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RFC 2642

 
 
 

Cabletron's VLS Protocol Specification

Part 4 of 4, p. 84 to 95
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11. Link State Advertisement Formats

   Link state advertisements are used to describe various pieces of the
   routing topology within the switch fabric.  Each switch in the fabric
   maintains a complete set of all link state advertisements generated
   throughout the fabric.  (Section 8.1 describes the circumstances
   under which a link state advertisement is originated.  Section 8.2
   describes how advertisements are distributed throughout the switch
   fabric.) This collection of advertisements, known as the link state
   (or topological) database, is used to calculate a set of best paths
   to all other switches in the fabric.

   There are two types of link state advertisement, as listed in Table
   8.

        Type   Name            Function             Description

        1      Switch link     Lists all network    Section 11.2
               advertisement   linksattached to
                               a switch

        2      Network link    Lists all adjacen-   Section 11.3
               advertisement   cies on a network
                               link

                Table 8: Link State Advertisement Types

   Each link state advertisement begins with a standard header,
   described in Section 11.1.

11.1 Link State Advertisement Headers

   All link state advertisements begin with a common 32-octet header.
   This header contains information that uniquely identifies the
   advertisement -- its type, link state ID, and the switch ID of its
   advertising switch.  Also, since multiple instances of a link state
   advertisement can exist concurrently in the switch fabric, the header
   contains information that permits a switch to determine which
   instance is the most recent -- the age, sequence number and checksum.

   The format of the link state advertisement header is shown below.

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       0                   1                   2                   3
       0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   00 |              Age              |    Options    |    LS Type    |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   04 |                                                               |
      +                         Link state ID                         +
   08 |                                                               |
      +                               +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   12 |                               |                               |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+                               +
   16 |                                                               |
      +                      Advertising switch ID                    +
   20 |                                                               |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   24 |                         Sequence number                       |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   28 |           Checksum            |             Length            |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+


   Age

      This 2-octet field contains the time, in seconds, since this
      instance of the link state advertisement was originated.

   Options

      This 1-octet field contains the optional capabilities supported by
      the advertising switch, as described in Section 10.5.

   LS type

      This 1-octet field contains the type of the link state
      advertisement.  Possible values are:

         1   Switch link advertisement
         2   Network link advertisement

   Link state ID

      This 10-octet field identifies the switch that originates
      advertisements for the link.  The content of this field depends on
      the advertisement's type.

      o  For a switch link advertisement, this field contains the switch
         ID of the originating switch

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         o  For a network link advertisement, this field contains the
         switch ID of the designated switch for the link

   Note:  In VLSP, the link state ID of an advertisement is always the
   same as the advertising switch.  This level of redundancy results
   from the fact that OSPF uses additional types of link state
   advertisements for which the originating switch is not the
   advertising switch.

   Advertising switch

      This 10-octet field contains the switch ID of the switch that
      originated the link state advertisement.

   Sequence number

      This 4-octet field is used to sequence the instances of a
      particular link state advertisement.  The number is incremented
      for each new instance.

   Checksum

      This 2-octet field contains the checksum of the complete contents
      of the link state advertisement, excluding the age field.  The
      checksum used is commonly referred to as the Fletcher checksum and
      is documented in [RFC905].  Note that since this checksum is
      calculated for each separate advertisement, a protocol packet
      containing lists of advertisements or advertisement headers will
      contain multiple checksum values.

   Length

      This 2-octet field contains the total length, in octets, of the
      link state advertisement, including the header.

11.2 Switch Link Advertisements

   A switch link advertisement is used to describe all functioning
   network links of a switch, including the cost of using each link.

   Each functioning switch in the fabric originates one, and only one,
   switch link advertisement -- all of the switch's links must be
   described in a single advertisement.  A switch originates its first
   switch link advertisement (containing no links) when it first becomes
   functional.  It then originates a new instance of the advertisement
   each time any of its neighbor states changes such that the contents
   of the advertisement changes.   See Section 8.1 for details on
   originating a switch link advertisement.

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   The format of a switch link advertisement is shown below.

       0                   1                   2                   3
       0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   00 |                                                               |
      :                       Link state header                       :
      |                                                               |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   32 |      (unused -- must be 0)    |            # links            |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   36 |                                                               |
      +                            Link ID                            +
   40 |                                                               |
      +                               +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   44 |                               |                               |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+                               +
   48 |                                                               |
      +                           Link data                           +
   52 |                                                               |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   56 |   Link type   |     # TOS     |         TOS 0 metric          |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   60 |                                                               |
      :                            . . .                              :
      |                                                               |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Link state header

      This 32-octet field contains the standard link state advertisement
      header.  The type field contains a 1, and the link state ID field
      contains the switch ID of the advertising switch.

   # links

      This 2-octet field contains the number of links described by this
      advertisement.  This value must be equal to the total number of
      functioning network links attached to the switch.

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   Link ID

      This 10-octet field identifies the other switch that originates
      link state advertisements for the link, providing a key for
      accessing other link state advertisements for the link.  The value
      here is based on the link type, as follows:

      o  For point-to-point links, this field contains the switch ID of
         the neighbor switch connected to the other end of the link.

      o  For multi-access links, this field contains the switch ID of
         the designated switch for the link.

   Link data

      This 10-octet field contains additional data necessary to
      calculate the set of best paths.  Typically, this field contains
      the interface ID of the link.

   Link type

      This 1-octet field contains the type of link being described.
      Possible values are as follows:

         1   Point-to-point link
         2   Multi-access link

   # TOS

      This 1-octet field contains the number of nonzero type of service
      metrics specified for the link.  Since the current version of VLSP
      does not support routing based on nonzero types of service, this
      field contains a value of zero.

   TOS 0 metric

      This 2-octet field contains the cost of using this link for the
      zero TOS.  This value is expressed in the link state metric and
      must be greater than zero.

   Note that the last five fields are repeated for all functioning
   network links attached to the advertising switch.  If the interface
   state of attached link changes, the switch must originate a new
   instance of the switch link advertisement.

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11.3 Network Link Advertisements

   A network link advertisement is originated by the designated switch
   of each multi-access network link.  The advertisement describes all
   switches attached to the link that are currently fully adjacent to
   the designated switch, including the designated switch itself.  See
   Section 8.1 for details on originating a switch link advertisement.

   Network link advertisements are not generated for point-to-point
   network links.

   The format of a network link advertisement is show below.

       0                   1                   2                   3
       0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   00 |                                                               |
      :                       Link state header                       :
      |                                                               |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   32 |                           (unused)                            |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   36 |                                                               |
      +                                                               +
      :                          Switch list                          :
      +                                                               +
      |                                                               |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Link state header

      This 32-octet field contains the standard link state advertisement
      header.  The type field contains a 2, and the link state ID field
      contains the switch ID of the designated switch.

   Switch list

      The switch IDs of all switches attached to the network link that
      are currently fully adjacent to the designated switch. The
      designated switch includes itself in this list.

12. Protocol Parameters

   This section contains a compendium of the parameters used in the VLS
   protocol.

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12.1 Architectural Constants

   Several VLS protocol parameters have fixed architectural values. The
   name of each architectural constant follows, together with its value
   and a short description of its function.

   AllSPFSwitches

      The multicast switch ID to which Hello packets and certain other
      protocol packets are addressed, as specified in the destination
      switch ID field of the network layer address information (see
      Section 10.3).  The value of AllSPFSwitches is E0-00-00-05-00-00-
      00-00.

   AllDSwitches

      The multicast switch ID to which Link State Update packets and
      Link State Acknowledgment packets are addressed, as specified in
      the destination switch ID field of the network layer address
      information (see Section 10.3), when they are destined for the
      designated switch or the backup designated switch of a network
      link.  The value of AllDSwitches is E0-00-00-06-00-00-00-00.

   LSRefreshTime

      The interval at which the set of best paths recalculated if no
      other state changes have forced a recalculation.  The value of
      LSRefreshTime is set to 1800 seconds (30 minutes).

   MinLSInterval

      The minimum time between distinct originations of any particular
      link state advertisement.  The value of MinLSInterval is set to 5
      seconds.

   MaxAge

      The maximum age that a link state advertisement can attain. When
      an advertisement's age reaches MaxAge, it is redistributed
      throughout the switch fabric.  When the originating switch
      receives an acknowledgment for the advertisement, indicating that
      the advertisement has been removed from all neighbor Link state
      retransmission lists, the advertisement is removed from the
      originating switch's database.  Advertisements having age MaxAge
      are not used to calculate the set of best paths.  The value of
      MaxAge must be greater than LSRefreshTime.  The value of MaxAge is
      set to 3600 seconds (1 hour).

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   MaxAgeDiff

      The maximum time disparity in ages that can occur for a single
      link state instance as it is distributed throughout the switch
      fabric.  Most of this time is accounted for by the time the
      advertisement sits on switch output queues (and therefore not
      aging) during the distribution process. The value of MaxAgeDiff is
      set to 900 seconds (15 minutes).

   LSInfinity

      The link state metric value indicating that the destination is
      unreachable.  It is defined to be a binary value of all ones.

12.2 Configurable Parameters

   Many of the switch interface parameters used by VLSP may be made
   configurable if the implementer so desires.  These parameters are
   listed below.  Sample default values are given for some of the
   parameters.

   Note that some of these parameters specify properties of the
   individual interfaces and their attached network links.  These
   parameters must be consistent across all the switches attached to
   that link.

   Interface output cost(s)

      The cost of sending a packet over the interface, expressed in the
      link state metric.  This is advertised as the link cost for this
      interface in the switch's switch link advertisement. The interface
      output cost must always be greater than zero.

   RxmtInterval

      The number of seconds between link state advertisement
      retransmissions for adjacencies established on this interface.
      This value is also used when retransmitting Database Description
      packets and Link State Request packets. This value must be greater
      than the expected round-trip delay between any two switches on the
      attached link.  However, the value should be conservative or
      needless retransmissions will result.  A typical value for a local
      area network would be 5 seconds.

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   InfTransDelay

      The estimated number of seconds it takes to transmit a Link State
      Update packet over this interface.  Link state advertisements
      contained in the Link State Update packet must have their age
      incremented by this amount before transmission.  This value must
      take into account the transmission and propagation delays for the
      interface and must be greater than zero.  A typical value for a
      local area network would be 1 second.

   Switch priority

      An 8-bit unsigned integer.  When two switches attached to the same
      network link contend for selection as the designated switch, the
      switch with the highest priority takes precedence.  If both
      switches have the same priority, the switch with the highest base
      MAC address becomes the designated switch.  A switch whose switch
      priority is set to zero is ineligible to become the designated
      switch on the attached link.

   HelloInterval

      The length of time, in seconds, between the Hello packets that the
      switch sends over the interface.  This value is advertised in the
      switch's Hello packets.  It must be the same for all switches
      attached to a common network link.  The smaller this value is set,
      the faster topological changes will be detected.  However, a
      smaller interval will also generate more routing traffic.  A
      typical value for a local area network would be 10 seconds.

   SwitchDeadInterval

      The length of time, in seconds, that neighboring switches will
      wait before declaring the interface down once they stop receiving
      Hello packets over the interface.  This value is advertised in the
      switch's Hello packets.  It must be the same for all switches
      attached to a common network link and should be some multiple of
      the HelloInterval parameter.  A typical value would be 4 times
      HelloInterval.

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13. End Notes

   [1] During calculation of the set of best paths, a network link
   advertisement must be located based solely on its link state ID.
   Note, however, that the lookup in this case is still well defined,
   since no two network advertisements can have the same link state ID.

   [2] It is instructive to see what happens when the designated switch
   for a network link fails.  Call the designated switch for the link S1
   and the backup designated switch S2.  If switch S1 fails (or its
   interface to the link goes down), the other switches on the link will
   detect S1's absence within SwitchDeadInterval seconds.  All switches
   may not detect this condition at precisely the same time.  The
   switches that detect S1's absence before S2 does will temporarily
   select S2 as both designated switch and backup designated switch.
   When S2 detects that S1 is down, it will move itself to designated
   switch.  At this time, the remaining switch with the highest switch
   priority will be selected as the backup designated switch.

   [3] Note that it is possible for a switch to resynchronize any of its
   fully established adjacencies by setting the neighbor state back to
   ExStart.  This causes the switch on the other end of the adjacency to
   process a SeqNumberMismatch event and also revert to the ExStart
   state.

   [4] When two advertisements have different checksum values, they are
   assumed to be separate instances.  This can occur when a switch
   restarts and loses track of its previous sequence number. In this
   case, since the two advertisements have the same sequence number, it
   is not possible to determine which advertisement is actually newer.
   If the wrong advertisement is accepted as newer, the originating
   switch will originate another instance.

   [5] An instance of an advertisement is originated with an age of
   MaxAge only when it is to be flushed from the database.  This is done
   either when the advertisement has naturally aged to MaxAge, or (more
   typically) when the sequence number must wrap. Therefore, a received
   instance with an age of MaxAge must be processed as the most recent
   in order to flush it properly from the database.

   [6] MaxAgeDiff is an architectural constant that defines the maximum
   disparity in ages, in seconds, that can occur for a single link state
   instance as it is distributed throughout the switch fabric.  If two
   advertisements differ by more than this amount, they are assumed to
   be different instances of the same advertisement. This can occur when
   a switch restarts and loses track of its previous sequence number.

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   [7] This is how the link state request list is emptied, causing the
   neighbor state to change to Full.

14. Security Considerations

   Security concerns are not addressed in this document.

15. References

   [Perlman]    Perlman, R.,  Interconnections: Bridges and Routers.
                Addison-Wesley Publishing Company.  1992.

   [RFC905]     McKenzie, A., "ISO Transport Protocol specification ISO
                DP 8073", RFC 905, April 1984.

   [RFC2328]    Moy, J., "OSPF Version 2", STD 54, RFC 2328, April 1998.

   [RFC1700]    Reynolds, J. and J. Postel, "Assigned Numbers", STD 2,
                RFC 1700, October 1994.

   [IDsfvlan]   Ruffen, D., Len, T. and J. Yanacek, "Cabletron's
                SecureFast VLAN Operational Model", RFC 2643, August
                1999.

   [IDhello]    Hamilton, D. and D. Ruffen, "Cabletron's VlanHello
                Protocol Specification", RFC 2641, August 1999.

16. Author's Address

   Laura Kane
   Cabletron Systems, Inc.
   Post Office Box 5005
   Rochester, NH  03866-5005

   Phone:(603) 332-9400
   EMail:  lkane@ctron.com

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17.  Full Copyright Statement

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

   This document and translations of it may be copied and furnished to
   others, and derivative works that comment on or otherwise explain it
   or assist in its implementation may be prepared, copied, published
   and distributed, in whole or in part, without restriction of any
   kind, provided that the above copyright notice and this paragraph are
   included on all such copies and derivative works.  However, this
   document itself may not be modified in any way, such as by removing
   the copyright notice or references to the Internet Society or other
   Internet organizations, except as needed for the purpose of
   developing Internet standards in which case the procedures for
   copyrights defined in the Internet Standards process must be
   followed, or as required to translate it into languages other than
   English.

   The limited permissions granted above are perpetual and will not be
   revoked by the Internet Society or its successors or assigns.

   This document and the information contained herein is provided on an
   "AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING
   TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING
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   HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF
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Acknowledgement

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