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Versions: 00 01 02 03 RFC 5171

Internet Engineering Task Force                            M. Foschiano
Internet Draft                                            Cisco Systems
Category: Informational                                      April 2007
Expires: October 2007

        Cisco Systems UniDirectional Link Detection (UDLD) Protocol

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   Copyright (C) The IETF Trust (2007).

Foschiano                                                     [Page 1]

   Cisco Systems UniDirectional Link Detection Protocol     April 2007


   This document describes a Cisco Systems protocol that can be used to
   detect and disable unidirectional Ethernet fiber or copper links
   caused for instance by mis-wiring of fiber strands, interface
   malfunctions, media converters' faults, etc.  It operates at Layer 2
   in conjunction with IEEE 802.3's existing Layer 1 fault detection

   This document explains the protocol objectives and applications,
   illustrates the specific premises the protocol was based upon and
   describes the protocol architecture and related deployment issues, to
   serve as a possible base for future standardization.

Table of Contents

   1. Introduction..................................................3
   2. Protocol Objectives and Applications..........................3
   3. Protocol Design Premises......................................4
   4. Protocol Background...........................................5
   5. Protocol Architecture.........................................5
      5.1 The Basics................................................5
      5.2 Neighbor Database Maintenance.............................5
      5.3 Event-driven Detection and Echoing........................6
      5.4 Event-based versus Event-less Detection...................6
   6. Packet Format.................................................7
      6.1 TLV Description...........................................9
   7. Protocol Logic...............................................10
      7.1 Protocol Timers..........................................11
   8. Comparison with Bidirectional Forwarding Detection...........11
   Security Considerations.........................................12
   IANA Considerations.............................................12
   Deployment Considerations.......................................12
   Changes from the Previous Version...............................12
   Normative References............................................12
   Author's Address................................................13
   IPR Notice......................................................13
   Full Copyright Notice...........................................14

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   Cisco Systems UniDirectional Link Detection Protocol     April 2007

1. Introduction

   Today's Ethernet-based switched networks often rely on the Spanning
   Tree Protocol (STP) defined in the IEEE 802.1D standard [1] to create
   a loop-free topology that is used to forward the traffic from a
   source to a destination based on the Layer 2 packet information
   learned by the switches and on the knowledge of the status of the
   physical links along the path.
   Issues arise when due to mis-wirings or to hardware faults the
   communication path behaves abnormally and generates forwarding
   anomalies. The simplest example of such anomalies is the case of a
   bidirectional link that stops passing traffic in one direction and
   therefore breaks one of the most basic assumptions most high-level
   protocols depend upon: reliable two-way communication between peers.

   The purpose of the UDLD protocol is to detect the presence of
   anomalous conditions in the Layer 2 communication channel, while
   relying on the mechanisms defined by the IEEE in the 802.3 standard
   [2] to properly handle conditions inherent to the physical layer.

2. Protocol Objectives and Applications

   The UniDirectional Link Detection protocol (often referred to in
   short as "UDLD") is a light-weight protocol that can be used to
   detect and disable one-way connections before they create dangerous
   situations such as Spanning Tree loops or other protocol

   The protocol's main goal is to advertise the identities of all the
   capable devices attached to the same LAN segment and to collect the
   information received on the ports of each device to determine if the
   Layer 2 communication is happening in the appropriate fashion.

   In a network that has an extensive fiber cabling plant, problems may
   arise when incorrect patching causes a switch port to have its RX
   fiber strand connected to one neighbor port and its TX fiber strand
   connected to another. In these cases, a port may be deemed active if
   it is receiving an optical signal on its RX strand. However, the
   problem is that this link does not provide a valid communication path
   at Layer 2 (and above).

   If this scenario of wrongly connected fiber strands is applied to
   multiple ports to create a fiber loop, each device in the loop could
   directly send packets to a neighbor but would not be able to receive
   from the same device to which it is sending to.

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   Cisco Systems UniDirectional Link Detection Protocol     April 2007

   Albeit the above scenario is rather extreme, it exemplifies how the
   lack of mutual identification of the neighbors can bring protocols to
   the wrong assumption that during a transmission the sender and the
   receiver at the other end of the link match.
   Another equally dangerous incorrect assumption is that the lack of
   reception of protocol messages on a port unmistakably indicates the
   absence of transmitting protocol entities at the other end of the

   The UDLD protocol was implemented to help correct certain assumptions
   made by other protocols, and in particular to help the Spanning Tree
   Protocol to function properly so as to avoid the creation of
   dangerous Layer 2 loops. It has been available on most Cisco Systems
   switches for several years and is now part of numerous network design
   best practices.

3. Protocol Design Premises

   The current implementation of UDLD is based on the following

     o  The protocol would have to be run in the control plane of a
        network device to be flexible enough to support upgrades and bug
        fixes. The control plane speed would ultimately be the limiting
        factor to the capability of fast fault detection of the protocol
        (CPU speed, task switching speed, event processing speed, etc.).
        The transmission medium's propagation delay at 10 Mbps speed (or
        higher) would instead be considered a negligible factor.
     o  Oftentimes network events tend to happen not with optimal
        timing, but rather at the speed determined by the
        software/firmware infrastructure that controls them (for
        psychological and practical reasons developers tend to choose
        round timer values rather than determine the optimal value for
        the specific software architecture in use; also, software bugs,
        coding oversights, slow process switching, implementation
        overhead can all affect the control plane responsiveness and
        event timings). Hence it was deemed necessary to adopt a
        conservative protocol design to minimize false positives during
        the detection process.
     o  If a fault were discovered, it was assumed that the user would
        want to keep the faulty port down for a predetermined amount of
        time to avoid unnecessary port state flapping. For that reason a
        time-based fault recovery mechanism was provided (although
        alternative recovery mechanisms are not implicitly precluded by
        the protocol itself).

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4. Protocol Background

   UDLD is meant to be a Layer 2 detection protocol that works on top of
   the existing Layer 1 detection mechanisms defined by the IEEE
   standards. For example, the Far End Fault Indication function (FEFI)
   for 100BaseFX interfaces and the Auto-Negotiation function for
   100BaseTX/1000BaseX interfaces represent standard physical-layer
   mechanisms to determine if the transmission media is bidirectional.
   (Please see [2] sections and for more details.) The
   typical case of a Layer 1 "fault" indication is the "loss of light"

   UDLD instead differs from the above-mentioned mechanisms insofar as
   it performs mutual neighbor identification; in addition it performs
   neighbor acknowledgement on top of the LLC layer and thus is able to
   discover logical one-way mis-communication between neighbors even
   when either one of the said PHY layer mechanisms has deemed the
   transmission medium bidirectional.

5. Protocol Architecture

5.1 The Basics

   UDLD uses two basic mechanisms:

     a. It advertises a port's identity and learns about its neighbors
        on a specific LAN segment; it keeps the acquired information on
        the neighbors in a cache table.
     b. It sends a train of echo messages in certain circumstances that
        require fast notifications or fast resynchronization of the
        cached information.

   Because of the above, the algorithm run by UDLD requires that all the
   devices connected to the same LAN segment be running the protocol in
   order for a potential misconfiguration to be detected and for a
   prompt corrective action to be taken.

5.2 Neighbor Database Maintenance

   UDLD sends periodical "hello" packets (also called "advertisements"
   or "probes") on every active interface to keep each device informed
   about its neighbors. When a hello message is received, it is cached
   and kept in memory at most for a defined time interval, called
   "holdtime" or "time-to-live", after which the cache entry is
   considered stale and is aged out.

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   If a new hello message is received when a correspondent old cache
   entry has not been aged out yet, then the old entry is dropped and is
   replaced by the new one with a reset time-to-live timer.
   Whenever an interface gets disabled and UDLD is running, or whenever
   UDLD is disabled on an interface, or whenever the device is reset,
   all existing cache entries for the interfaces affected by the
   configuration change are cleared and UDLD sends at least one message
   to inform the neighbors to flush the part of their caches also
   affected by the status change. This mechanism is meant to keep the
   caches coherent on all the connected devices.

5.3 Event-driven Detection and Echoing

   The echoing mechanism is the base of UDLD's detection algorithm:
   whenever a UDLD device learns about a new neighbor or receives a
   resynchronization request from an out-of-synch neighbor, it
   (re)starts the detection process on its side of the connection and
   sends N echo messages in reply. (This mechanism implicitly assumes
   that N packets are sufficient to get through a link and reach the
   other end, even though some of them might get dropped during the
   Since this behavior must be the same on all the neighbors, the sender
   of the echoes expects to receive after some time an echo in reply. If
   the detection process ends without the proper echo information being
   received, under specific conditions the link is considered to be

5.4 Event-based versus Event-less Detection

   UDLD can function in two modes: normal mode and aggressive mode.

   In normal mode a protocol determination at the end of the detection
   process is always based on information received in UDLD messages:
   whether it's the information about the exchange of proper neighbor
   identifications or the information about the absence of such proper
   identifications. Hence, albeit bound by a timer, normal mode
   determinations are always based on gleaned information, and as such
   are "event-based". If no such information can be obtained (e.g.,
   because of a bidirectional loss of connectivity), UDLD follows a
   conservative approach based on the considerations in Section 3 and
   deems a port to be in "undetermined" state. In other words, normal
   mode will shut down a port only if it can explicitly determine that
   the associated link is faulty for an extended period of time.

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   Cisco Systems UniDirectional Link Detection Protocol     April 2007

   In aggressive mode, instead, UDLD will shut down a port also in case
   it just loses bidirectional connectivity with the neighbor for the
   same extended period of time mentioned above and subsequently fails
   repeated last-resort attempts to re-establish communication with the
   other end of the link. This mode of operation assumes that loss of
   communication with the neighbor is a meaningful network event in
   itself and is a symptom of a serious connectivity problem. Because
   this type of detection can be event-less, and lack of information
   cannot always be associated to an actual malfunction of the link,
   this mode is optional and is recommended only in certain scenarios
   (typically only on point-to-point links where no communication
   failure between two neighbors is admissible).

6. Packet Format

   The UDLD protocol runs on top of the LLC sub-layer of the data link
   layer of the OSI stack. It uses a specially assigned multicast
   destination MAC address and encapsulates its messages using the
   standard SNAP format as described in the following:

         Destination MAC address            01-00-0C-CC-CC-CC

         UDLD SNAP format:
           LLC value                        0xAAAA03
           Org Id                           0x00000C
           HDLC protocol type               0x0111

   UDLD's Protocol Data Unit (PDU) format is as follows:

    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
   | Ver | Opcode  |     Flags     |           Checksum            |
   |               List of TLVs (variable length list)             |
   |                              ...                              |

Foschiano                                                     [Page 7]

   Cisco Systems UniDirectional Link Detection Protocol     April 2007

   The TLV format is the basic one described below:

    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                             |
   |                              ...                              |

   Type (16 bits): If an implementation does not understand a Type value
   it should skip over it using the length field.
   Length (16 bits): Length in bytes of the Type, Length, and Value
   fields. In order for this field value to be valid, it should be
   greater than or equal to the minimum allowed length, 4 bytes; if not,
   the whole packet is to be considered corrupted and therefore it must
   be discarded right away during the parsing process. TLVs should not
   be split across packet boundaries.
   Value (variable length): Object contained in the TLV.

   The protocol header fields are defined as follows:

   Ver (3 bits):
         0x01: UDLD PDU version number

   Opcode (5 bits):
         0x00: Reserved
         0x01: Probe message
         0x02: Echo message
         0x03: Flush message
         0x04-0x1F: Reserved for future use

   Flags (8 bits):
         bit 0: Recommended timeout flag (RT)
         bit 1: ReSynch flag (RSY)
         bit 2...7: Reserved for future use

   PDU Checksum (16 bits):
         IP-like checksum. Take the 1's complement of the 1's complement
   sum (with the modification that the odd byte at the end of an odd
   length message is used as the low 8 bits of an extra word, rather
   than as the high 8 bits.) N.B.: All UDLD implementations must comply
   with this specification.

Foschiano                                                     [Page 8]

   Cisco Systems UniDirectional Link Detection Protocol     April 2007

   The list of currently defined TLVs comprises:

     Name                   Type      Value format

     Device-ID TLV          0x0001    ASCII character string
     Port-ID TLV            0x0002    ASCII character string
     Echo TLV               0x0003    List of ID pairs
     Message Interval TLV   0x0004    8 bit unsigned integer
     Timeout Interval TLV   0x0005    8 bit unsigned integer
     Device Name TLV        0x0006    ASCII character string
     Sequence Number TLV    0x0007    32 bit unsigned integer
     Reserved TLVs          > 0x0007  Format unknown.
                                      To be skipped by parsing routine.

6.1 TLV Description

   Device-ID TLV:

     This TLV uniquely identifies the device that is sending the UDLD
     packet. The TLV length field determines the length of the carried
     identifier and must be greater than zero. In version 1 of the
     protocol the lack of this ID is considered a symptom of packet
     corruption that implies that the message is invalid and must be

   Port-ID TLV:

     This TLV uniquely identifies the physical port the UDLD packet is
     sent on. The TLV length field determines the length of the carried
     identifier and must be greater than zero. In version 1 of the
     protocol the lack of this ID is considered a symptom of packet
     corruption that implies that the message is invalid and must be

   Echo TLV:

     This TLV contains the list of valid DeviceID/PortID pairs received
     by a port from all its neighbors. If either one of the identifiers
     in a pair is corrupted the message will be ignored.
     This list includes only DeviceID's and PortID's extracted from UDLD
     messages received and cached on the same interface on which this
     TLV is sent. If no UDLD messages are received, then this TLV is
     sent containing zero pairs. Despite its name, this TLV must be
     present in both probe and echo messages, whereas in flush messages
     it's not required.

Foschiano                                                     [Page 9]

   Cisco Systems UniDirectional Link Detection Protocol     April 2007

   Message Interval TLV:

     This required TLV contains the 8-bit time interval value used by a
     neighbor to send UDLD probes after the linkup or detection phases.
     Its time unit is 1 second. The holdtime of a cache item for a
     received message is calculated as (advertised-message-interval x
     R), where R is a constant called "Ttl to message interval ratio".

   Timeout Interval TLV:

     This optional TLV contains the 8-bit timeout interval value (T)
     used by UDLD to decide the basic length of the detection phase. Its
     time unit is 1 second. If it's not present in an advertisement, T
     is assumed to be a hard-coded constant.

   Device Name TLV:

     This required TLV is meant to be used by the CLI or SNMP and
     typically contains the user-readable device name string.

   Sequence Number TLV:

     The purpose of this optional TLV is to inform the neighbors of the
     sequence number of the current message being transmitted. A counter
     from 1 to 2^32-1 is supposed to keep track of the sequence number;
     it is reset whenever a transition of phase occurs so that it will
     restart counting from one, for instance, whenever an echo message
     sequence is initiated, or whenever a linkup message train is
     No wraparound of the counter is supposed to happen.
     The zero value is reserved and can be used as a representation of a
     missing or invalid sequence number by the user interface.
     Therefore, the TLV should never contain zero.
     (N.B.: The use of this TLV is currently limited only to
     informational purposes.)

7. Protocol Logic

   UDLD's protocol logic relies on specific internal timers and is
   sensitive to certain network events.

   The type of messages that UDLD transmits and the timing intervals
   that it uses are dependent upon the internal state of the protocol,
   which changes based on network events such as:

Foschiano                                                    [Page 10]

   Cisco Systems UniDirectional Link Detection Protocol     April 2007

     o  Link up
     o  Link down
     o  Protocol enabled
     o  Protocol disabled
     o  New neighbor discovery
     o  Neighbor state change
     o  Neighbor resynchronization requests

7.1 Protocol Timers

   UDLD timer values could vary within certain "safety" ranges based on
   the considerations in Section 3. However, in practice, in the current
   implementation timers use only certain values verified during
   testing. Their time unit is one second.

   During the detection phase, messages are exchanged at the maximum
   possible rate of one per second. After that, if the protocol reaches
   a stable state and can make a certain determination on the
   "bidirectionality" of the link, the message interval is increased to
   a configurable value based on a curve known as M1(t), a time-based

   In case the link is deemed anything other than bidirectional at the
   end of the detection, this curve is a flat line with a fixed value of
   Mfast (7 seconds in the current implementation).

   In case the link is instead deemed bidirectional, the curve will use
   Mfast for the first 4 subsequent message transmissions and then will
   transition to an Mslow value for all other steady-state
   transmissions. Mslow can be either a fixed value (60 seconds in some
   obsolete implementations) or a user configurable value (between Mfast
   and 90 seconds with a default of 15 seconds in the current

8. Comparison with Bidirectional Forwarding Detection

   Similarly to UDLD, the Bidirectional Forwarding Detection (BFD) [3]
   protocol is intended to detect faults in the path between two network
   nodes. However, BFD is supposed to operate independently of media,
   data protocols and routing protocols. There is no address discovery
   mechanism in BFD, which is left to the application to determine.

   On the other hand, UDLD works exclusively on top of a L2 transport
   supporting the SNAP encapsulation and operates independently of the
   other bridge protocols (UDLD's main "applications"), with which it
   has limited interaction. It also performs full neighbor discovery on
   point-to-point and point-to-multipoint media.

Foschiano                                                    [Page 11]

   Cisco Systems UniDirectional Link Detection Protocol     April 2007

Security Considerations

   In a heterogeneous Layer 2 network that is built with different
   models of network devices or with devices running different software
   images, the UDLD protocol should be supported and configured on all
   ports interconnecting said devices in order to achieve a complete
   coverage of its detection process. Instead, UDLD is not supposed to
   be used on ports connected to untrusted devices or incapable devices;
   hence, it should be disabled on such ports.

IANA Considerations

   This document has no actions for IANA.

Deployment Considerations

   Cisco Systems has supported the UDLD protocol in its Catalyst family
   of switches since year 1999.

Changes from the Previous Version

   Updated as per RFC 4748 and edited as per reviewers' comments. Added
   a section with a comparison between UDLD and BFD.

Normative References

   [1]  IEEE 802.1D-2004 Standard -- Media access control (MAC) Bridges

   [2]  IEEE 802.3-2002 IEEE Standard -- Local and metropolitan area
        networks Specific requirements--Part 3: Carrier Sense Multiple
        Access with Collision Detection (CSMA/CD) Access Method and
        Physical Layer Specifications

   [3]  Katz, D., and Ward, D., "Bidirectional Forwarding Detection",
        draft-ietf-bfd-base-06.txt, March, 2007.

Foschiano                                                    [Page 12]

   Cisco Systems UniDirectional Link Detection Protocol     April 2007

Author's Address

   Marco Foschiano
   Cisco Systems, Inc.
   Via Torri Bianche 7, Vimercate, MI, 20059, Italy
   Email: foschia@cisco.com

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   Cisco Systems UniDirectional Link Detection Protocol     April 2007

Full Copyright Notice

   Copyright (C) The IETF Trust (2007).

   This document is subject to the rights, licenses and restrictions
   contained in BCP 78, and except as set forth therein, the authors
   retain all their rights.

   This document and the information contained herein are provided on an


   Funding for the RFC Editor function is currently provided by the
   Internet Society.

   This Internet-Draft will expire in October, 2007.

Foschiano                                                    [Page 14]

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