tech-invite   World Map
3GPP     Specs     Glossaries     UICC       IETF     RFCs     Groups     SIP     ABNFs       T+       Search     Home

RFC 2642

 
 
 

Cabletron's VLS Protocol Specification

Part 3 of 4, p. 51 to 83
Prev RFC Part       Next RFC Part

 


prevText      Top      Up      ToC       Page 51 
8. Maintaining the Databases

   Each switch advertises its state (also known as its link state) by
   originating switch link advertisements.  In addition, the designated
   switch on each network link advertises the state of the link by
   originating network link advertisements.

   As described in Section 7.1, link state advertisements are uniquely
   identified by their type, link state ID, and advertising switch.

   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.

Top      Up      ToC       Page 52 
8.1 Originating Link State Advertisements

   A new instance of each link state advertisement is originated any
   time the state of the switch or link changes.  When a new instance of
   a link state advertisement is originated, its sequence number is
   incremented, its age is set to zero, and its checksum is calculated.
   The advertisement is then installed into the local link state
   database and forwarded out all fully operational interfaces (that is,
   those interfaces with a state greater than Waiting) for distribution
   throughout the switch fabric.  See Section 8.2.4 for a description of
   the installation of the advertisement into the link state database
   and Section 8.2.5 for a description of how advertisements are
   forwarded.

   A switch originates a new instance of a link state advertisement as a
   result of the following events:

   o  The state of one of the switch's interfaces changes such that the
      contents of the associated switch link advertisement changes.

   o  The designated switch on any of the switch's attached network
      links changes.  The switch originates a new switch link
      advertisement.  Also, if the switch itself is now the designated
      switch, it originates a new network link advertisement for the
      link.

   o  One of the switch's neighbor states changes to or from Full. If
      this changes the contents of the associated switch link
      advertisement, a new instance is generated.  Also, if the switch
      is the designated switch for the attached network link, it
      originates a new network link advertisement for the link.

   Two instances of the same link state advertisement must not be
   originated within the time period MinLSInterval.  Note that this may
   require that the generation of the second instance to be delayed up
   to MinLSInterval seconds.

8.1.1 Switch Link Advertisements

   A switch link advertisement describes the collected states of all
   functioning links attached to the originating switch -- that is, all
   attached links with an interface state greater than Down.  A switch
   originates an empty switch link advertisement when it first becomes
   functional.  It then generates a new instance of the advertisement
   each time one of its interfaces reaches a fully functioning state
   (Point-to-Point or better).

Top      Up      ToC       Page 53 
   Each link in the advertisement is assigned a type, based on the state
   of interface, as shown in Table 4.

            Interface state     Link type     Description

            Point-to-Point      1             Point-to-point link
            DS Other*           2             Multi-access link
            Backup*             2             Multi-access link
            DS**                2             Multi-access link

              *If a full adjacency has been formed with the designated
               switch.

             **If a full adjacency has been formed with at least one
               other switch on the link.

               Table 4: Link Types in a Switch Link Advertisement

   Each link in the advertisement is also assigned a link identifier
   based on its link type.  In general, this value identifies another
   switch that also originates advertisements for the link, thereby
   providing a key for accessing other link state advertisements for the
   link.  The relationship between link type and ID is shown in Table 5.


             Type  Description           Link ID

            1     Point-to-point link   Switch ID of neighbor switch
            2     Multi-access link     Switch ID of designated switch

               Table 5: Link IDs in a Switch Link Advertisement


   In addition to a type and an identifier, the description of each link
   specifies the interface ID of the associated network link.

   Finally, each link description includes the cost of sending a packet
   over the link.  This output cost is expressed in the link state
   metric and must be greater than zero.

   To illustrate the format of a switch link advertisement, consider the
   switch fabric shown in Figure 4.

   In this example, switch SW1 has 5 neighboring switches (shown as
   boxes) distributed over 3 network links (shown as lines).  The base
   MAC address of each switch is also shown adjacent to each box.  On
   switch SW1, ports 01 and 02 attach to point-to-point network links,

Top      Up      ToC       Page 54 
   while port 03 attaches to a multi-access network link with three
   attached switches.  The interface state of each port is shown next to
   the line representing the corresponding link.


                            00-00-1d-22-23-c5
                                +-------+
                                |  SW2  |
                                +-------+
                                    |
                                    | Point-to-Point
                                    |
                                    | 01
       +-------+    Loopback    +-------+
       |  SW3  |----------------|  SW1  | 00-00-1d-1f-05-81
       +-------+             02 +-------+
   00-00-1d-17-35-a4                | 03
                                    |
                                    | DS Other
                                    |
               +--------------------+--------------------+
               |                    |                    |
               | DS Other           | Backup             | DS
               |                    |                    |
           +-------+            +-------+            +-------+
           |  SW4  |            |  SW5  |            |  SW6  |
           +-------+            +-------+            +-------+
        00-00-1d-4a-26-b3    00-00-1d-4a-27-1c    00-00-1d-7e-84-2e


                    Figure 4: Sample Switch Fabric

   The switch link advertisement generated by switch SW1 would contain
   the following data items:

      ; switch link advertisement for switch SW1

      LS age = 0               ; always true on origination
      Options = (T-bit|E-bit)  ; options
      LS type = 1              ; this is a switch link advert

Top      Up      ToC       Page 55 
                               ; SW1's switch ID
      Link State ID = 00-00-1d-1f-05-81-00-00-00-00
      Advertising switch = 00-00-1d-1f-05-81-00-00-00-00
      # links = 2

         ; link on interface port 1
         Link ID = 00-00-1d-22-23-c5-00-00-00-00    ; switch ID

         Link Data = 00-00-1d-1f-05-81-00-00-00-01  ; interface ID
         Type = 1                                   ; pt-to-pt link
         # other metrics = 0                        ; TOS 0 only
         TOS 0 metric = 1

         ; link on interface port 2 is not fully functional

         ; link on interface port 3
         Link ID = 00-00-1d-7e-84-2e-00-00-00-00    ; switch ID of DS
         Link Data = 00-00-1d-1f-05-81-00-00-00-03  ; interface ID
         Type = 2                                   ; multi-access
         # other metrics = 0                        ; TOS 0 only
         TOS 0 metric = 2

   (See Section 11.2 for a detailed description of the format of a
   switch link advertisement.)

8.1.2 Network Link Advertisements

   Network link advertisements are used to describe the switches
   attached to each multi-access network link.

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

   A network link advertisement is originated by the designated switch
   for the associated multi-access link once the switch has established
   a full adjacency with at least one other switch on the link.  Each
   advertisement lists the switch IDs of those switches that are fully
   adjacent to the designated switch.  The designated switch includes
   itself in this list.

   To illustrate the format of a network link advertisement, consider
   again the switch fabric shown in Figure 4.  In this example, network
   link advertisements will be generated only by switch SW6, the
   designated switch of the multi-access network link between switches
   SW1 and switches SW4, SW5, and SW6.

   The network link advertisement generated by switch SW6 would contain
   the following data items:

Top      Up      ToC       Page 56 
      ; network link advertisement for switch SW6

      LS age = 0               ; always true on origination
      Options = (T-bit|E-bit)  ; options
      LS type = 2              ; this is a network link advert

                                 ; SW6's switch ID
      Link State ID = 00-00-1d-73-84-2e-00-00-00-00
      Advertising switch = 00-00-1d-73-84-2e-00-00-00-00

         Attached switch = 00-00-1d-7e-84-2e-00-00-00-00
         Attached switch = 00-00-1d-4a-26-b3-00-00-00-00
         Attached switch = 00-00-1d-1f-05-81-00-00-00-00
         Attached switch = 00-00-1d-4a-27-1c-00-00-00-00

      (See Section 11.3 for a detailed description of the format of a
      network link advertisement.)

8.2 Distributing Link State Advertisements

   Link state advertisements are distributed throughout the switch
   fabric encapsulated within Link State Update packets.  A single Link
   State Update packet may contain several distinct advertisements.

   To make the distribution process reliable, each advertisement must be
   explicitly acknowledged in a Link State Acknowledgment packet.  Note,
   however, that multiple acknowledgments can be grouped together into a
   single Link State Acknowledgment packet. A sending switch retransmits
   unacknowledged Link State Update packets at regular intervals until
   they are acknowledged.

   The remainder of this section is structured as follows:

   o  Section 8.2.1 presents an overview of the distribution process.

   o  Section 8.2.2 describes how an incoming Link State Update packet
      is processed.

   o  Section 8.2.3 describes how a Link State Packet is forwarded --
      both by the originating switch and an intermediate receiving
      switch.

   o  Section 8.2.4 describes how advertisements are installed into the
      local database.

   o  Section 8.2.5 describes the retransmission of unacknowledged
      advertisements.

Top      Up      ToC       Page 57 
    o  Section 8.2.6 describes how advertisements are acknowledged.

8.2.1 Overview

   The philosophy behind the distribution of link state advertisements
   is based on the concept of adjacencies -- that is, each switch is
   only required to remain synchronized with its adjacent neighbors.

   When a switch originates a new instance of a link state
   advertisement, it formats the advertisement into a Link State Update
   packet and floods the packet out each fully operational interface --
   that is, each interface with a state greater than Waiting.  However,
   only those neighbors that are adjacent to the sending switch need to
   process the packet.

   The sending switch indicates which of its neighbor switches should
   process the advertisement by specifying a particular multicast
   destination in the network layer address information (see Section
   10.3).  The sending switch sets the value of the network layer
   destination switch ID field according to the state of the interface
   over which the packet is sent:

   o  If the interface state is Point-to-Point, DS, or Backup, the
      switch is adjacent to all other switches on the link and all
      neighboring switches must process the packet.  Therefore, the
      destination field is set to the multicast switch ID
      AllSPFSwitches.

   o  If the interface state is DS Other, the switch is only adjacent to
      the designated switch and the backup designated switch and only
      those two neighboring switches must process the packet.
      Therefore, the destination field is set to the multicast switch ID
      AllDSwitches.

   A similar logic is used when a switch receives a Link State Update
   packet containing a new instance of a link state advertisement.
   After processing and acknowledging the packet, the receiving switch
   forwards the Link State Update packet as

   o  On the interface over which the original Link State Update packet
      was received:

Top      Up      ToC       Page 58 
      o  If the receiving switch is the designated switch for the
         attached network link, the packet is forwarded to all other
         switches on the link.  (The destination field is set to
         AllSPFSwitches.)  The originating switch will recognize that it
         was the advertisement originator and discard the packet.

      o  If the receiving switch is not the designated switch for the
         attached network link, the packet is not sent back out the
         interface over which it was received.

   o  On all other interfaces:

      o  If the receiving switch is the designated switch for the
         attached network link, the packet is forwarded to all switches
         on the link.  (The destination field is set to AllSPFSwitches.)

      o  If the receiving switch is neither the designated switch or the
         backup designated switch for the attached network link, the
         packet is forwarded only to the designated switch and the
         backup designated switch.  (The destination field is set to
         AllDSwitches.)

   Each Link State Update packet is forwarded and processed in this
   fashion until all switches in the fabric have received notification
   of the new instance of the link state advertisement.

8.2.2 Processing an Incoming Link State Update Packet

   When the a Link State Update packet is received, it is first
   subjected to a number of consistency checks.  In particular, the Link
   State Update packet is associated with a specific neighbor. If the
   state of that neighbor is less than Exchange, the entire Link State
   Update packet is discarded.

   Each link state advertisement contained in the packet is processed as
   follows:

   1. Validate the advertisement's link state checksum and type. If the
      checksum is invalid or the type is unknown, discard the
      advertisement without acknowledging it.

   2. If the advertisement's age is equal to MaxAge and there is
      currently no instance of the advertisement in the local link state
      database, then do the following:

Top      Up      ToC       Page 59 
      a) Acknowledge the advertisement by sending a Link State
         Acknowledgment packet to the sending neighbor (see Section
         8.2.6).

      b) Purge all outstanding requests for equal or previous instances
         of the advertisement from the sending neighbor's Link State
         Request list.

      c) If the neighbor is Exchange or Loading, install the
         advertisement in the link state database (see Section 8.2.4).
         Otherwise, discard the advertisement.

   3. If the advertisement's age is equal to MaxAge and there is an
      instance of the advertisement in the local link state database,
      then do the following:

      a) If the advertisement is listed in the link state retransmission
         list of any neighbor, remove the advertisement from the
         retransmission list(s) and delete the database copy of the
         advertisement.

      b) Discard the received (MaxAge) advertisement without
         acknowledging it.

   4. If the advertisement's age is less than MaxAge, attempt to locate
      an instance of the advertisement in the local link state database.
      If there is no database copy of this advertisement, or the
      received advertisement is more recent than the database copy (see
      Section 7.1.1), do the following:

      a) If there is already a database copy, and if the database copy
         was installed less than MinLSInterval seconds ago, discard the
         new advertisement without acknowledging it.

      b) Otherwise, forward the new advertisement out some subset of the
         local interfaces (see Section 8.2.3).  Note whether the
         advertisement was sent back out the receiving interface for
         later use by the acknowledgment process.

      c) Remove the current database copy from the Link state
         retransmission lists of all neighbors.

      d) Install the new advertisement in the link state database,
         replacing the current database copy.  (Note that this may cause
         the calculation of the set of best paths to be scheduled.  See
         Section 9.)  Timestamp the new advertisement with the time that
         it was received to prevent installation of another instance
         within MinLSInterval seconds.

Top      Up      ToC       Page 60 
      e) Acknowledge the advertisement, if necessary, by sending a Link
         State Acknowledgment packet back out the receiving interface.
         (See Section 8.2.6.)

      f) If the link state advertisement was initially advertised by the
         local switch itself, advance the advertisement sequence number
         and issue a new instance of the advertisement. (Receipt of a
         newer instance of an advertisement means that the local copy of
         the advertisement is left over from before the last time the
         switch was restarted.)

   5. If the received advertisement is the same instance as the database
      copy (as determined by the algorithm described in Section 7.1.1),
      do the following:

      a) If the advertisement is listed in the neighbor's link state
         retransmission list, the local switch is expecting an
         acknowledgment for this advertisement.  Treat the received
         advertisement as an implied acknowledgment, and remove the
         advertisement from the link state retransmission list. Note
         this implied acknowledgment for later use by the acknowledgment
         process (Section 8.2.6).

      b) Acknowledge the advertisement, if necessary, by sending a Link
         State Acknowledgment packet back out the receiving interface.
         (See Section 8.2.6.)

   If the database copy of the advertisement is more recent than the
      instance just received, do the following:

      a) Determine whether the instance is listed in the neighbor link
         state request list.  If so, an error has occurred in the
         database exchange process.  Restart the database exchange
         process by generating a neighbor BadLSReq event for the sending
         neighbor and terminate processing of the Link State Update
         packet.

      b) Otherwise, generate an unusual event to network management and
         discard the advertisement.

8.2.3 Forwarding Link State Advertisements

   When a new instance of an advertisement is originated or after an
   incoming advertisement has been processed, the switch must decide
   over which interfaces and to which neighbors the advertisement will
   be forwarded.  In some instances, the switch may decide not to
   forward the advertisement over a particular interface because it is
   able to determine that the neighbors on that attached link have or

Top      Up      ToC       Page 61 
   will receive the advertisement from another switch on the link.

   The decision of whether to forward an advertisement over each of the
   switch's interfaces is made as follows:

   1. Each neighboring switch attached to the interface is examined to
      determine whether it should receive and process the new
      advertisement.  For each neighbor, the following steps are
      executed:

      a) If the neighbor state is less than Exchange, the neighbor need
         not receive or process the new advertisement.

      b) If the neighbor state is Exchange or Loading, examine the link
         state request list associated with the neighbor.  If an
         instance of the new advertisement is on the list, the
         neighboring switch already has an instance of the
         advertisement.  Compare the new advertisement to the neighbor's
         copy:

         o  If the new advertisement is less recent, the neighbor need
            not receive or process the new advertisement.

         o  If the two copies are the same instance, delete the
            advertisement from the link state request list.  The
            neighbor need not receive or process the new advertisement
            [7].

         o  Otherwise, the new advertisement is more recent.  Delete the
            advertisement from the link state request list.  The
            neighbor may need to receive and process the new
            advertisement.

      c) If the new advertisement was received from this neighbor, the
         neighbor need not receive or process the advertisement.

      d) Add the new advertisement to the link state retransmission list
         for the neighbor.

   2. The switch must now decide whether to forward the new
      advertisement out the interface.

      a) If the link state advertisement was not added to any of the
         link state retransmission lists for neighbors attached to the
         interface, there is no need to forward the advertisement out
         the interface.

Top      Up      ToC       Page 62 
      b) If the new advertisement was received on this interface, and it
         was received from either the designated switch or the backup
         designated switch, there is no need to forward the
         advertisement out the interface.  Chances are all neighbors on
         the attached network link have also received the advertisement
         already.

      c) If the new advertisement was received on this interface and the
         state of the interface is Point-to-Point, there is no need to
         forward the advertisement since the received advertisement was
         originated by the neighbor switch.

      d) If the new advertisement was received on this interface, and
         the interface state is Backup -- that is, the switch itself is
         the backup designated switch -- there is no need to forward the
         advertisement out the interface.  The designated switch will
         distribute advertisements on the attached network link.

      e) Otherwise, the advertisement must be forwarded out the
         interface.

      To forward a link state advertisement, the switch first increments
      the advertisement's age by InfTransDelay seconds to account for
      the transmission time over the link.  The switch then copies the
      advertisement into a Link State Update packet

      Forwarded advertisements are sent to all adjacent switches
      associated with the interface.  If the interface state is Point-
      to-Point, DS, or Backup, the destination switch ID field of the
      network layer address information is set to the multicast switch
      ID AllSPFSwitches.  If the interface state is DS Other, the
      destination switch ID field is set to the multicast switch ID
      AllDSwitches.

8.2.4 Installing Link State Advertisements in the Database

   When a new link state advertisement is installed into the link state
   database, as the result of either originating or receiving a new
   instance of an advertisement, the switch must determine whether the
   best paths need to be recalculated.  To make this determination, do
   the following:

   1. Compare the contents of the new instance with the contents of the
      old instance (assuming the older instance is available). Note that
      this comparison does not include any data from the link state
      header.  Differences in fields within the header (such as the
      sequence number and checksum, which are guaranteed to be different
      in different instances of an advertisement) are of no consequence

Top      Up      ToC       Page 63 
      when deciding whether or not to recalculate the set of best paths.

   2. If there are no differences in the contents of the two
      advertisement instances, there is no need to recalculate the set
      of best paths.

   3. Otherwise, the set of best paths must be recalculated.

   Note also that the older instance of the advertisement must be
   removed from the link state database when the new advertisement is
   installed.  The older instance must also be removed from the link
   state retransmission lists of all neighbors.

8.2.5 Retransmitting Link State Advertisements

   When a switch sends a link state advertisement to an adjacent
   neighbor, it records the advertisement in the neighbor's link state
   retransmission list.  To ensure the reliability of the distribution
   process, the switch continues to periodically retransmit the
   advertisements specified in the list until they are acknowledged.

   The interval timer used to trigger retransmission of the
   advertisements is set to RxmtInterval seconds, as found in the
   interface data structure. Note that if this value is too low,
   needless retransmissions will ensue.  If the value is too high, the
   speed with which the databases synchronize across adjacencies may be
   affected if there are lost packets.

   When the interval timer expires, entries in the retransmission list
   are formatted into one or more Link State Update packets. (Remember
   that multiple advertisements can fit into a single Link State Update
   packet.)  The age field of each advertisement is incremented by
   InfTransDelay, as found in the interface data structure, before the
   advertisement is copied into the outgoing packet.

   Link State Update packets containing retransmitted advertisements are
   always sent directly to the adjacent switch. That is, the destination
   field of the network layer addressing information is set to the
   switch ID of the neighboring switch.

   If the adjacent switch goes down, retransmissions will continue until
   the switch failure is detected and the adjacency is torn down by the
   VLSP discovery process.  When the adjacency is torn down, the link
   state retransmission list is cleared.

Top      Up      ToC       Page 64 
8.2.6 Acknowledging Link State Advertisements

   Each link state advertisement received by a switch must be
   acknowledged.  In most cases, this is done by sending a Link State
   Acknowledgment packet.  However, acknowledgments can also be done
   implicitly by sending Link State Update packets (see step 4a of
   Section 8.2.2).

   Multiple acknowledgments can be grouped together into a single Link
   State Acknowledgment packet.

   Sending an acknowledgment

      Link State Acknowledgment packets are sent back out the interface
      over which the advertisement was received.  The packet can be sent
      immediately to the sending neighbor, or it can be delayed and sent
      when an interval timer expires.

      o  Sending delayed acknowledgments facilitates the formatting of
         multiple acknowledgments into a single packet.  This enables a
         single packet to send acknowledgments to several neighbors at
         once by using a multicast switch ID in the destination field of
         the network layer addressing information (see below).  Delaying
         acknowledgments also randomizes the acknowledgment packets sent
         by the multiple switches attached to a multi-access network
         link.

         Note that the interval used to time delayed acknowledgments
         must be short (less than RxmtInterval) or needless
         retransmissions will ensue.

         Delayed acknowledgments are sent to all adjacent switches
         associated with the interface.  If the interface state is
         Point-to-Point, DS, or Backup, the destination field of the
         network layer addressing information is set to the multicast
         switch ID AllSPFSwitches.  If the interface state is DS Other,
         the destination field is set to the multicast switch ID
         AllDSwitches.

      o  Immediate acknowledgments are sent directly to a specific
         neighbor in response to the receipt of duplicate link state
         advertisements.  These acknowledgments are sent immediately
         when the duplicate is received.

      The method used to send a Link State Acknowledgment packet --
      either delayed or immediate -- depends on the circumstances
      surrounding the receipt of the advertisement, as shown in Table 6.
      Note that switches with an interface state of Backup send

Top      Up      ToC       Page 65 
      acknowledgments differently than other switches because they play
      a slightly different role in the distribution process (see Section
      8.2.3).

                                  Action taken in state
      Circumstances           Backup               Other states

      Advertisement was       No ack sent          No ack sent
      forwarded back out
      receiving interface

      Advertisement is        Delayed ack sent     Delayed ack
      more recent than        if advertisement     sent
      database copy, but      received from DS,
      was not forwarded       else do nothing
      back out receiving
      interface

      Advertisement was a     Delayed ack sent     No ack sent
      duplicate treated       if advertisement
      as an implied acknow-   received from DS,
      ledgment (step 4a of    else do nothing
      Section 8.2.2)

      Advertisement was a     Immediate ack        Immediate ack
      duplicate not treated   sent                 sent
      as an implied acknow-
      ledgment

      Advertisement age       Immediate ack        Immediate ack
      equal to MaxAge and     sent                 sent
      no current instance
      found in database

               Table 6: Sending Link State Acknowledgments

   Receiving an acknowledgment

      When the a Link State Acknowledgment packet is received, it is
      first subjected to a number of consistency checks.  In particular,
      the packet is associated with a specific neighbor. If the state of
      that neighbor is less than Exchange, the entire Link State
      Acknowledgment packet is discarded.

      Each acknowledgment contained in the packet is processed as
      follows:

Top      Up      ToC       Page 66 
      o  If the advertisement being acknowledged has an instance in the
         link state retransmission list for the sending neighbor, do the
         following:

         o  If the acknowledgment is for the same instance as that
            specified in the list (as determined by the procedure
            described in Section 7.1.1), remove the instance from the
            retransmission list.

         o  Otherwise, log the acknowledgment as questionable.

8.3 Aging the Link State Database

   Each link state advertisement has an age field, containing the
   advertisement's age, expressed in seconds.  When the advertisement is
   copied into a Link State Update packet for forwarding out a
   particular interface, the age is incremented by InfTransDelay seconds
   to account for the transmission time over the link.  An
   advertisement's age is never incremented past the value MaxAge.
   Advertisements with an age of MaxAge are not used to calculate best
   paths.

   If a link state advertisement's age reaches MaxAge, the switch
   flushes the advertisement from the switch fabric by doing the
   following:

   o  Originate a new instance of the advertisement with the age field
      set to MaxAge.  The distribution process will eventually result in
      the advertisement being removed from the retransmission lists of
      all switches in the fabric.

   o  Once the advertisement is no longer contained in the link state
      retransmission list of any neighbor and no neighbor is in a state
      of Exchange or Loading, remove  the advertisement from the local
      link state database.

8.3.1 Premature Aging of Advertisements

   A link state advertisement can be prematurely flushed from the switch
   fabric by forcing its age to MaxAge and redistributing the
   advertisement.

   A switch that was previously the designated switch for a multi-access
   network link but has lost that status due to a failover to the backup
   designated switch prematurely ages the network link advertisements it
   originated for the link.

Top      Up      ToC       Page 67 
   Premature aging also occurs when an advertisement's sequence number
   must wrap -- that is, when the current advertisement instance has a
   sequence number of 0x7fffffff.  In this circumstance, the
   advertisement is prematurely aged so that the next instance of the
   advertisement can be originated with a sequence number of 0x80000001
   and be recognized as the most recent instance.

   A switch may only prematurely age those link state advertisements for
   which it is the advertising switch.

9. Calculating the Best Paths

   Once an adjacency has been formed and the two switches have
   synchronized their databases, each switch in the adjacency calculates
   the best path(s) to all other switches in the fabric, using itself as
   the root of each path.  In this context, "best" path means that path
   with the lowest total cost metric across all hops.  If there are
   multiple paths with the same (lowest) total cost metric, they are all
   calculated.  Best paths are stored in the area data structure.

   Paths are calculated using the well-known Dijkstra algorithm. For a
   detailed description of this algorithm, the reader is referred to
   [Perlman], or any of a number of standard textbooks dealing with
   network routing.

   Note that whenever there is a change in an adjacency relationship, or
   any change that alters the topology of the switch fabric, the set of
   best paths must be recalculated.

10. Protocol Packets

   This section describes VLS protocol packets and link state
   advertisements.

Top      Up      ToC       Page 68 
   There are five distinct VLSP packet types, as listed in Table 7.

     Type  Packet Name       Function              Description

     1     Hello             Select DS/Backup DS   Section 10.6.1
     2     Database          Summarize database
             Description     contents              Section 10.6.2
     3     LS Request        Database download     Section 10.6.3
     4     LS Update         Database update       Section 10.6.4
     5     LS Ack            Flooding acknow-
                             ledgment              Section 10.6.5

                      Table 7: VLSP Packet Types


   All VLSP packets are encapsulated within a standard ISMP packet, with
   the VLS packet carried in the ISMP message body.  The ISMP packet is
   described in Section 10.1.

   Since it is important that the link state databases remain
   synchronized throughout the switch fabric, processing of both
   incoming and outgoing routing protocol packets should take priority
   over ordinary data packets.  Section 10.2 describes packet
   processing.

   All VLSP packets begin with network layer addressing information,
   described in Section 10.3, followed by a standard header, described
   in Section 10.4.

   With the exception of Hello packets, all VLSP packets deal with lists
   of link state advertisements.  The format of a link state
   advertisement is described in Section 11.

10.1 ISMP Packet Format

   All VLSP packets are encapsulated within a standard ISMP packet. ISMP
   packets are of variable length and have the following general
   structure:

   o  Frame header
   o  ISMP packet header
   o  ISMP message body

Top      Up      ToC       Page 69 
10.1.1 Frame Header

   ISMP packets are encapsulated within an IEEE 802-compliant frame
   using a standard header as 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 |                                                               |
      +      Destination address      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   04 |                               |                               |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+        Source address         +
   08 |                                                               |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   12 |             Type              |                               |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+                               +
   16 |                                                               |
      +                                                               +
      :                                                               :

   Destination address

      This 6-octet field contains the Media Access Control (MAC) address
      of the multicast channel over which all switches in the fabric
      receive ISMP packets.  The destination address of all ISMP packets
      contain a value of 01-00-1D-00-00-00.

   Source address

      This 6-octet field contains the physical (MAC) address of the
      switch originating the ISMP packet.

   Type

      This 2-octet field identifies the type of data carried within the
      frame.  The type field of ISMP packets contains the value 0x81FD.

Top      Up      ToC       Page 70 
10.1.2 ISMP Packet Header

   The ISMP packet header consists of 6 octets, as 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 |///////////////////////////////////////////////////////////////|
      ://////// Frame header /////////////////////////////////////////:
      +//////// (14 octets)  /////////+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   12 |///////////////////////////////|            Version            |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   16 |       ISMP message type       |        Sequence number        |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   20 |                                                               |
      +                                                               +
      :                                                               :


   Frame header

      This 14-octet field contains the frame header.

      Version

      This 2-octet field contains the version number of the InterSwitch
      Message Protocol to which this ISMP packet adheres.  This document
      describes ISMP Version 2.0.           ISMP message type

      This 2-octet field contains a value indicating which type of ISMP
      message is contained within the message body.  Valid values are as
      follows:

         1    (reserved)
         2    Interswitch Keepalive messages
         3    Interswitch Link State messages
         4    Interswitch Spanning Tree BPDU messages and
              Interswitch Remote Blocking messages
         5    Interswitch Resolve and New User messages
         6    (reserved)
         7    Tag-Based Flood messages
         8    Interswitch Tap messages

      All VLS protocol messages have an ISMP message type of 3.

Top      Up      ToC       Page 71 
   Sequence number

      This 2-octet field contains an internally generated sequence
      number used by the various protocol handlers for internal
      synchronization of messages.

10.1.3 ISMP Message Body

   The ISMP message body is a variable-length field containing the
   actual data of the ISMP message.  The length and content of this
   field are determined by the value found in the message type field.
   VLSP packets are contained in the ISMP message body.

10.2 VLSP Packet Processing

   Note that with the exception of Hello packets, VLSP packets are sent
   only between adjacent neighbors.  Therefore, all packets travel a
   single hop.

   VLSP does not support fragmentation and reassembly of packets.
   Therefore, packets containing lists of link state advertisements or
   advertisement headers must be formatted such that they contain only
   as many advertisements or headers as will fit within the size
   constraints of a standard ethernet frame.

   When a protocol packet is received by a switch, it must first pass
   the following criteria before being accepted for further processing:

   o  The checksum number must be correct.

   o  The destination switch ID (as found in the network layer address
      information) must be the switch ID of the receiving switch, or one
      of the multicast switch IDs AllSPFSwitches or AllDSwitches.

      If the destination switch ID is the multicast switch ID
      AllDSwitches, the state of the receiving interface must be Point-
      to-Point, DS, or Backup.

   o  The source switch ID (as found in the network layer address
      information) must not be that of the receiving switch.  (That is,
      locally originated packets should be discarded.)

   At this point, if the packet is a Hello packet, it is accepted for
   further processing.

Top      Up      ToC       Page 72 
   Since all other packet types are only sent between adjacent
   neighbors, the packet must have been sent by one of the switch's
   active neighbors.  If the source switch ID matches the switch ID of
   one of the receiving switch's active neighbors (as stored in the
   interface data structure associated with the inport interface), the
   packet is accepted for further processing.  Otherwise, the packet is
   discarded.

10.3 Network Layer Address Information

   As mentioned in Section 2.2.1, portions 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 VSLP packets.  This
   information immediately follows the ISMP frame and packet header and
   immediately precedes the VLSP packet header, as 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
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                                                               |
      :                  frame header / ISMP header                   :
      |                                                               |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   00 |                                                               |
      :                      Unused (20 octets)                       :
      |                                                               |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   20 |                                                               |
      +                       Source switch ID                        +
   24 |                                                               |
      +                               +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   28 |                               |                               |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+                               +
   32 |                                                               |
      +                     Destination switch ID                     +
   36 |                                                               |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   40 |                                                               |
      :                          VLSP header                          :
      |                                                               |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Top      Up      ToC       Page 73 
   Source switch ID

      This 10-octet field contains the switch ID of the sending switch.

   Destination switch ID

      This 10-octet field contains the switch ID of the packet
      destination.  The value here is set as follows:

      o  Hello packets are addressed to the multicast switch ID
         AllSPFSwitches.

      o  The designated switch and the backup designated switch address
         initial Link State Update packets and Link State Acknowledgment
         packets to the multicast switch ID AllSPFSwitches.

      o  All other switches address initial Link State Update packets
         and Link State Acknowledgment packets to the multicast switch
         ID AllDSwitches.

      o  Retransmissions of Link State Update packets are always
         addressed directly to the nonresponding switch.

      o  Database Description packets and Link State Request are always
         addressed directly to the other switch participating in the
         database exchange process.

   VLSP header

      This 30-octet field contains the VLSP standard header.  See
      Section 10.4.

10.4 VLSP Packet Header

   Every VLSP packet starts with a common 30-octet header.  This header,
   along with the data found in the network layer address information,
   contains all the data necessary to determine whether the packet
   should be accepted for further processing. (See Section 10.1.)

   The format of the VLSP header is shown below.  Note that the header
   starts at offset 36 of the ISMP message body, following the network
   layer address information.

Top      Up      ToC       Page 74 
        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
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                                                               |
      :                  frame header / ISMP header                   :
      |                                                               |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   00 |                                                               |
      :               Network layer address information               :
      |                                                               |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   40 |    (unused)   |     Type      |         Packet length         |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   44 |                                                               |
      +                       Source switch ID                        +
   48 |                                                               |
      +                               +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   52 |                               |         Area ID . . .         |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   56 |         Area ID . . .         |           Checksum            |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   60 |            Autype             |                               |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+        Authentication         +
   64 |                                                               |
      +                               +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   68 |                               |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+


   Type

      This 1-octet field contains the packet type.  Possible values are
      as follows:

         1   Hello
         2   Database Description
         3   Link State Request
         4   Link State Update
         5   Link State Acknowledgment

   Packet length

      This 2-octet field contains the length of the protocol packet, in
      bytes, calculated from the start of the VLSP header, at offset 20
      of the ISMP message body.  If the packet length is not an integral
      number of 16-bit words, the packet is padded with an octet of zero
      (see the description of the checksum field, below).

Top      Up      ToC       Page 75 
   Switch ID

      This 10-octet field contains the switch ID of the sending switch.

   Area ID

      This 4-octet field contains the area identifier.  Since VLSP does
      not support multiple areas, the value here is always zero.

   Checksum

      This 2-octet field contains the packet checksum value.  The
      checksum is calculated as the 16-bit one's complement of the one's
      complement sum of all the 16-bit words in the packet, beginning
      with the VLSP header, excluding the authentication field.  If the
      packet length is not an integral number of 16-bit words, the
      packet is padded with an octet of zero before calculating the
      checksum.

   AuType

      This 2-octet field identifies the authentication scheme to be used
      for the packet.  Since authentication is not supported by this
      version of VLSP, this field contains zero.

   Authentication

      This 8-octet field is reserved for use by the authentication
      scheme.  Since authentication is not supported by this version of
      VLSP, this field contains zeroes.

10.5 Options Field

   Hello packets and Database Description packets, as well as link state
   advertisements, contain a 1-octet options field.  Using this field, a
   switch can communicate its optional capabilities to other VLSP
   switches.  The receiving switch can then choose whether or not to
   support those optional capabilities.  Thus, switches of differing
   capabilities potentially can be mixed within a single VLSP routing
   domain.

   Two optional capabilities are currently defined in the options field:
   routing based on Type of Service (TOS) and support for external
   routing beyond the local switch fabric.  These two capabilities are
   specified in the options field as shown below.

Top      Up      ToC       Page 76 
                             +-+-+-+-+-+-+-+-+
                             |0|0|0|0|0|0|E|T|
                             +-+-+-+-+-+-+-+-+

                             The options field

   T-bit

      The T-bit specifies the switch's Type of Service (TOS) capability.
      If the T-bit is set, the switch supports routing based on nonzero
      types of service.

   E-bit

      The E-bit specifies the switch's external routing capability. If
      the E-bit is set, the switch supports external routing.

   Note:  The current version of VLSP supports neither of these
   capabilities.  Therefore, both the T-bit and the E-bit are clear and
   the options field contains a value of zero.

10.6 Packet Formats

   This section contains detailed descriptions of the five VLS protocol
   packets.

10.6.1 Hello Packets

   Hello packets are sent periodically over multi-access switch
   interfaces in order to discover and maintain neighbor relationships.

   Note:  Hello packets are not sent over point-to-point network links.
   For point-to-point links, the VLS protocol relies on the VlanHello
   protocol [IDhello] to notify it of neighboring switches.

   Since all switches connected to a common network link must agree on
   certain interface parameters, these parameters are included in each
   Hello packet.  A switch receiving a Hello packet that contains
   parameters inconsistent with its own view of the interface will not
   establish a neighbor relationship with the sending switch.

   The format of a Hello packet is shown below.

Top      Up      ToC       Page 77 
        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 |                                                               |
       :              Network layer addressing / VLSP header           :
       |                                                               |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    70 |                      (unused -- must be 0)                    |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    74 |         HelloInt              |    Options    |   Priority    |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    78 |                            DeadInt                            |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    82 |                                                               |
       +                      Designated switch ID                     +
    86 |                                                               |
       +                               +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    90 |                               |                               |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+                               +
    94 |                                                               |
       +                   Backup designated switch ID                 +
    98 |                                                               |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   102 |                                                               |
       +                                                               +
       :                          Neighbor list                        :
       +                                                               +
       |                                                               |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Network layer addressing / VLSP header

      This 70-octet field contains the network layer addressing
      information and the standard VLS protocol packet header.  The
      packet header type field contains a value of 1.

   HelloInt

      This 2-octet field contains the interval, in seconds, at which
      this switch sends Hello packets.

   Options

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

Top      Up      ToC       Page 78 
   Priority

      This 1-octet field contains the switch priority used in selecting
      the designated switch and backup designated switch (see Section
      6.3.1).  If the value here is zero, the switch is ineligible to
      become the designated switch or the backup designated switch.

   DeadInt

      This 4-octet field contains 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.  The
      value here is equal to the value of SwitchDeadInterval, as found
      in the interface data structure.

   Designated switch

      This 10-octet field contains the switch ID of the designated
      switch for this network link, as currently understood by the
      sending switch.  This value is set to zero if the designated
      switch selection process has not yet begun.

   Backup designated switch

      This 10-octet field contains the switch ID of the backup
      designated switch for the network link, as currently understood by
      the sending switch.  This value is set to zero if the backup
      designated switch selection process has not yet begun.

   Neighbor list

      This variable-length field contains a list of switch IDs of each
      switch from which the sending switch has received a valid Hello
      packet within the last SwitchDeadInterval seconds.

10.6.2 Database Description Packets

   Database Description packets are exchanged while an adjacency is
   being formed between two neighboring switches and are used to
   describe the contents of the topological database.  For a complete
   description of the database exchange process, see Section 7.2.

   The format of a Database Description packet is shown below.

Top      Up      ToC       Page 79 
       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 |                                                               |
      :              Network layer addressing / VLSP header           :
      |                                                               |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   70 |     (unused -- must be 0)     |    Options    |     Flags     |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   74 |                        Sequence number                        |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   78 |                                                               |
      +                                                               +
      :                 Link state advertisement headers              :
      +                                                               +
      |                                                               |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+


   Network layer addressing / VLSP header

      This 70-octet field contains the network layer addressing
      information and the standard VLS protocol packet header.  The
      packet header type field contains a value of 2.

   Options

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

   Flags

      This 1-octet field contains a set of bit flags that are used to
      coordinate the database exchange process.  The format of this
      octet is as follows:

                          +-+-+-+-+-+-+-+-+
                          |0|0|0|0|0|I|M|MS
                          +-+-+-+-+-+-+-+-+

   I-bit (Init)

      The I-bit is used to signal the start of the exchange.  It is set
      while the two switches negotiate the master/slave relationship and
      the starting sequence number.

Top      Up      ToC       Page 80 
   M-bit (More)

      The M-bit is set to indicate that more Database Description
      packets to follow.

   MS-bit (Master/Slave)

      The MS-bit is used to indicate which switch is the master of the
      exchange.  If the bit is set, the sending switch is the master
      during the database exchange process.  If the bit is clear, the
      switch is the slave.

   Sequence number

      This 4-octet field is used to sequence the Database Description
      packets during the database exchange process.  The two switches
      involved in the exchange process agree on the initial value of the
      sequence number during the master/slave negotiation.  The number
      is then incremented for each Database Description packet in the
      exchange.

      To acknowledge each Database Description packet sent by the
      master, the slave sends a Database Description packet that echoes
      the sequence number of the packet being acknowledged.

   Link state advertisement headers

      This variable-length field contains a list of link state headers
      that describe a portion of the master's topological database.
      Each header uniquely identifies a link state advertisement and its
      current instance.  (See Section 11.1 for a detailed description of
      a link state advertisement header.)  The number of headers
      included in the list is calculated implicitly from the length of
      the packet, as stored in the VLSP packet header (see Section
      10.4).

10.6.3 Link State Request Packets

   Link State Request packets are used to request those pieces of the
   neighbor's database that the sending switch has discovered (during
   the database exchange process) are more up-to-date than instances in
   its own database.  Link State Request packets are sent as the last
   step in bringing up an adjacency.  (See Section 7.3.)

   The format of a Link State Request packet is shown below.

Top      Up      ToC       Page 81 
       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 |                                                               |
      :              Network layer addressing / VLSP header           :
      |                                                               |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   70 |                        Link state type                        |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   74 |                                                               |
      +                         Link state ID                         +
   88 |                                                               |
      +                               +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   82 |                               |                               |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+                               +
   86 |                                                               |
      +                      Advertising switch ID                    +
   90 |                                                               |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   94 |                                                               |
      :                            . . .                              :
      |                                                               |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+


   Network layer addressing / VLSP header

      This 70-octet field contains the network layer addressing
      information and the standard VLS protocol packet header.  The
      packet header type field contains a value of 3.

   Link state type

      This 4-octet field contains the link state type of the requested
      link state advertisement, as stored in the advertisement header.

   Link state ID

      This 10-octet field contains the link state ID of the requested
      link state advertisement, as stored in the advertisement header.

   Advertising switch

      This 10-octet field contains the switch ID of advertising switch
      for the requested link state advertisement, as stored in the
      advertisement header.

Top      Up      ToC       Page 82 
      Note that the last three fields uniquely identify the
      advertisement, but not its instance.  The receiving switch will
      respond with its most recent instance of the specified
      advertisement.

      Multiple link state advertisements can be requested in a single
      Link State Request packet by repeating the link state type, ID,
      and advertising switch for each requested advertisement.  The
      number of advertisements requested is calculated implicitly from
      the length of the packet, as stored in the VLSP packet header.

10.6.4 Link State Update Packets

   Link State Update packets are used to respond to a Link State Request
   packet or to advertise a new instance of one or more link state
   advertisements.  Link State Update packets are acknowledged with Link
   State Acknowledgment packets.  For more information on the use of
   Link State Update packets, see Section 7 and Section 8.

   The format of a Link State Update packet 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 |                                                               |
      :              Network layer addressing / VLSP header           :
      |                                                               |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   70 |                        # advertisements                       |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   74 |                                                               |
      +                                                               +
      :                    Link state advertisements                  :
      +                                                               +
      |                                                               |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Network layer addressing / VLSP header

      This 70-octet field contains the network layer addressing
      information and the standard VLS protocol packet header.  The
      packet header type field contains a value of 4.

   # advertisements

      This 4-octet field contains the number of link state
      advertisements included in the packet.

Top      Up      ToC       Page 83 
   Link state advertisements

      This variable-length field contains a list of link state
      advertisements.  For a detailed description of the different types
      of link state advertisements, see Section 11.

10.6.5 Link State Acknowledgment Packets

   Link State Acknowledgment Packets are used to explicitly acknowledge
   one or more Link State Update packets, thereby making the
   distribution of link state advertisements reliable.  (See Section
   8.2.6.)

   The format of a Link State Acknowledgment packet 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 |                                                               |
      :              Network layer addressing / VLSP header           :
      |                                                               |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   70 |                                                               |
      +                                                               +
      :                 Link state advertisement headers              :
      +                                                               +
      |                                                               |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Network layer addressing / VLSP header

      This 70-octet field contains the network layer addressing
      information and the standard VLS protocol packet header.  The
      packet header type field contains a value of 5.

   Link state advertisement headers

      This variable-length field contains a list of link state headers
      that are being acknowledged by this packet.  Each header uniquely
      identifies a link state advertisement and its current instance.
      (See Section 11.1 for a detailed description of a link state
      advertisement header.)  The number of headers included in the list
      is calculated implicitly from the length of the packet, as stored
      in the VLSP packet header (see Section 10.4).


Next RFC Part