Network Working Group K. McCloghrie Request for Comments: 2863 Cisco Systems Obsoletes: 2233 F. Kastenholz Category: Standards Track Argon Networks June 2000 The Interfaces Group MIB 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 (2000). All Rights Reserved. 1 Introduction ................................................. 2 2 The SNMP Network Management Framework ........................ 2 3 Experience with the Interfaces Group ......................... 3 3.1 Clarifications/Revisions ................................... 4 3.1.1 Interface Sub-Layers ..................................... 4 3.1.2 Guidance on Defining Sub-layers .......................... 7 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 .............................................. 17 3.1.10 Addition of New ifType values ........................... 18 3.1.11 InterfaceIndex Textual Convention ....................... 18 3.1.12 New states for IfOperStatus ............................. 18 3.1.13 IfAdminStatus and IfOperStatus .......................... 19 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 ......................... 23 3.1.18 All Values Must be Known ................................ 24 4 Media-Specific MIB Applicability ............................. 24 5 Overview ..................................................... 25 6 Interfaces Group Definitions ................................. 26
7 Acknowledgements ............................................. 64 8 References ................................................... 64 9 Security Considerations ...................................... 66 10 Authors' Addresses .......................................... 67 11 Changes from RFC 2233 ....................................... 67 12 Notice on Intellectual Property ............................. 68 13 Full Copyright Statement .................................... 69 17], 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 MIB-II model of the ' interfaces' group. This memo obsoletes RFC 2233, the previous version of the Interfaces Group MIB. The key words "MUST" and "MUST NOT" in this document are to be interpreted as described in RFC 2119 . RFC 2571 . o Mechanisms for describing and naming objects and events for the purpose of management. The first version of this Structure of Management Information (SMI) is called SMIv1 and described in STD 16, RFC 1155 , STD 16, RFC 1212  and RFC 1215 . The second version, called SMIv2, is described in STD 58, which consists of RFC 2578 , RFC 2579  and RFC 2580 . o Message protocols for transferring management information. The first version of the SNMP message protocol is called SNMPv1 and described in STD 15, RFC 1157 . A second version of the SNMP message protocol, which is not an Internet standards track protocol, is called SNMPv2c and described in RFC 1901  and RFC 1906 . The third version of the message protocol is called SNMPv3 and described in RFC 1906 , RFC 2572  and RFC 2574 .
o Protocol operations for accessing management information. The first set of protocol operations and associated PDU formats is described in STD 15, RFC 1157 . A second set of protocol operations and associated PDU formats is described in RFC 1905 . o A set of fundamental applications described in RFC 2573  and the view-based access control mechanism described in RFC 2575 . A more detailed introduction to the current SNMP Management Framework can be found in RFC 2570 . Managed objects are accessed via a virtual information store, termed the Management Information Base or MIB. Objects in the MIB are defined using the mechanisms defined in the SMI. This memo specifies a MIB module that is compliant to the SMIv2. A MIB conforming to the SMIv1 can be produced through the appropriate translations. The resulting translated MIB must be semantically equivalent, except where objects or events are omitted because no translation is possible (e.g., use of Counter64). Some machine readable information in SMIv2 will be converted into textual descriptions in SMIv1 during the translation process. However, this loss of machine readable information is not considered to change the semantics of the MIB. 18] 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
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 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: (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 interface. (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 interface. 21].
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. 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 specification.
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 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 SMIv2 (in contrast to SMIv1) refer to object groups, where object groups are explicitly defined by listing the objects they contain. Thus, with SMIv2, 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 interface: (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 interfaces. 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.
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 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 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 added/removed. 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 turn out to be inadequate. Instead, implementors must choose what it means in their particular situation, subject to the following rules:
(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 value. 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. 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 re-initialization of the agent (such as a reboot). 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 interfaces) 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.
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 supported. 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 supported and 64-bit octet counters MUST be supported. For interfaces that operate at 650,000,000 bits/second or faster, 64-bit packet counters AND 64-bit octet counters MUST be supported. 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 the SMIv2. 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, - 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.
(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. 19] 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.
The default setting should limit the number of traps generated to one per interface per linkUp/linkDown 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. 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.
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, and/or (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 operational. 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. 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.
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 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. (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 appropriate. (7) Remain in the notPresent state if interface components are missing.
(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 traps. Therefore, this memo defines that LinkUp and linkDown traps are generated just after ifOperStatus leaves, or just before it enters, the down state, respectively; except that LinkUp and linkDown traps are never generated on transitions to/from the notPresent state. For the purpose of deciding when these traps occur, the lowerLayerDown state and the down state are considered to be equivalent, i.e., there is no trap on transition from lowerLayerDown into down, and there is a trap on transition from any other state except down (and notPresent) into lowerLayerDown. 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.) 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 limit the rate at which it generates traps.
6] 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  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.
20]). 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). 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.
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. ifRcvAddressTable The media-specific MIB designer MUST specify the applicability of the ifRcvAddressTable. ifType 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 modules. ifXxxOctets The definitions of ifInOctets and ifOutOctets (and similarly, ifHCInOctets and ifHCOutOctets) specify that their values include framing characters. The media-specific MIB designer MUST specify any special conditions of the media concerning the inclusion of framing characters, especially with respect to frames with errors. 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 applicability.
because the semantics of said objects have significantly changed. This table also contains objects that were previously in the ifExtnsTable. ifStackTable This table contains objects that define the relationships among the sub-layers of an interface. ifRcvAddressTable 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.