rfc2642.txt

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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



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       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



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       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:



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      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.



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   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.















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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.








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