tech-invite   World Map     

3GPP     Specs     Glossaries     Architecture     IMS     UICC       IETF     RFCs     Groups     SIP     ABNFs       Search

RFC 2642

Informational
Pages: 95
Top     in Index     Prev     Next
in Group Index     Prev in Group     Next in Group     Group: ~cabletron

Cabletron's VLS Protocol Specification

Part 1 of 4, p. 1 to 22
None       Next RFC Part

 


Top       ToC       Page 1 
Network Working Group                                            L. Kane
Request for Comments: 2642                Cabletron Systems Incorporated
Category: Informational                                      August 1999


                 Cabletron's VLS Protocol Specification

Status of this Memo

   This memo provides information for the Internet community.  It does
   not specify an Internet standard of any kind.  Distribution of this
   memo is unlimited.

Copyright Notice

   Copyright (C) The Internet Society (1999).  All Rights Reserved.

Abstract

   The Virtual LAN Link State Protocol (VLSP) is part of the InterSwitch
   Message Protocol (ISMP) which provides interswitch communication
   between switches running Cabletron's SecureFast VLAN (SFVLAN)
   product.  VLSP is used to determine and maintain a fully connected
   mesh topology graph of the switch fabric.  Each switch maintains an
   identical database describing the topology. Call-originating switches
   use the topology database to determine the path over which to route a
   call connection.

   VLSP provides support for equal-cost multipath routing, and
   recalculates routes quickly in the face of topological changes,
   utilizing a minimum of routing protocol traffic.

Table of Contents

    1. Introduction............................................  3
       1.1 Acknowledgments.....................................  3
       1.2 Data Conventions....................................  3
       1.3 ISMP Overview.......................................  4
    2. VLS Protocol Overview...................................  5
       2.1 Definitions of Commonly Used Terms..................  6
       2.2 Differences Between VLSP and OSPF...................  7
           2.2.1 Operation at the Physical Layer...............  8
           2.2.2 All Links Treated as Point-to-Point...........  8
           2.2.3 Routing Path Information......................  9
           2.2.4 Configurable Parameters.......................  9
           2.2.5 Features Not supported........................  9
       2.3 Functional Summary.................................. 10
       2.4 Protocol Packets.................................... 11

Top      ToC       Page 2 
       2.5 Protocol Data Structures............................ 12
       2.6 Basic Implementation Requirements................... 12
       2.7 Organization of the Remainder of This Document...... 13
    3. Interface Data Structure................................ 14
       3.1 Interface States.................................... 16
       3.2 Events Causing Interface State Changes.............. 18
       3.3 Interface State Machine............................. 21
    4. Neighbor Data Structure................................. 23
       4.1 Neighbor States..................................... 25
       4.2 Events Causing Neighbor State Changes............... 27
       4.3 Neighbor State Machine.............................. 29
    5. Area Data Structure..................................... 33
       5.1 Adding and Deleting Link State Advertisements....... 34
       5.2 Accessing Link State Advertisements................. 35
       5.3 Best Path Lookup.................................... 35
    6. Discovery Process....................................... 35
       6.1 Neighbor Discovery.................................. 36
       6.2 Bidirectional Communication......................... 37
       6.3 Designated Switch................................... 38
           6.3.1 Selecting the Designated Switch............... 39
       6.4 Adjacencies......................................... 41
    7. Synchronizing the Databases............................. 42
       7.1 Link State Advertisements........................... 43
           7.1.1 Determining Which
                 Link State Advertisement Is Newer............. 44
       7.2 Database Exchange Process........................... 44
           7.2.1 Database Description Packets.................. 44
           7.2.2 Negotiating the Master/Slave Relationship..... 45
           7.2.3 Exchanging Database Description Packets....... 46
       7.3 Updating the Database............................... 48
       7.4 An Example.......................................... 49
    8. Maintaining the Databases............................... 51
       8.1 Originating Link State Advertisements............... 52
           8.1.1 Switch Link Advertisements.................... 52
           8.1.2 Network Link Advertisements................... 55
       8.2 Distributing Link State Advertisements.............. 56
           8.2.1 Overview...................................... 57
           8.2.2 Processing an
                 Incoming Link State Update Packet............. 58
           8.2.3 Forwarding Link State Advertisements.......... 60
           8.2.4 Installing Link
                 State Advertisements in the Database.......... 62
           8.2.5 Retransmitting Link State Advertisements...... 63
           8.2.6 Acknowledging Link State Advertisements....... 64
       8.3 Aging the Link State Database....................... 66
           8.3.1 Premature Aging of Advertisements............. 66
    9. Calculating the Best Paths.............................. 67
   10. Protocol Packets........................................ 67

Top      ToC       Page 3 
       10.1 ISMP Packet Format................................. 68
            10.1.1 Frame Header................................ 69
            10.1.2 ISMP Packet Header.......................... 70
            10.1.3 ISMP Message Body........................... 71
       10.2 VLSP Packet Processing............................. 71
       10.3 Network Layer Address Information.................. 72
       10.4 VLSP Packet Header................................. 73
       10.5 Options Field...................................... 75
       10.6 Packet Formats..................................... 76
            10.6.1 Hello Packets............................... 76
            10.6.2 Database Description Packets................ 78
            10.6.3 Link State Request Packets.................. 80
            10.6.4 Link State Update Packets................... 82
            10.6.5 Link State Acknowledgment Packets........... 83
   11. Link State Advertisement Formats........................ 84
       11.1 Link State Advertisement Headers................... 84
       11.2 Switch Link Advertisements......................... 86
       11.3 Network Link Advertisements........................ 89
   12. Protocol Parameters..................................... 89
       12.1 Architectural Constants............................ 90
       12.2 Configurable Parameters............................ 91
   13. End Notes............................................... 93
   14. Security Considerations................................. 94
   15. References.............................................. 94
   16. Author's Address........................................ 94
   17. Full Copyright Statement................................ 95

1. Introduction

   This memo is being distributed to members of the Internet community
   in order to solicit reactions to the proposals contained herein.
   While the specification discussed here may not be directly relevant
   to the research problems of the Internet, it may be of interest to
   researchers and implementers.

1.1 Acknowledgments

   VLSP is derived from the OSPF link-state routing protocol described
   in [RFC2328], written by John Moy, formerly of Proteon, Inc.,
   Westborough, Massachusetts.  Much of the current memo has been drawn
   from [RFC2328].  Therefore, this author wishes to acknowledge the
   contribution Mr. Moy has (unknowingly) made to this document.

1.2 Data Conventions

   The methods used in this memo to describe and picture data adhere to
   the standards of Internet Protocol documentation [RFC1700].  In
   particular:

Top      ToC       Page 4 
      The convention in the documentation of Internet Protocols is to
      express numbers in decimal and to picture data in "big-endian"
      order.  That is, fields are described left to right, with the most
      significant octet on the left and the least significant octet on
      the right.  The order of transmission of the header and data
      described in this document is resolved to the octet level.
      Whenever a diagram shows a group of octets, the order of
      transmission of those octets is the normal order in which they are
      read in English.

      Whenever an octet represents a numeric quantity the left most bit
      in the diagram is the high order or most significant bit.  That
      is, the bit labeled 0 is the most significant bit.

      Similarly, whenever a multi-octet field represents a numeric
      quantity the left most bit of the whole field is the most
      significant bit.  When a multi-octet quantity is transmitted the
      most significant octet is transmitted first.

1.3 ISMP Overview

   The InterSwitch Message Protocol (ISMP) provides a consistent method
   of encapsulating and transmitting control messages exchanged between
   switches running Cabletron's SecureFast VLAN (SFVLAN) product, as
   described in [IDsfvlan].  ISMP provides the following services:

   o  Topology services.  Each switch maintains a distributed topology
      of the switch fabric by exchanging the following interswitch
      control messages with other switches:

   o  Interswitch Keepalive messages are sent by each switch to announce
      its existence to its neighboring switches and to establish the
      topology of the switch fabric.  (Interswitch Keepalive messages
      are exchanged in accordance with Cabletron's VlanHello protocol,
      described in [IDhello].)

   o  Interswitch Spanning Tree BPDU messages and Interswitch Remote
      Blocking messages are used to determine and maintain a loop-free
      flood path between all network switches in the fabric.  This flood

      path is used for all undirected interswitch messages -- that is,
      messages that are (potentially) sent to all switches in the switch
      fabric.

   o  Interswitch Link State messages (VLS protocol) are used to
      determine and maintain a fully connected mesh topology graph of
      the switch fabric.  Call-originating switches use the topology
      graph to determine the path over which to route a call connection.

Top      ToC       Page 5 
   o  Address resolution services.  Interswitch Resolve messages are
      used to resolve a packet destination address when the packet
      source and destination pair does not match a known connection.
      Interswitch New User messages are used to provide end-station
      address mobility between switches.

   o  Tag-based flooding.  A tag-based broadcast method is used to
      restrict the broadcast of unresolved packets to only those ports
      within the fabric that belong to the same VLAN as the source.

   o  Call tapping services.  Interswitch Tap messages are used to
      monitor traffic moving between two end stations.  Traffic can be
      monitored in one or both directions along the connection path.

   Note:  Previous versions of VLSP treated all links as if they were
   broadcast (multi-access).  Thus, if VLSP determines that a neighbor
   switch is running an older version of the protocol software (see
   Section 6.1), it will change the interface type to broadcast and
   begin exchanging Hello packets with the single neighbor switch.

2. VLS Protocol Overview

   VLSP is a dynamic routing protocol.  It quickly detects topological
   changes in the switch fabric (such as, switch interface failures) and
   calculates new loop-free routes after a period of convergence.  This
   period of convergence is short and involves a minimum of routing
   traffic.

   All switches in the fabric run the same algorithm and maintain
   identical databases describing the switch fabric topology.  This
   database contains each switch's local state, including its usable
   interfaces and reachable neighbors.  Each switch distributes its
   local state throughout the switch fabric by flooding.  From the
   topological database, each switch constructs a set of best path trees
   (using itself as the root) that specify routes to all other switches
   in the fabric.

Top      ToC       Page 6 
2.1 Definitions of Commonly Used Terms

   This section contains a collection of definitions for terms that have
   a specific meaning to the protocol and that are used throughout the
   text.

   Switch ID

      A 10-octet value that uniquely identifies the switch within the
      switch fabric.  The value consists of the 6-octet base MAC address
      of the switch, followed by 4 octets of zeroes.

   Network link

      The physical connection between two switches.  A link is
      associated with a switch interface.

      There are two physical types of network links supported by VLSP:

      o  Point-to-point links that join a single pair of switches.  A
         serial line is an example of a point-to-point network link.

      o  Multi-access broadcast links that support the attachment of
         multiple switches, along with the capability to address a
         single message to all the attached switches.  An attached
         ethernet is an example of a multi-access broadcast network
         link.

         A single topology can contain both types of links.  At startup,
         all links are assumed to be point-to-point.  A link is
         determined to be multi-access when more than one neighboring
         switch is discovered on the link.

   Interface

      The port over which a switch accesses one of its links.
      Interfaces are identified by their interface ID, a 10-octet value
      consisting of the 6-octet base MAC address of the switch, followed
      by the 4-octet local port number of the interface.

   Neighboring switches

      Two switches attached to a common link.

Top      ToC       Page 7 
   Adjacency

      A relationship formed between selected neighboring switches for
      the purpose of exchanging routing information.  Not every pair of
      neighboring switches become adjacent.

   Link state advertisement

      Describes the local state of a switch or a link.  Each link state
      advertisement is flooded throughout the switch fabric.  The
      collected link state advertisements of all switches and links form
      the protocol's topological database.

   Designated switch

      Each multi-access network link has a designated switch.  The
      designated switch generates a link state advertisement for the
      link and has other special responsibilities in the running of the
      protocol.

      The use of a designated switch permits a reduction in the number
      of adjacencies required on multi-access links.  This in turn
      reduces the amount of routing protocol traffic and the size of the
      topological database.

      The designated switch is selected during the discovery process.  A
      designated switch is not selected for a point-to-point network
      link.

   Backup designated switch

      Each multi-access network link has a backup designated switch.
      The backup designated switch maintains adjacencies with the same
      switches on the link as the designated switch.  This optimizes the
      failover time when the backup designated switch must take over for
      the (failed) designated switch.

      The backup designated switch is selected during the Discovery
      process.  A backup designated switch is not selected for a point-
      to-point network link.

2.2 Differences Between VLSP and OSPF

   The VLS protocol is derived from the OSPF link-state routing protocol
   described in [RFC2328].

Top      ToC       Page 8 
2.2.1 Operation at the Physical Layer

   The primary differences between the VLS and OSPF protocols stem from
   the fact that OSPF runs over the IP layer, while VLSP runs at the
   physical MAC layer.  This difference has the following repercussions:

   o  VLSP does not support features (such as fragmentation) that are
      typically provided by network layer service providers.

   o  Due to the unrelated nature of MAC address assignments, VLSP
      provides no summarization of the address space (such as, classical
      IP subnet information) or level 2 routing (such as,

      IS-IS Phase V DECnet).  Thus, VLSP does not support grouping
      switches into areas.  All switches exist in a single area.  Since
      a single domain exists within any switch fabric, there is no need
      for VLSP to provide interdomain reachability.

   o  As mentioned in Section 10.1.1, ISMP uses a single well-known
      multicast address for all packets.  However, parts of the VLS
      protocol (as derived from OSPF) are dependent on certain network
      layer addresses -- in particular, the AllSPFSwitches and
      AllDSwitches multicast addresses that drive the distribution of
      link state advertisements throughout the switch fabric.  In order
      to facilitate the implementation of the protocol at the physical
      MAC layer, network layer address information is encapsulated in
      the protocol packets (see Section 10.3).  This information is
      unbundled and packets are then processed as if they had been sent
      or received on that multicast address.

2.2.2 All Links Treated as Point-to-Point

   When the switch first comes on line, VLSP assumes all network links
   are point-to-point and no more than one neighboring switch will be
   discovered on any one port.  Therefore, at startup, VLSP does not
   send its own Hello packets over its network ports, but instead,
   relies on the VlanHello protocol [IDhello] for the discovery of its
   neighbor switches.  If a second neighbor is detected on a link, the
   link is then deemed multi-access and the interface type is changed to
   broadcast.  At that point, VLSP exchanges its own Hello packets with
   the switches on the link in order to select a designated switch and
   designated backup switch for the link.

   This method eliminates unnecessary duplication of message traffic and
   processing, thereby increasing the overall efficiency of the switch
   fabric.

Top      ToC       Page 9 
   Note:  Previous versions of VLSP treated all links as if they were
   broadcast (multi-access).  Thus, if VLSP determines that a neighbor
   switch is running an older version of the protocol software (see
   Section 6.1), it will change the interface type to broadcast and
   begin exchanging Hello packets with the single neighbor switch.

2.2.3 Routing Path Information

   Instead of providing the next hop to a destination, VLSP calculates
   and maintains complete end-to-end path information. On request, a
   list of individual port identifiers is generated describing a
   complete path from the source switch to the destination switch.  If
   multiple equal-cost routes exist to a destination switch, up to three
   paths are calculated and returned.

2.2.4 Configurable Parameters

   OSPF supports (and requires) configurable parameters.  In fact, even
   the default OSPF configuration requires that IP address assignments
   be specified.  On the other hand, no configuration information is
   ever required for the VLS protocol.  Switches are uniquely identified
   by their base MAC addresses and ports are uniquely identified by the
   base MAC address of the switch and a port number.

   While a developer is free to implement configurable parameters for
   the VLS protocol, the current version of VLSP supports configurable
   path metrics only.  Note that this has the following repercussions:

   o  All switches are assigned a switch priority of 1.  This forces the
      selection of the designated switch to be based solely on base MAC
      address.

   o  Authentication is not supported.

2.2.5 Features Not supported

   In addition to those features mentioned in the previous sections, the
   following OSPF features are not supported by the current version of
   VLSP:

   o  Periodic refresh of link state advertisements.  (This optimizes
      performance by eliminating unnecessary traffic between the
      switches.)

   o  Routing based on non-zero type of service (TOS).

   o  Use of external routing information for destinations outside the
      switch fabric.

Top      ToC       Page 10 
2.3 Functional Summary

   There are essentially four operational stages of the VLS protocol.

   o  Discovery Process The discovery process involves two steps:

      o  Neighboring switches are detected by the VlanHello protocol
         [IDhello] which then notifies VLSP of the neighbor.

      o  If more than one neighbor switch is detected on a single port,
         the link is determined to be multi-access.  VLSP then sends its
         own Hello packets over the link in order to discover the full
         set of neighbors on the link and select a designated switch and
         designated backup switch for the link.  Note that this
         selection process is unnecessary on point-to-point links.

      The discovery process is described in more detail in Section 6.

   o  Synchronizing the Databases

      Adjacencies are used to simplify and speed up the process of
      synchronizing the topological database (also known as the link
      state database) maintained by each switch in the fabric.  Each
      switch is only required to synchronize its database with those
      neighbors to which it is adjacent. This reduces the amount of
      routing protocol traffic across the fabric, particularly for
      multi-access links with multiple switches.

      The process of synchronizing the databases is described in more
      detail in Section 7.

   o  Maintaining the Databases

      Each switch advertises its state (also known as its link state)
      any time its link state changes.  Link state advertisements are
      distributed throughout the switch fabric using a reliable flooding
      algorithm that ensures that all switches in the fabric are
      notified of any link state changes.

      The process of maintaining the databases is described in more
      detail in Section 8.

Top      ToC       Page 11 
   o  Calculating the Best Paths

      The link state database consists of the collection of link state
      advertisements received from each switch.  Each switch uses its
      link state database to calculate a set of best paths, using itself
      as root, to all other switches in the fabric.

      The process of recalculating the set of best paths is described in
      more detail in Section 9.

2.4 Protocol Packets

   In addition to the frame header and the ISMP packet header described
   in Section 10.1, all VLS protocol packets share a common protocol
   header, described in Section 10.4.

   The VLSP packet types are listed below in Table 1.  Their formats are
   described in Section 10.6.

      Type   Packet Name            Protocol Function

      1      Hello                  Select DS and Backup DS
      2      Database Description   Summarize database contents
      3      Link State Request     Database download
      4      Link State Update      Database update
      5      Link State Ack         Flooding acknowledgment

                  Table 1: VLSP Packet Types

   The Hello packets are used to select the designated switch and the
   backup designated switch on multi-access links.  The Database
   Description and Link State Request packets are used to form
   adjacencies.  Link State Update and Link State Acknowledgment packets
   are used to update the topological database.

   Each Link State Update packet carries a set of link state
   advertisements.  A single Link State Update packet may contain the
   link state advertisements of several switches.  There are two
   different types of link state advertisement, as shown below in Table
   2.

Top      ToC       Page 12 
         LS     Advertisement    Advertisement Description
         Type   Name

         1      Switch link      Originated by all switches. This
                advertisements   advertisement describes the collected
                                 states of the switch's interfaces.

         2      Network link     Originated by the designated switch.
                advertisements   This advertisement contains the list
                                 of switches connected to the network
                                 link.

                  Table 2: VLSP Link State Advertisements

2.5 Protocol Data Structures

   The VLS protocol is described in this specification in terms of its
   operation on various protocol data structures.  Table 3 lists the
   primary VLSP data structures, along with the section in which they
   are described in detail.

         Structure Name                        Description

         Interface Data Structure              Section 3
         Neighbor Data Structure               Section 4
         Area Data Structure                   Section 5

                     Table 3: VLSP Data Structures

2.6 Basic Implementation Requirements

   An implementation of the VLS protocol requires the following pieces
   of system support:

   Timers

      Two types of timer are required.  The first type, known as a one-
      shot timer, expires once and triggers an event.  The second type,
      known as an interval timer, expires at preset intervals.  Interval
      timers are used to trigger events at periodic intervals.  The
      granularity of both types of timers is one second.

      Interval timers should be implemented in such a way as to avoid
      drift.  In some switch implementations, packet processing can
      affect timer execution.  For example, on a multi-access link with
      multiple switches, regular broadcasts can lead to undesirable
      synchronization of routing packets unless the interval timers have
      been implemented to avoid drift.  If it is not possible to

Top      ToC       Page 13 
      implement drift-free timers, small random amounts of time should
      be added to or subtracted from the timer interval at each firing.

   List manipulation primitives

      Much of the functionality of VLSP is described here in terms of
      its operation on lists of link state advertisements.  Any
      particular advertisement may be on many such lists. Implementation
      of VLSP must be able to manipulate these lists, adding and
      deleting constituent advertisements as necessary.

   Tasking support

      Certain procedures described in this specification invoke other
      procedures.  At times, these other procedures should be executed
      in-line -- that is, before the current procedure has finished.
      This is indicated in the text by instructions to "execute" a
      procedure.  At other times, the other procedures are to be
      executed only when the current procedure has finished.  This is
      indicated by instructions to "schedule" a task.  Implementation of
      VLSP must provide these two types of tasking support.

2.7 Organization of the Remainder of This Document

   The remainder of this document is organized as follows:

   o  Section 3 through Section 5 describe the primary data structures
      used by the protocol.  Note that this specification is presented
      in terms of these data structures in order to make explanations
      more precise.  Implementations of the protocol must support the
      functionality described, but need not use the exact data
      structures that appear in this specification.

   o  Section 6 through Section 9 describe the four operational stages
      of the protocol:  the discovery process, synchronizing the
      databases, maintaining the databases, and calculating the set of
      best paths.

   o  Section 10 describes the processing of VLSP packets and presents
      detailed descriptions of their formats.

   o  Section 11 presents detailed descriptions of link state
      advertisements.

   o  Section 12 summarizes the protocol parameters.

Top      ToC       Page 14 
3. Interface Data Structure

   The port over which a switch accesses a network link is known as the
   link interface.  Each switch maintains a separate interface data
   structure for each network link.

   The following data items are associated with each interface:

   Type

      The type of network to which the interface is attached -- point-
      to-point or broadcast (multi-access).  This data item is
      initialized to point-to-point when the interface becomes
      operational.  If a second neighbor is detected on the link after
      the first neighbor has been discovered, the link interface type is
      changed to broadcast.  The type remains as broadcast until the
      interface is declared down, at which time the type reverts to
      point-to-point.

   Note:  Previous versions of VLSP treated all links as if they were
   multi-access.  Thus, if VLSP determines that a neighbor switch is
   running an older version of the protocol software (see Section 6.1),
   it will change the interface type to broadcast.

   State

      The functional level of the interface.  The state of the interface
      is included in all switch link advertisements generated by the
      switch, and is also used to determine whether full adjacencies are
      allowed on the interface.  See Section 3.1 for a complete
      description of interface states.

   Interface identifier

      A 10-octet value that uniquely identifies the interface. This
      value consists of the 6-octet base MAC address of the neighbor
      switch, followed by the 4-octet local port number of the
      interface.

   Area ID

      A 4-octet value identifying the area.  Since VLSP does not support
      multiple areas, the value here is always zero.

Top      ToC       Page 15 
   HelloInterval

      The interval, in seconds, at which the switch sends VLSP Hello
      packets over the interface.  This parameter is not used on point-
      to-point links.

   SwitchDeadInterval

      The length of time, in seconds, that neighboring switches will
      wait before declaring the local switch dNeighboring switches

      A list of the neighboring switches attached to this network link.
      This list is created during the discovery process. Adjacencies are
      formed to one or more of these neighbors. The set of adjacent
      neighbors can be determined by examining the states of the
      neighboring switches as shown in their link state advertisements.

   Designated switch

      The designated switch selected for the multi-access network link.
      (A designated switch is not selected for a point-to-point link.)
      This data item is initialized to zero when the switch comes on-
      line, indicating that no designated switch has been chosen for the
      link.

   Backup designated switch

      The backup designated switch selected for the multi-access network
      link.  (A backup designated switch is not selected for a point-
      to-point link.)  This data item is initialized to zero when the
      switch comes on-line, indicating that no backup designated switch
      has been chosen for the link.

   Interface output cost(s)

      The cost of sending a packet over the interface.  The link cost is
      expressed in the link state metric and must be greater than zero.

   RxmtInterval

      The number of seconds between link state advertisement
      retransmissions, for adjacencies belonging to this interface. This
      value is also used to time the retransmission of Database
      Description and Link State Request packets.

Top      ToC       Page 16 
3.1 Interface States

   This section describes the various states of a switch interface. The
   states are listed in order of progressing functionality. For example,
   the inoperative state is listed first, followed by a list of the
   intermediate states through which the interface passes before
   attaining the final, fully functional state.  The specification makes
   use of this ordering by references such as "those interfaces in state
   greater than X".

   Figure 1 represents the interface state machine, showing the
   progression of interface state changes.  The arrows on the graph
   represent the events causing each state change.  These events are
   described in Section 3.2.  The interface state machine is described
   in detail in Section 3.3.

   Down

      This is the initial state of the interface.  In this state, the
      interface is unusable, and no protocol traffic is sent or received
      on the interface.  In this state, interface parameters are set to
      their initial values, all interface timers are disabled, and no
      adjacencies are associated with the interface.

Top      ToC       Page 17 
       +-------+
       |  any  |  Interface   +----------+  Unloop Ind  +----------+
       | state | -----------> |   Down   | <----------- | Loopback |
       +-------+    Down      +----------+              +----------+
                                   |                         ^
                                   | Interface Up            |
           +-------+  [pt-to-pt]   |                         |
           | Point |<------------type?              Loop Ind |
           |  to   |               |                         |
           | Point |               | [broadcast]             |
           +-------+               V                     +-------+
                             +-----------+               |  any  |
                             |  Waiting  |               | state |
                             +-----------+               +-------+
                                   |
                       Backup Seen |
                                   | Wait Timer
                                   |
                                   |
      +----------+    Neighbor     V     Neighbor    +----------+
      |    DS    | <------------> [ ] <------------> | DS Other |
      +----------+     Change      ^      Change     +----------+
                                   |
                                   |
                   Neighbor Change |
                                   |
                                   V
                              +----------+
                              |  Backup  |
                              +----------+

                   Figure 1:  Interface State Machine


   Loopback

      In this state, the switch interface is looped back, either in
      hardware or in software.  The interface is unavailable for regular
      data traffic.

   Point-to-Point

      In this state, the interface is operational and is connected to a
      physical point-to-point link.  On entering this state, the switch
      attempts to form an adjacency with the neighboring switch.

Top      ToC       Page 18 
   Waiting

      In this state, the switch is attempting to identify the backup
      designated switch for the link by monitoring the Hello packets it
      receives.  The switch does not attempt to select a designated
      switch or a backup designated switch until it changes out of this
      state, thereby preventing unnecessary changes of the designated
      switch and its backup.

   DS Other

      In this state, the interface is operational and is connected to a
      multi-access broadcast link on which other switches have been
      selected as the designated switch and the backup designated
      switch.   On entering this state, the switch attempts to form
      adjacencies with both the designated switch and the backup
      designated switch.

   Backup

      In this state, the switch itself is the backup designated switch
      on the attached multi-access broadcast link.  It will be promoted
      to designated switch if the current designated switch fails.  The
      switch establishes adjacencies with all other switches attached to
      the link.  (See Section 6.3 for more information on the functions
      performed by the backup designated switch.)

   DS

      In this state, this switch itself is the designated switch on the
      attached multi-access broadcast link.  The switch establishes
      adjacencies with all other switches attached to the link.  The
      switch is responsible for originating network link advertisements
      for the link, containing link information for all switches
      attached to the link, including the designated switch itself.
      (See Section 6.3 for more information on the functions performed
      by the designated switch.)

3.2 Events Causing Interface State Changes

   The state of an interface changes due to an interface event.  This
   section describes these events.

   Interface events are shown as arrows in Figure 1, the graphic
   representation of the interface state machine.  For more information
   on the interface state machine, see Section 3.3.

Top      ToC       Page 19 
   Interface Up

      This event is generated by the VlanHello protocol [IDhello] when
      it discovers a neighbor switch on the interface.  The interface is
      now operational.  This event causes the interface to change out of
      the Down state.  The state it enters is determined by the
      interface type.  If the interface type is broadcast (multi-
      access), this event also causes the switch to begin sending
      periodic Hello packets out over the interface.

   Wait Timer

      This event is generated when the one-shot Wait timer expires,
      triggering the end of the required waiting period before the
      switch can begin the process of selecting a designated switch and
      a backup designated switch on a multi-access link.

   Backup Seen

      This event is generated when the switch has detected the existence
      or non-existence of a backup designated switch for the link, as
      determined in one of the following two ways:

      o  A Hello packet has been received from a neighbor that claims to
         be the backup designated switch.

      o  A Hello packet has been received from a neighbor that claims to
         be the designated switch.  In addition, the packet indicated
         that there is no backup.

   In either case, the interface must have bidirectional communication
   with its neighbor -- that is, the local switch must be listed in the
   neighbor's Hello packet.

   This event signals the end of the Waiting state.

   Neighbor change

      This event is generated when there has been one of the following
      changes in the set of bidirectional neighbors associated with the
      interface.  (See Section 4.1 for information on neighbor states.)

      o  Bidirectional communication has been established with a
         neighbor -- the state of the neighbor has changed to 2-Way or
         higher.

      o  Bidirectional communication with a neighbor has been lost --
         the state of the neighbor has changed to Init or lower.

Top      ToC       Page 20 
      o  A bidirectional neighbor has just declared itself to be either
         the designated switch or the backup designated switch, as
         detected by examination of that neighbor's Hello packets.

      o  A bidirectional neighbor is no longer declaring itself to be
         either the designated switch or the backup designated switch,
         as detected by examination of that neighbor's Hello packets.

      o  The advertised switch priority of a bidirectional neighbor has
         changed, as detected by examination of that neighbor's Hello
         packets.

      When this event occurs, the designated switch and the backup
      designated switch must be reselected.

      Loop Ind

         This event is generated when an interface enters the Loopback
         state.  This event can be generated by either the network
         management service or by the lower-level protocols.

      Unloop Ind

         This event is generated when an interface leaves the Loopback
         state.  This event can be generated by either the network
         management service or by the lower-level protocols.

      Interface Down

         This event is generated under the following two circumstances:

         o  The VlanHello [IDhello] protocol has determined that the
            interface is no longer functional.

         o  The neighbor state machine has detected a second neighboring
            switch on a link presumed to be of type point-to-point. In
            addition to generating the Interface Down event, the
            neighbor state machine changes the interface type to
            broadcast.

      In both instances, this event forces the interface state to Down.
      However, when the event is generated by the neighbor state
      machine, it is immediately followed by an Interface Up event.
      (See Section 4.3.)

Top      ToC       Page 21 
3.3 Interface State Machine

   This section presents a detailed description of the interface state
   machine.

   Interface states (see Section 3.1) change as the result of various
   events (see Section 3.2).  However, the effect of each event can
   vary, depending on the current state of the interface. For this
   reason, the state machine described in this section is organized
   according to the current interface state and the occurring event.
   For each state/event pair, the new interface state is listed, along
   with a description of the required processing.

   Note that when the state of an interface changes, it may be necessary
   to originate a new switch link advertisement.  See Section 8.1 for
   more information.

   Some of the processing described here includes generating events for
   the neighbor state machine.  For example, when an interface becomes
   inoperative, all neighbor connections associated with the interface
   must be destroyed.  For more information on the neighbor state
   machine, see Section 4.3.

   State(s):  Down
   Event:  Interface Up
   New state:  Depends on action routine
   Action:
      If the interface is a point-to-point link, set the interface state
      to Point-to-Point.  Otherwise, start the Hello interval timer,
      enabling the periodic sending of Hello packets over the interface.
      If the switch is not eligible to become the designated switch,
      change the interface state to DS Other. Otherwise, set the
      interface state to Waiting and start the one-shot wait timer.
      Create a new neighbor data structure for the neighbor switch,
      initialize all neighbor parameters and set the stateof the
      neighbor to Down.

   State(s):  Waiting
   Event:  Backup Seen
   New state:  Depends on action routine
   Action:
      Select the designated switch and backup designated switch for the
      attached link, as described in Section 6.3.1.  As a result of this
      selection, set the new state of the interface to either DS Other,
      Backup or DS.

Top      ToC       Page 22 
   State(s):  Waiting
   Event:  Wait Timer
   New state:  Depends on action routine
   Action:
      Select the designated switch and backup designated switch for the
      attached link, as described in Section 6.3.1.  As a result of this
      selection, set the new state of the interface to either DS Other,
      Backup or DS.

   State(s):  DS Other, Backup or DS
   Event:  Neighbor Change
   New state:  Depends on action routine
   Action:
      Reselect the designated switch and backup designated switch for
      the attached link, as described in Section 6.3.1.  As a result of
      this selection, set the new state of the interface to either DS
      Other, Backup or DS.

   State(s):  Any State
   Event:  Interface Down
   New state:  Down
   Action:
      Reset all variables in the interface data structure and disable
      all timers.  In addition, destroy all neighbor connections
      associated with the interface by generating the KillNbr event on
      all neighbors listed in the interface data structure.

   State(s):  Any State
   Event:  Loop Ind
   New state:  Loopback
   Action:
      Reset all variables in the interface data structure and disable
      all timers.  In addition, destroy all neighbor connections
      associated with the interface by generating the KillNbr event on
      all neighbors listed in the interface data structure.

   State(s):  Loopback
   Event:  Unloop Ind
   New state:  Down
   Action:
      No action is necessary beyond changing the interface state to Down
      because the interface was reset on entering the Loopback state.


Next RFC Part