV defined within this document describes an addition to the
IS-IS Hello protocol. Inappropriate use of this TLV could prevent an
IS-IS adjacency from forming or lead to failure to detect
bidirectional forwarding failures -- each of which is a form of
denial of service. However, a party who can manipulate the contents
of this TLV is already in a position to create such a denial of
service by disrupting IS-IS routing in other ways.
Note that the introduction of this TLV has no impact on the use/
non-use of authentication either by IS-IS or by BFD.
8. IANA Considerations
The following IS-IS TLV type is defined by this document.
Name Value IIH LSP SNP Purge
---------------------- ----- --- --- --- -----
BFD-Enabled TLV 148 y n n n
The IS-IS TLV Codepoint registry has been updated accordingly.
9. Acknowledgements
The authors wish to thank Jeffrey Haas, Matthew Jones, Dave Katz,
Jonathan Moon, Stefano Previdi, Mike Shand, Michael Shiplett, and
David Ward for various input on this document.
10. Normative References
[ISO9577] International Organization for Standardization, "Protocol
identification in the network layer(ISO/IEC 9577)", ISO/
IEC 9577:1999, Fourth Edition, December 1999.
[RFC1195] Callon, R., "Use of OSI IS-IS for routing in TCP/IP and
dual environments", RFC 1195, December 1990.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC5120] Przygienda, T., Shen, N., and N. Sheth, "M-ISIS: Multi
Topology (MT) Routing in Intermediate System to
Intermediate Systems (IS-ISs)", RFC 5120, February 2008.
[RFC5303] Katz, D., Saluja, R., and D. Eastlake, "Three-Way
Handshake for IS-IS Point-to-Point Adjacencies", RFC 5303,
October 2008.
[RFC5306] Shand, M. and L. Ginsberg, "Restart Signaling for IS-IS",
RFC 5306, October 2008.
[RFC5880] Katz, D. and D. Ward, "Bidirectional Forwarding Detection
(BFD)", RFC 5880, June 2010.
[RFC5882] Katz, D. and D. Ward, "Generic Application of
Bidirectional Forwarding Detection (BFD)", RFC 5882,
June 2010.
Authors' Addresses
Christian E. Hopps
Cisco Systems
170 W. Tasman Dr.
San Jose, California 95134
USA
EMail: chopps@cisco.com
Les Ginsberg
Cisco Systems
510 McCarthy Blvd.
Milpitas, California 95035
USA
EMail: ginsberg@cisco.com
AI ELI5
On this page
§1Introduction
The Bidirectional Forwarding Detection (BFD) protocol, defined in RFC 5880, lets a router detect a forwarding plane failure between two routers. RFC 5882 describes using BFD to verify the forwarding ability of the adjacencies of an IS-IS router (IS-IS itself is in RFC 1195), but its method does not cover certain failure scenarios.
RFC 6213 defines a type-length-value (TLV) that allows a proper response to all forwarding failures when BFD is used with IS-IS.
BFD is a fast way for two routers to check that the road between them works. This RFC fixes a gap in how IS-IS uses that check.
§2The Problem
To allow mixed use, where some routers run BFD and some do not, RFC 5882 does not require a BFD session before an IS-IS adjacency is formed. If router A has neighbors B and C, and B does not support BFD, A still forms adjacencies with both and sets up a BFD session only with C.
This assumes that IS-IS Hellos (IIHs) share fate with forwarded data packets. That is not a fair assumption, because BFD mainly protects IPv4 and IPv6 forwarding while IS-IS does not use IPv4 or IPv6 to send or receive its hellos. If C has an IPv4 forwarding failure that still lets hellos through, then when A starts or restarts it will assume C simply lacks BFD, form an adjacency, and may incorrectly forward IPv4 traffic through C.
If the walkie-talkie is silent, you cannot tell whether your friend has none or whether their bridge is broken. The hellos still get through even when the real road is broken.
§3The Solution: Advertise That BFD Is Enabled
An IS-IS router advertises that BFD is enabled on an interface by including a TLV in its IIHs. When sending an IIH on a BFD-enabled interface, a router that supports this extension MUST include the BFD-enabled TLV, listing the Multi-Topology Identifier (MTID) and Network Layer Protocol Identifier (NLPID) pairs that have BFD enabled. On an interface where BFD is NOT enabled, it MUST NOT include the TLV.
The receiver tracks state for each neighbor with a set of variables:
ISIS_TOPO_NLPID_BFD_REQUIREDis TRUE for an MTID/NLPID pair when both the local system and the neighbor support BFD for it.ISIS_TOPO_BFD_REQUIREDis the OR of those values for an MTID, andISIS_BFD_REQUIREDis the AND of the per-topology values.ISIS_TOPO_NLPID_STATEfollows the BFD session state when BFD is required, and is otherwise TRUE.ISIS_TOPO_USEABLEis the AND of the states for a topology, andISIS_NEIGHBOR_USEABLEis the OR of all topologies.
Each router puts up a sign saying what it checks with BFD. The neighbor keeps a scoreboard of which roads are both promised and confirmed working.
§4Adjacency Rules and Transitions
Whenever ISIS_BFD_REQUIRED is TRUE, these rules MUST apply. ISIS_NEIGHBOR_USEABLE MUST be TRUE before an adjacency moves from INIT to UP, and if it becomes FALSE while the adjacency is UP, the adjacency MUST go DOWN. On a point-to-point circuit the Three-Way adjacency state is set to DOWN in the Three-Way Adjacency TLV of RFC 5303 while it is FALSE. On a LAN circuit the IS Neighbors TLV with the neighbor's MAC address MUST be omitted. A topology-specific neighbor is advertised only when ISIS_TOPO_USEABLE is TRUE for that topology.
For a non-disruptive transition, some time should be allowed before bringing down an UP adjacency when a neighbor's TLV newly requires BFD. A simple way is to not update the adjacency hold time on such hellos until ISIS_NEIGHBOR_USEABLE becomes TRUE. If a BFD session is administratively shut down, IS-IS SHOULD allow time for the MTID/NLPID to be removed from the neighbor's TLV, again by not updating the hold time.
A road only counts as open when the walkie-talkie check works. If a friend newly says they are using one, you give them a little time to get it working before you close the road.
§5The BFD-Enabled TLV Format
The TLV has type 148. Its length is the number of octets in the value field, from 3 to 255. The value holds 3 octets for each topology and data protocol with BFD enabled: 2 octets with four reserved bits and the MTID, then 1 octet with the NLPID, as defined in ISO 9577. The TLV SHALL only be included in an IIH, and only when BFD is enabled for one or more supported MTID/protocols on the sending interface.
The sign is a list. Each entry is three small pieces of information saying which road and which kind of traffic is being checked.
| Field | Name | Size | Value | Description |
|---|---|---|---|---|
RESERVED | R R R R | 4 bits | Reserved bits. | |
MTID | MTID | 12 bits | Multi-Topology Identifier. | |
NLPID | NLPID | 1 byte | Network Layer Protocol Identifier, as defined in ISO 9577. |
§6Graceful Restart, Security and IANA
When IS-IS graceful restart (RFC 5306) and BFD are used together and BFD shares fate with the control plane, a BFD failure can coincide with a restart. Aborting graceful restart because of it is undesirable, so ignoring the BFD failure for a limited time would be beneficial. How to do this is outside the scope of the document.
Misusing the TLV could prevent an adjacency from forming or cause bidirectional forwarding failures to go undetected, both forms of denial of service. However, a party that can manipulate the TLV can already disrupt IS-IS in other ways. The TLV does not change the use of authentication by IS-IS or BFD. IANA registered TLV type 148 for IIH only (not LSP, SNP or purge).
If a router is restarting, a failed check might just be the restart, so it is smart to be patient. Someone who could mess with the sign could already cause trouble in other ways.