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RFC 2233

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The Interfaces Group MIB using SMIv2

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Network Working Group                                      K. McCloghrie
Request for Comments: 2233                                 Cisco Systems
Obsoletes: 1573                                            F. Kastenholz
Category: Standards Track                                   FTP Software
                                                           November 1997

                  The Interfaces Group MIB using SMIv2

Status of this Memo

   This document specifies an Internet standards track protocol for the
   Internet community, and requests discussion and suggestions for
   improvements.  Please refer to the current edition of the "Internet
   Official Protocol Standards" (STD 1) for the standardization state
   and status of this protocol.  Distribution of this memo is unlimited.

Copyright Notice

   Copyright (C) The Internet Society (1997).  All Rights Reserved.

Table of Contents

   1 Introduction ..............................................    2
   2 The SNMP Network Management Framework .....................    2
   2.1 Object Definitions ......................................    3
   3 Experience with the Interfaces Group ......................    3
   3.1 Clarifications/Revisions ................................    3
   3.1.1 Interface Sub-Layers ..................................    4
   3.1.2 Guidance on Defining Sub-layers .......................    6
   3.1.3 Virtual Circuits ......................................    8
   3.1.4 Bit, Character, and Fixed-Length Interfaces ...........    8
   3.1.5 Interface Numbering ...................................   10
   3.1.6 Counter Size ..........................................   14
   3.1.7 Interface Speed .......................................   16
   3.1.8 Multicast/Broadcast Counters ..........................   17
   3.1.9 Trap Enable ...........................................   18
   3.1.10 Addition of New ifType values ........................   18
   3.1.11 InterfaceIndex Textual Convention ....................   18
   3.1.12 New states for IfOperStatus ..........................   19
   3.1.13 IfAdminStatus and IfOperStatus .......................   20
   3.1.14 IfOperStatus in an Interface Stack ...................   21
   3.1.15 Traps ................................................   21
   3.1.16 ifSpecific ...........................................   23
   3.1.17 Creation/Deletion of Interfaces ......................   24
   3.1.18 All Values Must be Known .............................   24
   4 Media-Specific MIB Applicability ..........................   25

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   5 Overview ..................................................   26
   6 Interfaces Group Definitions ..............................   26
   7 Acknowledgements ..........................................   64
   8 References ................................................   64
   9 Security Considerations ...................................   65
   10 Authors' Addresses .......................................   65
   11 Full Copyright Statement .................................   66

1.  Introduction

   This memo defines a portion of the Management Information Base
   (MIB) for use with network management protocols in the Internet
   community.  In particular, it describes managed objects used for
   managing Network Interfaces.

   This memo discusses the 'interfaces' group of MIB-II, especially the
   experience gained from the definition of numerous media- specific MIB
   modules for use in conjunction with the 'interfaces' group for
   managing various sub-layers beneath the internetwork- layer.  It
   specifies clarifications to, and extensions of, the architectural
   issues within the previous model used for the 'interfaces' group.

   This memo also includes a MIB module.  As well as including new
   MIB definitions to support the architectural extensions, this MIB
   module also re-specifies the 'interfaces' group of MIB-II in a
   manner that is both compliant to the SNMPv2 SMI and semantically-
   identical to the existing SNMPv1-based definitions.

   The key words "MUST" and "MUST NOT" in this document are to be
   interpreted as described in RFC 2119 [10].

2.  The SNMP Network Management Framework

   The SNMP Network Management Framework presently consists of three
   major components.  They are:

   o    RFC 1902 which defines the SMI, the mechanisms used for
        describing and naming objects for the purpose of management.

   o    STD 17, RFC 1213 defines MIB-II, the core set of managed
        objects for the Internet suite of protocols.

   o    STD 15, RFC 1157 and RFC 1905 which define two versions of
        the protocol used for network access to managed objects.

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   The Framework permits new objects to be defined for the purpose of
   experimentation and evaluation.

2.1.  Object Definitions

   Managed objects are accessed via a virtual information store,
   termed the Management Information Base or MIB.  Objects in the MIB
   are defined using the subset of Abstract Syntax Notation One
   (ASN.1) defined in the SMI.  In particular, each object object
   type is named by an OBJECT IDENTIFIER, an administratively
   assigned name.  The object type together with an object instance
   serves to uniquely identify a specific instantiation of the
   object.  For human convenience, we often use a textual string,
   termed the descriptor, to refer to the object type.

3.  Experience with the Interfaces Group

   One of the strengths of internetwork-layer protocols such as IP
   [6] is that they are designed to run over any network interface.
   In achieving this, IP considers any and all protocols it runs over
   as a single "network interface" layer.  A similar view is taken by
   other internetwork-layer protocols.  This concept is represented
   in MIB-II by the 'interfaces' group which defines a generic set of
   managed objects such that any network interface can be managed in
   an interface-independent manner through these managed objects.
   The 'interfaces' group provides the means for additional managed
   objects specific to particular types of network interface (e.g., a
   specific medium such as Ethernet) to be defined as extensions to
   the 'interfaces' group for media-specific management.  Since the
   standardization of MIB-II, many such media-specific MIB modules
   have been defined.

   Experience in defining these media-specific MIB modules has shown
   that the model defined by MIB-II is too simplistic and/or static
   for some types of media-specific management.  As a result, some of
   these media-specific MIB modules assume an evolution or loosening
   of the model.  This memo documents and standardizes that evolution
   of the model and fills in the gaps caused by that evolution.  This
   memo also incorporates the interfaces group extensions documented
   in RFC 1229 [7].

3.1.  Clarifications/Revisions

   There are several areas for which experience has indicated that
   clarification, revision, or extension of the model would be
   helpful.  The following sections discuss the changes in the
   interfaces group adopted by this memo in each of these areas.

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   In some sections, one or more paragraphs contain discussion of
   rejected alternatives to the model adopted in this memo.  Readers
   not familiar with the MIB-II model and not interested in the
   rationale behind the new model may want to skip these paragraphs.

3.1.1.  Interface Sub-Layers

   Experience in defining media-specific management information has
   shown the need to distinguish between the multiple sub-layers
   beneath the internetwork-layer.  In addition, there is a need to
   manage these sub-layers in devices (e.g., MAC-layer bridges) which
   are unaware of which, if any, internetwork protocols run over
   these sub-layers.  As such, a model of having a single conceptual
   row in the interfaces table (MIB-II's ifTable) represent a whole
   interface underneath the internetwork-layer, and having a single
   associated media-specific MIB module (referenced via the ifType
   object) is too simplistic.  A further problem arises with the
   value of the ifType object which has enumerated values for each
   type of interface.

   Consider, for example, an interface with PPP running over an HDLC
   link which uses a RS232-like connector.  Each of these sub-layers
   has its own media-specific MIB module.  If all of this is
   represented by a single conceptual row in the ifTable, then an
   enumerated value for ifType is needed for that specific
   combination which maps to the specific combination of media-
   specific MIBs.  Furthermore, such a model still lacks a method to
   describe the relationship of all the sub-layers of the MIB stack.

   An associated problem is that of upward and downward multiplexing
   of the sub-layers.  An example of upward multiplexing is MLP
   (Multi-Link-Procedure) which provides load-sharing over several
   serial lines by appearing as a single point-to-point link to the
   sub-layer(s) above.  An example of downward multiplexing would be
   several instances of PPP, each framed within a separate X.25
   virtual circuit, all of which run over one fractional T1 channel,
   concurrently with other uses of the T1 link.  The MIB structure
   must allow these sorts of relationships to be described.

   Several solutions for representing multiple sub-layers were
   rejected.  One was to retain the concept of one conceptual row for
   all the sub-layers of an interface and have each media-specific
   MIB module identify its "superior" and "subordinate" sub-layers
   through OBJECT IDENTIFIER "pointers".  This scheme would have
   several drawbacks: the superior/subordinate pointers would be
   contained in the media-specific MIB modules; thus, a manager could
   not learn the structure of an interface without inspecting
   multiple pointers in different MIB modules; this would be overly

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   complex and only possible if the manager had knowledge of all the
   relevant media-specific MIB modules; MIB modules would all need to
   be retrofitted with these new "pointers"; this scheme would not
   adequately address the problem of upward and downward
   multiplexing; and finally, enumerated values of ifType would be
   needed for each combination of sub-layers.  Another rejected
   solution also retained the concept of one conceptual row for all
   the sub-layers of an interface but had a new separate MIB table to
   identify the "superior" and "subordinate" sub-layers and to
   contain OBJECT IDENTIFIER "pointers" to the media-specific MIB
   module for each sub-layer.  Effectively, one conceptual row in the
   ifTable would represent each combination of sub-layers between the
   internetwork-layer and the wire.  While this scheme has fewer
   drawbacks, it still would not support downward multiplexing, such
   as PPP over MLP: observe that MLP makes two (or more) serial
   lines appear to the layers above as a single physical interface,
   and thus PPP over MLP should appear to the internetwork-layer as a
   single interface; in contrast, this scheme would result in two (or
   more) conceptual rows in the ifTable, both of which the
   internetwork-layer would run over.  This scheme would also require
   enumerated values of ifType for each combination of sub-layers.

   The solution adopted by this memo is to have an individual
   conceptual row in the ifTable to represent each sub-layer, and
   have a new separate MIB table (the ifStackTable, see section 6
   below) to identify the "superior" and "subordinate" sub-layers
   through INTEGER "pointers" to the appropriate conceptual rows in
   the ifTable.  This solution supports both upward and downward
   multiplexing, allows the IANAifType to Media-Specific MIB mapping
   to identify the media-specific MIB module for that sub-layer, such
   that the new table need only be referenced to obtain information
   about layering, and it only requires enumerated values of ifType
   for each sub-layer, not for combinations of them.  However, it
   does require that the descriptions of some objects in the ifTable
   (specifically, ifType, ifPhysAddress, ifInUcastPkts, and
   ifOutUcastPkts) be generalized so as to apply to any sub-layer
   (rather than only to a sub-layer immediately beneath the network
   layer as previously), plus some (specifically, ifSpeed) which need
   to have appropriate values identified for use when a generalized
   definition does not apply to a particular sub-layer.

   In addition, this adopted solution makes no requirement that a
   device, in which a sub-layer is instrumented by a conceptual row
   of the ifTable, be aware of whether an internetwork protocol runs
   on top of (i.e., at some layer above) that sub-layer.  In fact,
   the counters of packets received on an interface are defined as
   counting the number "delivered to a higher-layer protocol".  This
   meaning of "higher-layer" includes:

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   (1)  Delivery to a forwarding module which accepts
        packets/frames/octets and forwards them on at the same
        protocol layer.  For example, for the purposes of this
        definition, the forwarding module of a MAC-layer bridge is
        considered as a "higher-layer" to the MAC-layer of each port
        on the bridge.

   (2)  Delivery to a higher sub-layer within a interface stack.  For
        example, for the purposes of this definition, if a PPP module
        operated directly over a serial interface, the PPP module
        would be considered the higher sub-layer to the serial

   (3)  Delivery to a higher protocol layer which does not do packet
        forwarding for sub-layers that are "at the top of" the
        interface stack.  For example, for the purposes of this
        definition, the local IP module would be considered the
        higher layer to a SLIP serial interface.

   Similarly, for output, the counters of packets transmitted out an
   interface are defined as counting the number "that higher-level
   protocols requested to be transmitted".  This meaning of "higher-
   layer" includes:

   (1)  A forwarding module, at the same protocol layer, which
        transmits packets/frames/octets that were received on an
        different interface.  For example, for the purposes of this
        definition, the forwarding module of a MAC-layer bridge is
        considered as a "higher-layer" to the MAC-layer of each port
        on the bridge.

   (2)  The next higher sub-layer within an interface stack.  For
        example, for the purposes of this definition, if a PPP module
        operated directly over a serial interface, the PPP module
        would be a "higher layer" to the serial interface.

   (3)  For sub-layers that are "at the top of" the interface stack,
        a higher element in the network protocol stack.  For example,
        for the purposes of this definition, the local IP module
        would be considered the higher layer to an Ethernet

3.1.2.  Guidance on Defining Sub-layers

   The designer of a media-specific MIB must decide whether to divide
   the interface into sub-layers or not, and if so, how to make the
   divisions.  The following guidance is offered to assist the
   media-specific MIB designer in these decisions.

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   In general, the number of entries in the ifTable should be kept to
   the minimum required for network management.  In particular, a
   group of related interfaces should be treated as a single
   interface with one entry in the ifTable providing that:

   (1)  None of the group of interfaces performs multiplexing for any
        other interface in the agent,
   (2)  There is a meaningful and useful way for all of the ifTable's
        information (e.g., the counters, and the status variables),
        and all of the ifTable's capabilities (e.g., write access to
        ifAdminStatus), to apply to the group of interfaces as a

   Under these circumstances, there should be one entry in the
   ifTable for such a group of interfaces, and any internal structure
   which needs to be represented to network management should be
   captured in a MIB module specific to the particular type of

   Note that application of bullet 2 above to the ifTable's ifType
   object requires that there is a meaningful media-specific MIB and
   a meaningful ifType value which apply to the group of interfaces
   as a whole.  For example, it is not appropriate to treat an HDLC
   sub-layer and an RS-232 sub-layer as a single ifTable entry when
   the media-specific MIBs and the ifType values for HDLC and RS-232
   are separate (rather than combined).

   Subject to the above, it is appropriate to assign an ifIndex value
   to any interface that can occur in an interface stack (in the
   ifStackTable) where the bottom of the stack is a physical
   interface (ifConnectorPresent has the value 'true') and there is a
   layer-3 or other application that "points down" to the top of this
   stack.  An example of an application that points down to the top
   of the stack is the Character MIB [9].

   Note that the sub-layers of an interface on one device will
   sometimes be different from the sub-layers of the interconnected
   interface of another device; for example, for a frame-relay DTE
   interface connected a frameRelayService interface, the inter-
   connected DTE and DCE interfaces have different ifType values and
   media-specific MIBs.

   These guidelines are just that, guidelines.  The designer of a
   media-specific MIB is free to lay out the MIB in whatever SMI
   conformant manner is desired.  However, in doing so, the media-
   specific MIB MUST completely specify the sub-layering model used
   for the MIB, and provide the assumptions, reasoning, and rationale
   used to develop that model.

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3.1.3.  Virtual Circuits

   Several of the sub-layers for which media-specific MIB modules
   have been defined are connection oriented (e.g., Frame Relay,
   X.25).  Experience has shown that each effort to define such a MIB
   module revisits the question of whether separate conceptual rows
   in the ifTable are needed for each virtual circuit.  Most, if not
   all, of these efforts to date have decided to have all virtual
   circuits reference a single conceptual row in the ifTable.

   This memo strongly recommends that connection-oriented sub-layers
   do not have a conceptual row in the ifTable for each virtual
   circuit.  This avoids the proliferation of conceptual rows,
   especially those which have considerable redundant information.
   (Note, as a comparison, that connection-less sub-layers do not
   have conceptual rows for each remote address.)  There may,
   however, be circumstances under which it is appropriate for a
   virtual circuit of a connection-oriented sub-layer to have its own
   conceptual row in the ifTable; an example of this might be PPP
   over an X.25 virtual circuit.  The MIB in section 6 of this memo
   supports such circumstances.

   If a media-specific MIB wishes to assign an entry in the ifTable
   to each virtual circuit, the MIB designer must present the
   rationale for this decision in the media-specific MIB's

3.1.4.  Bit, Character, and Fixed-Length Interfaces

   RS-232 is an example of a character-oriented sub-layer over which
   (e.g., through use of PPP) IP datagrams can be sent.  Due to the
   packet-based nature of many of the objects in the ifTable,
   experience has shown that it is not appropriate to have a
   character-oriented sub-layer represented by a whole conceptual row
   in the ifTable.

   Experience has also shown that it is sometimes desirable to have
   some management information for bit-oriented interfaces, which are
   similarly difficult to represent by a whole conceptual row in the
   ifTable.  For example, to manage the channels of a DS1 circuit,
   where only some of the channels are carrying packet-based data.

   A further complication is that some subnetwork technologies
   transmit data in fixed length transmission units.  One example of
   such a technology is cell relay, and in particular Asynchronous
   Transfer Mode (ATM), which transmits data in fixed-length cells.
   Representing such a interface as a packet-based interface produces

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   redundant objects if the relationship between the number of
   packets and the number of octets in either direction is fixed by
   the size of the transmission unit (e.g., the size of a cell).

   About half the objects in the ifTable are applicable to every type
   of interface: packet-oriented, character-oriented, and bit-
   oriented.  Of the other half, two are applicable to both
   character-oriented and packet-oriented interfaces, and the rest
   are applicable only to packet-oriented interfaces.  Thus, while it
   is desirable for consistency to be able to represent any/all types
   of interfaces in the ifTable, it is not possible to implement the
   full ifTable for bit- and character-oriented sub-layers.

   A rejected solution to this problem would be to split the ifTable
   into two (or more) new MIB tables, one of which would contain
   objects that are relevant only to packet-oriented interfaces
   (e.g., PPP), and another that may be used by all interfaces.  This
   is highly undesirable since it would require changes in every
   agent implementing the ifTable (i.e., just about every existing
   SNMP agent).

   The solution adopted in this memo builds upon the fact that
   compliance statements in SNMPv2 (in contrast to SNMPv1) refer to
   object groups, where object groups are explicitly defined by
   listing the objects they contain.  Thus, in SNMPv2, multiple
   compliance statements can be specified, one for all interfaces and
   additional ones for specific types of interfaces.  The separate
   compliance statements can be based on separate object groups,
   where the object group for all interfaces can contain only those
   objects from the ifTable which are appropriate for every type of
   interfaces.  Using this solution, every sub-layer can have its own
   conceptual row in the ifTable.

   Thus, section 6 of this memo contains definitions of the objects
   of the existing 'interfaces' group of MIB-II, in a manner which is
   both SNMPv2-compliant and semantically-equivalent to the existing
   MIB-II definitions.  With equivalent semantics, and with the BER
   ("on the wire") encodings unchanged, these definitions retain the
   same OBJECT IDENTIFIER values as assigned by MIB-II.  Thus, in
   general, no rewrite of existing agents which conform to MIB-II and
   the ifExtensions MIB is required.

   In addition, this memo defines several object groups for the
   purposes of defining which objects apply to which types of

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   (1)  the ifGeneralInformationGroup.  This group contains those
        objects applicable to all types of network interfaces,
        including bit-oriented interfaces.

   (2)  the ifPacketGroup.  This group contains those objects
        applicable to packet-oriented network interfaces.

   (3)  the ifFixedLengthGroup.  This group contains the objects
        applicable not only to character-oriented interfaces, such as
        RS-232, but also to those subnetwork technologies, such as
        cell-relay/ATM, which transmit data in fixed length
        transmission units.  As well as the octet counters, there are
        also a few other counters (e.g., the error counters) which
        are useful for this type of interface, but are currently
        defined as being packet-oriented.  To accommodate this, the
        definitions of these counters are generalized to apply to
        character-oriented interfaces and fixed-length-transmission

   It should be noted that the octet counters in the ifTable
   aggregate octet counts for unicast and non-unicast packets into a
   single octet counter per direction (received/transmitted).  Thus,
   with the above definition of fixed-length-transmission interfaces,
   where such interfaces which support non-unicast packets, separate
   counts of unicast and multicast/broadcast transmissions can only
   be maintained in a media-specific MIB module.

3.1.5.  Interface Numbering

   MIB-II defines an object, ifNumber, whose value represents:

        "The number of network interfaces (regardless of their
        current state) present on this system."

   Each interface is identified by a unique value of the ifIndex
   object, and the description of ifIndex constrains its value as

        "Its value ranges between 1 and the value of ifNumber.  The
        value for each interface must remain constant at least from
        one re-initialization of the entity's network management
        system to the next re-initialization."

   This constancy requirement on the value of ifIndex for a
   particular interface is vital for efficient management.  However,
   an increasing number of devices allow for the dynamic
   addition/removal of network interfaces.  One example of this is a
   dynamic ability to configure the use of SLIP/PPP over a

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   character-oriented port.  For such dynamic additions/removals, the
   combination of the constancy requirement and the restriction that
   the value of ifIndex is less than ifNumber is problematic.

   Redefining ifNumber to be the largest value of ifIndex was
   rejected since it would not help.  Such a re-definition would
   require ifNumber to be deprecated and the utility of the redefined
   object would be questionable.  Alternatively, ifNumber could be
   deprecated and not replaced.  However, the deprecation of ifNumber
   would require a change to that portion of ifIndex's definition
   which refers to ifNumber.  So, since the definition of ifIndex
   must be changed anyway in order to solve the problem, changes to
   ifNumber do not benefit the solution.

   The solution adopted in this memo is just to delete the
   requirement that the value of ifIndex must be less than the value
   of ifNumber, and to retain ifNumber with its current definition.
   This is a minor change in the semantics of ifIndex; however, all
   existing agent implementations conform to this new definition, and
   in the interests of not requiring changes to existing agent
   implementations and to the many existing media-specific MIBs, this
   memo assumes that this change does not require ifIndex to be
   deprecated.  Experience indicates that this assumption does
   "break" a few management applications, but this is considered
   preferable to breaking all agent implementations.

   This solution also results in the possibility of "holes" in the
   ifTable, i.e., the ifIndex values of conceptual rows in the
   ifTable are not necessarily contiguous, but SNMP's GetNext (and
   SNMPv2's GetBulk) operation easily deals with such holes.  The
   value of ifNumber still represents the number of conceptual rows,
   which increases/decreases as new interfaces are dynamically

   The requirement for constancy (between re-initializations) of an
   interface's ifIndex value is met by requiring that after an
   interface is dynamically removed, its ifIndex value is not re-used
   by a *different* dynamically added interface until after the
   following re-initialization of the network management system.
   This avoids the need for assignment (in advance) of ifIndex values
   for all possible interfaces that might be added dynamically.  The
   exact meaning of a "different" interface is hard to define, and
   there will be gray areas.  Any firm definition in this document
   would likely to turn out to be inadequate.  Instead, implementors
   must choose what it means in their particular situation, subject
   to the following rules:

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   (1)  a previously-unused value of ifIndex must be assigned to a
        dynamically added interface if an agent has no knowledge of
        whether the interface is the "same" or "different" to a
        previously incarnated interface.

   (2)  a management station, not noticing that an interface has gone
        away and another has come into existence, must not be
        confused when calculating the difference between the counter
        values retrieved on successive polls for a particular ifIndex

   When the new interface is the same as an old interface, but a
   discontinuity in the value of the interface's counters cannot be
   avoided, the ifTable has (until now) required that a new ifIndex
   value be assigned to the returning interface.  That is, either all
   counter values have had to be retained during the absence of an
   interface in order to use the same ifIndex value on that
   interface's return, or else a new ifIndex value has had to be
   assigned to the returning interface.  Both alternatives have
   proved to be burdensome to some implementations:

   (1)  maintaining the counter values may not be possible (e.g., if
        they are maintained on removable hardware),

   (2)  using a new ifIndex value presents extra work for management
        applications.  While the potential need for such extra work
        is unavoidable on agent re-initializations, it is desirable
        to avoid it between re-initializations.

   To address this, a new object, ifCounterDiscontinuityTime, has
   been defined to record the time of the last discontinuity in an
   interface's counters.  By monitoring the value of this new object,
   a management application can now detect counter discontinuities
   without the ifIndex value of the interface being changed.  Thus,
   an agent which implements this new object should, when a new
   interface is the same as an old interface, retain that interface's
   ifIndex value and update if necessary the interface's value of
   ifCounterDiscontinuityTime.  With this new object, a management
   application must, when calculating differences between counter
   values retrieved on successive polls, discard any calculated
   difference for which the value of ifCounterDiscontinuityTime is
   different for the two polls.  (Note that this test must be
   performed in addition to the normal checking of sysUpTime to
   detect an agent re-initialization.)  Since such discards are a
   waste of network management processing and bandwidth, an agent
   should not update the value of ifCounterDiscontinuityTime unless
   absolutely necessary.

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   While defining this new object is a change in the semantics of the
   ifTable counter objects, it is impractical to deprecate and
   redefine all these counters because of their wide deployment and
   importance.  Also, a survey of implementations indicates that many
   agents and management applications do not correctly implement this
   aspect of the current semantics (because of the burdensome issues
   mentioned above), such that the practical implications of such a
   change is small.  Thus, this breach of the SMI's rules is
   considered to be acceptable.

   Note, however, that the addition of ifCounterDiscontinuityTime
   does not change the fact that:

        It is necessary at certain times for the assignment of ifIndex
        values to change on a reinitialization of the agent (such as a

   The possibility of ifIndex value re-assignment must be
   accommodated by a management application whenever the value of
   sysUpTime is reset to zero.

   Note also that some agents support multiple "naming scopes", e.g.,
   for an SNMPv1 agent, multiple values of the SNMPv1 community
   string.  For such an agent (e.g., a CNM agent which supports a
   different subset of interfaces for different customers), there is
   no required relationship between the ifIndex values which identify
   interfaces in one naming scope and those which identify interfaces
   in another naming scope.  It is the agent's choice as to whether
   the same or different ifIndex values identify the same or
   different interfaces in different naming scopes.

   Because of the restriction of the value of ifIndex to be less than
   ifNumber, interfaces have been numbered with small integer values.
   This has led to the ability by humans to use the ifIndex values as
   (somewhat) user-friendly names for network interfaces (e.g.,
   "interface number 3").  With the relaxation of the restriction on
   the value of ifIndex, there is now the possibility that ifIndex
   values could be assigned as very large numbers (e.g., memory
   addresses).  Such numbers would be much less user-friendly.
   Therefore, this memo recommends that ifIndex values still be
   assigned as (relatively) small integer values starting at 1, even
   though the values in use at any one time are not necessarily
   contiguous.  (Note that this makes remembering which values have
   been assigned easy for agents which dynamically add new

Top      ToC       Page 14 
   A new problem is introduced by representing each sub-layer as an
   ifTable entry.  Previously, there usually was a simple, direct,
   mapping of interfaces to the physical ports on systems.  This
   mapping would be based on the ifIndex value.  However, by having
   an ifTable entry for each interface sub-layer, mapping from
   interfaces to physical ports becomes increasingly problematic.

   To address this issue, a new object, ifName, is added to the MIB.
   This object contains the device's local name (e.g., the name used
   at the device's local console) for the interface of which the
   relevant entry in the ifTable is a component.  For example,
   consider a router having an interface composed of PPP running over
   an RS-232 port.  If the router uses the name "wan1" for the
   (combined) interface, then the ifName objects for the
   corresponding PPP and RS-232 entries in the ifTable would both
   have the value "wan1".  On the other hand, if the router uses the
   name "wan1.1" for the PPP interface and "wan1.2" for the RS-232
   port, then the ifName objects for the corresponding PPP and RS-232
   entries in the ifTable would have the values "wan1.1" and
   "wan1.2", respectively.  As an another example, consider an agent
   which responds to SNMP queries concerning an interface on some
   other (proxied) device: if such a proxied device associates a
   particular identifier with an interface, then it is appropriate to
   use this identifier as the value of the interface's ifName, since
   the local console in this case is that of the proxied device.

   In contrast, the existing ifDescr object is intended to contain a
   description of an interface, whereas another new object, ifAlias,
   provides a location in which a network management application can
   store a non-volatile interface-naming value of its own choice.
   The ifAlias object allows a network manager to give one or more
   interfaces their own unique names, irrespective of any interface-
   stack relationship.  Further, the ifAlias name is non-volatile,
   and thus an interface must retain its assigned ifAlias value
   across reboots, even if an agent chooses a new ifIndex value for
   the interface.

3.1.6.  Counter Size

   As the speed of network media increase, the minimum time in which
   a 32 bit counter will wrap decreases.  For example, a 10Mbs stream
   of back-to-back, full-size packets causes ifInOctets to wrap in
   just over 57 minutes; at 100Mbs, the minimum wrap time is 5.7
   minutes, and at 1Gbs, the minimum is 34 seconds.  Requiring that
   interfaces be polled frequently enough not to miss a counter wrap
   is increasingly problematic.

Top      ToC       Page 15 
   A rejected solution to this problem was to scale the counters; for
   example, ifInOctets could be changed to count received octets in,
   say, 1024 byte blocks.  While it would provide acceptable
   functionality at high rates of the counted-events, at low rates it
   suffers.  If there is little traffic on an interface, there might
   be a significant interval before enough of the counted-events
   occur to cause the scaled counter to be incremented.  Traffic
   would then appear to be very bursty, leading to incorrect
   conclusions of the network's performance.

   Instead, this memo adopts expanded, 64 bit, counters.  These
   counters are provided in new "high capacity" groups.  The old,
   32-bit, counters have not been deprecated.  The 64-bit counters
   are to be used only when the 32-bit counters do not provide enough
   capacity; that is, when the 32 bit counters could wrap too fast.

   For interfaces that operate at 20,000,000 (20 million) bits per
   second or less, 32-bit byte and packet counters MUST be used.  For
   interfaces that operate faster than 20,000,000 bits/second, and
   slower than 650,000,000 bits/second, 32-bit packet counters MUST
   be used and 64-bit octet counters MUST be used.  For interfaces
   that operate at 650,000,000 bits/second or faster, 64-bit packet
   counters AND 64-bit octet counters MUST be used.

   These speed thresholds were chosen as reasonable compromises based
   on the following:

   (1)  The cost of maintaining 64-bit counters is relatively high,
        so minimizing the number of agents which must support them is
        desirable.  Common interfaces (such as 10Mbs Ethernet) should
        not require them.

   (2)  64-bit counters are a new feature, introduced in SNMPv2.  It
        is reasonable to expect that support for them will be spotty
        for the immediate future.  Thus, we wish to limit them to as
        few systems as possible.  This, in effect, means that 64-bit
        counters should be limited to higher speed interfaces.
        Ethernet (10,000,000 bps) and Token Ring (16,000,000 bps) are
        fairly wide-spread so it seems reasonable to not require 64-
        bit counters for these interfaces.

   (3)  The 32-bit octet counters will wrap in the following times,
        for the following interfaces (when transmitting maximum-sized
        packets back-to-back):

        -   10Mbs Ethernet: 57 minutes,

        -   16Mbs Token Ring: 36 minutes,

Top      ToC       Page 16 
        -   a US T3 line (45 megabits): 12 minutes,

        -   FDDI: 5.7 minutes

   (4)  The 32-bit packet counters wrap in about 57 minutes when 64-
        byte packets are transmitted back-to-back on a 650,000,000
        bit/second link.

   As an aside, a 1-terabit/second (1,000 Gbs) link will cause a 64 bit
   octet counter to wrap in just under 5 years.  Conversely, an
   81,000,000 terabit/second link is required to cause a 64-bit counter
   to wrap in 30 minutes.  We believe that, while technology rapidly
   marches forward, this link speed will not be achieved for at least
   several years, leaving sufficient time to evaluate the introduction
   of 96 bit counters.

   When 64-bit counters are in use, the 32-bit counters MUST still be
   available.  They will report the low 32-bits of the associated 64-bit
   count (e.g., ifInOctets will report the least significant 32 bits of
   ifHCInOctets).  This enhances inter-operability with existing
   implementations at a very minimal cost to agents.

   The new "high capacity" groups are:

   (1)  the ifHCFixedLengthGroup for character-oriented/fixed-length
        interfaces, and the ifHCPacketGroup for packet-based interfaces;
        both of these groups include 64 bit counters for octets, and

   (2)  the ifVHCPacketGroup for packet-based interfaces; this group
        includes 64 bit counters for octets and packets.

3.1.7.  Interface Speed

   Network speeds are increasing.  The range of ifSpeed is limited to
   reporting a maximum speed of (2**31)-1 bits/second, or approximately
   2.2Gbs.  SONET defines an OC-48 interface, which is defined at
   operating at 48 times 51 Mbs, which is a speed in excess of 2.4Gbs.
   Thus, ifSpeed is insufficient for the future, and this memo defines
   an additional object: ifHighSpeed.

   The ifHighSpeed object reports the speed of the interface in
   1,000,000 (1 million) bits/second units.  Thus, the true speed of the
   interface will be the value reported by this object, plus or minus
   500,000 bits/second.

Top      ToC       Page 17 
   Other alternatives considered (but rejected) were:

   (1)  Making the interface speed a 64-bit gauge.  This was rejected
        since the current SMI does not allow such a syntax.

        Furthermore, even if 64-bit gauges were available, their use
        would require additional complexity in agents due to an
        increased requirement for 64-bit operations.

   (2)  We also considered making "high-32 bit" and "low-32-bit"
        objects which, when combined, would be a 64-bit value.  This
        simply seemed overly complex for what we are trying to do.

        Furthermore, a full 64-bits of precision does not seem
        necessary.  The value of ifHighSpeed will be the only report of
        interface speed for interfaces that are faster than
        4,294,967,295 bits per second.  At this speed, the granularity
        of ifHighSpeed will be 1,000,000 bits per second, thus the error
        will be 1/4294, or about 0.02%.  This seems reasonable.

   (3)  Adding a "scale" object, which would define the units which
        ifSpeed's value is.

        This would require two additional objects; one for the scaling
        object, and one to replace the current ifSpeed.  This later
        object is required since the semantics of ifSpeed would be
        significantly altered, and manager stations which do not
        understand the new semantics would be confused.

3.1.8.  Multicast/Broadcast Counters

   In MIB-II, the ifTable counters for multicast and broadcast packets
   are combined as counters of non-unicast packets.  In contrast, the
   ifExtensions MIB [7] defined one set of counters for multicast, and a
   separate set for broadcast packets.  With the separate counters, the
   original combined counters become redundant.  To avoid this
   redundancy, the non-unicast counters are deprecated.

   For the output broadcast and multicast counters defined in RFC 1229,
   their definitions varied slightly from the packet counters in the
   ifTable, in that they did not count errors/discarded packets.  Thus,
   this memo defines new objects with better aligned definitions.
   Counters with 64 bits of range are also needed, as explained above.

Top      ToC       Page 18 
3.1.9.  Trap Enable

   In the multi-layer interface model, each sub-layer for which there is
   an entry in the ifTable can generate linkUp/Down Traps.  Since
   interface state changes would tend to propagate through the interface
   (from top to bottom, or bottom to top), it is likely that several
   traps would be generated for each linkUp/Down occurrence.

   It is desirable to provide a mechanism for manager stations to
   control the generation of these traps.  To this end, the
   ifLinkUpDownTrapEnable object has been added.  This object allows
   managers to limit generation of traps to just the sub-layers of

   The default setting should limit the number of traps generated to one
   per interface per linkUp/Down event.  Furthermore, it seems that the
   state changes of most interest to network managers occur at the
   lowest level of an interface stack.  Therefore we specify that by
   default, only the lowest sub-layer of the interface generate traps.

3.1.10.  Addition of New ifType values

   Over time, there is the need to add new ifType enumerated values for
   new interface types.  If the syntax of ifType were defined in the MIB
   in section 6, then a new version of this MIB would have to be re-
   issued in order to define new values.  In the past, re- issuing of a
   MIB has occurred only after several years.

   Therefore, the syntax of ifType is changed to be a textual
   convention, such that the enumerated integer values are now defined
   in the textual convention, IANAifType, defined in a different
   document.  This allows additional values to be documented without
   having to re-issue a new version of this document.  The Internet
   Assigned Number Authority (IANA) is responsible for the assignment of
   all Internet numbers, including various SNMP-related numbers, and
   specifically, new ifType values.

3.1.11.  InterfaceIndex Textual Convention

   A new textual convention, InterfaceIndex, has been defined.  This
   textual convention "contains" all of the semantics of the ifIndex
   object.  This allows other mib modules to easily import the semantics
   of ifIndex.

Top      ToC       Page 19 
3.1.12.  New states for IfOperStatus

   Three new states have been added to ifOperStatus: 'dormant', 
   'notPresent', and 'lowerLayerDown'.

   The dormant state indicates that the relevant interface is not
   actually in a condition to pass packets (i.e., it is not "up") but is
   in a "pending" state, waiting for some external event.  For "on-
   demand" interfaces, this new state identifies the situation where the
   interface is waiting for events to place it in the up state.
   Examples of such events might be:

   (1)  having packets to transmit before establishing a connection
        to a remote system;

   (2)  having a remote system establish a connection to the
        interface (e.g. dialing up to a slip-server).

   The notPresent state is a refinement on the down state which
   indicates that the relevant interface is down specifically because
   some component (typically, a hardware component) is not present in
   the managed system.  Examples of use of the notPresent state are:

   (1)  to allow an interface's conceptual row including its counter
        values to be retained across a "hot swap" of a card/module,

   (2)  to allow an interface's conceptual row to be created, and
        thereby enable interfaces to be pre-configured prior to
        installation of the hardware needed to make the interface

   Agents are not required to support interfaces in the notPresent
   state.  However, from a conceptual viewpoint, when a row in the
   ifTable is created, it first enters the notPresent state and then
   subsequently transitions into the down state; similarly, when a row
   in the ifTable is deleted, it first enters the notPresent state and
   then subsequently the object instances are deleted.  For an agent
   with no support for notPresent, both of these transitions (from the
   notPresent state to the down state, and from the notPresent state to
   the instances being removed) are immediate, i.e., the transition does
   not last long enough to be recorded by ifOperStatus.  Even for those
   agents which do support interfaces in the notPresent state, the
   length of time and conditions under which an interface stays in the
   notPresent state is implementation-specific.

Top      ToC       Page 20 
   The lowerLayerDown state is also a refinement on the down state.
   This new state indicates that this interface runs "on top of" one or
   more other interfaces (see ifStackTable) and that this interface is
   down specifically because one or more of these lower-layer interfaces
   are down.

3.1.13.  IfAdminStatus and IfOperStatus

   The down state of ifOperStatus now has two meanings, depending on the
   value of ifAdminStatus.

   (1)  if ifAdminStatus is not down and ifOperStatus is down then a
        fault condition is presumed to exist on the interface.

   (2)  if ifAdminStatus is down, then ifOperStatus will normally
        also be down (or notPresent) i.e., there is not (necessarily) a
        fault condition on the interface.

   Note that when ifAdminStatus transitions to down, ifOperStatus will
   normally also transition to down.  In this situation, it is possible
   that ifOperStatus's transition will not occur immediately, but rather
   after a small time lag to complete certain operations before going
   "down"; for example, it might need to finish transmitting a packet.
   If a manager station finds that ifAdminStatus is down and
   ifOperStatus is not down for a particular interface, the manager
   station should wait a short while and check again.  If the condition
   still exists, only then should it raise an error indication.
   Naturally, it should also ensure that ifLastChange has not changed
   during this interval.

   Whenever an interface table entry is created (usually as a result of
   system initialization), the relevant instance of ifAdminStatus is set
   to down, and presumably ifOperStatus will be down or notPresent.

   An interface may be enabled in two ways: either as a result of
   explicit management action (e.g. setting ifAdminStatus to up) or as a
   result of the managed system's initialization process.  When
   ifAdminStatus changes to the up state, the related ifOperStatus
   should do one of the following:

   (1)  Change to the up state if and only if the interface is able
        to send and receive packets.

   (2)  Change to the lowerLayerDown state if and only if the
        interface is prevented from entering the up state because of the
        state of one or more of the interfaces beneath it in the
        interface stack.

Top      ToC       Page 21 
   (3)  Change to the dormant state if and only if the interface is
        found to be operable, but the interface is waiting for other,
        external, events to occur before it can transmit or receive
        packets.  Presumably when the expected events occur, the
        interface will then change to the up state.

   (4)  Remain in the down state if an error or other fault condition
        is detected on the interface.

   (5)  Change to the unknown state if, for some reason, the state of
        the interface can not be ascertained.

   (6)  Change to the testing state if some test(s) must be performed
        on the interface. Presumably after completion of the test, the
        interface's state will change to up, dormant, or down, as

   (7)  Remain in the notPresent state if interface components are

3.1.14.  IfOperStatus in an Interface Stack

   When an interface is a part of an interface-stack, but is not the
   lowest interface in the stack, then:

   (1)  ifOperStatus has the value 'up' if it is able to pass packets
        due to one or more interfaces below it in the stack being 'up',
        irrespective of whether other interfaces below it are 'down', 
        'dormant', 'notPresent', 'lowerLayerDown', 'unknown' or

   (2)  ifOperStatus may have the value 'up' or 'dormant' if one or
        more interfaces below it in the stack are 'dormant', and all
        others below it are either 'down', 'dormant', 'notPresent',
        'lowerLayerDown', 'unknown' or 'testing'.

   (3)  ifOperStatus has the value 'lowerLayerDown' while all
        interfaces below it in the stack are either 'down',
        'notPresent', 'lowerLayerDown', or 'testing'.

3.1.15.  Traps

   The exact definition of when linkUp and linkDown traps are generated
   has been changed to reflect the changes to ifAdminStatus and

Top      ToC       Page 22 
   Operational experience indicates that management stations are most
   concerned with an interface being in the down state and the fact that
   this state may indicate a failure.  Thus, it is most useful to
   instrument transitions into/out of either the up state or the down

   Instrumenting transitions into or out of the up state was rejected
   since it would have the drawback that a demand interface might have
   many transitions between up and dormant, leading to many linkUp traps
   and no linkDown traps.  Furthermore, if a node's only interface is
   the demand interface, then a transition to dormant would entail
   generation of a linkDown trap, necessitating bringing the link to the
   up state (and a linkUp trap)!!

   On the other hand, instrumenting transitions into or out of the down
   state (to/from all other states except notPresent) has the

   (1)  A transition into the down state (from a state other than
        notPresent) will occur when an error is detected on an
        interface.  Error conditions are presumably of great interest to
        network managers.

   (2)  Departing the down state (to a state other than the
        notPresent state) generally indicates that the interface is
        going to either up or dormant, both of which are considered
        "healthy" states.

   Furthermore, it is believed that generating traps on transitions into
   or out of the down state (except to/from the notPresent state) is
   generally consistent with current usage and interpretation of these
   traps by manager stations.

   Transitions to/from the notPresent state are concerned with the
   insertion and removal of hardware, and are outside the scope of these

   Therefore, this memo defines that LinkUp and linkDown traps are
   generated on just after ifOperStatus leaves, or just before it
   enters, the down state, respectively; except that LinkUp and linkDown
   traps never generated on transitions to/from the notPresent state.

   Note that this definition allows a node with only one interface to
   transmit a linkDown trap before that interface goes down.  (Of
   course, when the interface is going down because of a failure
   condition, the linkDown trap probably cannot be successfully
   transmitted anyway.)

Top      ToC       Page 23 
   Some interfaces perform a link "training" function when trying to
   bring the interface up.  In the event that such an interface were
   defective, then the training function would fail and the interface
   would remain down, and the training function might be repeated at
   appropriate intervals.  If the interface, while performing this
   training function, were considered to the in the testing state, then
   linkUp and linkDown traps would be generated for each start and end
   of the training function.  This is not the intent of the linkUp and
   linkDown traps, and therefore, while performing such a training
   function, the interface's state should be represented as down.

   An exception to the above generation of linkUp/linkDown traps on
   changes in ifOperStatus, occurs when an interface is "flapping",
   i.e., when it is rapidly oscillating between the up and down states.
   If traps were generated for each such oscillation, the network and
   the network management system would be flooded with unnecessary
   traps.  In such a situation, the agent should rate- limit its
   generation of traps.

3.1.16.  ifSpecific

   The original definition of the OBJECT IDENTIFIER value of ifSpecific
   was not sufficiently clear.  As a result, different implementors used
   it differently, and confusion resulted.  Some implementations set the
   value of ifSpecific to the OBJECT IDENTIFIER that defines the media-
   specific MIB, i.e., the "foo" of:

              foo OBJECT IDENTIFIER ::= { transmission xxx }

   while others set it to be OBJECT IDENTIFIER of the specific table or
   entry in the appropriate media-specific MIB (i.e., fooTable or
   fooEntry), while still others set it be the OBJECT IDENTIFIER of the
   index object of the table's row, including instance identifier,
   (i.e., fooIfIndex.ifIndex).  A definition based on the latter would
   not be sufficient unless it also allowed for media- specific MIBs
   which include several tables, where each table has its own
   (different) indexing.

   The only definition that can both be made explicit and can cover all
   the useful situations is to have ifSpecific be the most general value
   for the media-specific MIB module (the first example given above).
   This effectively makes it redundant because it contains no more
   information than is provided by ifType.  Thus, ifSpecific has been

Top      ToC       Page 24 
3.1.17.  Creation/Deletion of Interfaces

   While some interfaces, for example, most physical interfaces, cannot
   be created via network management, other interfaces such as logical
   interfaces sometimes can be.  The ifTable contains only generic
   information about an interface.  Almost all 'create-able' interfaces
   have other, media-specific, information through which configuration
   parameters may be supplied prior to creating such an interface.
   Thus, the ifTable does not itself support the creation or deletion of
   an interface (specifically, it has no RowStatus [2] column).  Rather,
   if a particular interface type supports the dynamic creation and/or
   deletion of an interface of that type, then that media-specific MIB
   should include an appropriate RowStatus object (see the ATM LAN-
   Emulation Client MIB [8] for an example of a MIB which does this).
   Typically, when such a RowStatus object is created/deleted, then the
   conceptual row in the ifTable appears/disappears as a by-product, and
   an ifIndex value (chosen by the agent) is stored in an appropriate
   object in the media-specific MIB.

3.1.18.  All Values Must be Known

   There are a number of situations where an agent does not know the
   value of one or more objects for a particular interface.  In all such
   circumstances, an agent MUST NOT instantiate an object with an
   incorrect value; rather, it MUST respond with the appropriate
   error/exception condition (e.g., noSuchInstance for SNMPv2).

   One example is where an agent is unable to count the occurrences
   defined by one (or more) of the ifTable counters.  In this
   circumstance, the agent MUST NOT instantiate the particular counter
   with a value of, say, zero.  To do so would be to provide mis-
   information to a network management application reading the zero
   value, and thereby assuming that there have been no occurrences of
   the event (e.g., no input errors because ifInErrors is always zero).

   Sometimes the lack of knowledge of an object's value is temporary.
   For example, when the MTU of an interface is a configured value and a
   device dynamically learns the configured value through (after)
   exchanging messages over the interface (e.g., ATM LAN- Emulation
   [8]).  In such a case, the value is not known until after the ifTable
   entry has already been created.  In such a case, the ifTable entry
   should be created without an instance of the object whose value is
   unknown; later, when the value becomes known, the missing object can
   then be instantiated (e.g., the instance of ifMtu is only
   instantiated once the interface's MTU becomes known).

Top      ToC       Page 25 
   As a result of this "known values" rule, management applications MUST
   be able to cope with the responses to retrieving the object instances
   within a conceptual row of the ifTable revealing that some of the
   row's columnar objects are missing/not available.

4.  Media-Specific MIB Applicability

   The exact use and semantics of many objects in this MIB are open to
   some interpretation.  This is a result of the generic nature of this
   MIB.  It is not always possible to come up with specific,
   unambiguous, text that covers all cases and yet preserves the generic
   nature of the MIB.

   Therefore, it is incumbent upon a media-specific MIB designer to,
   wherever necessary, clarify the use of the objects in this MIB with
   respect to the media-specific MIB.

   Specific areas of clarification include

   Layering Model
        The media-specific MIB designer MUST completely and
        unambiguously specify the layering model used.  Each individual
        sub-layer must be identified, as must the ifStackTable's
        portrayal of the relationship(s) between the sub-layers.

   Virtual Circuits
        The media-specific MIB designer MUST specify whether virtual
        circuits are assigned entries in the ifTable or not.  If they
        are, compelling rationale must be presented.

        The media-specific MIB designer MUST specify the applicability
        of the ifRcvAddressTable.

        For each of the ifType values to which the media-specific MIB
        applies, it must specify the mapping of ifType values to media-
        specific MIB module(s) and instances of MIB objects within those

   However, wherever this interface MIB is specific in the semantics,
   DESCRIPTION, or applicability of objects, the media-specific MIB
   designer MUST NOT change said semantics, DESCRIPTION, or

Top      ToC       Page 26 
5.  Overview

   This MIB consists of 4 tables:

        This table is the ifTable from MIB-II.

        This table contains objects that have been added to the
        Interface MIB as a result of the Interface Evolution effort, or
        replacements for objects of the original (MIB-II) ifTable that
        were deprecated because the semantics of said objects have
        significantly changed.  This table also contains objects that
        were previously in the ifExtnsTable.

        This table contains objects that define the relationships among
        the sub-layers of an interface.

        This table contains objects that are used to define the media-
        level addresses which this interface will receive.  This table
        is a generic table.  The designers of media- specific MIBs must
        define exactly how this table applies to their specific MIB.

(page 26 continued on part 2)

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