rfc2642.txt

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   Adjacency

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

   Link state advertisement

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

   Designated switch

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

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

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

   Backup designated switch

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

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

2.2 Differences Between VLSP and OSPF

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






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2.2.1 Operation at the Physical Layer

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

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

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

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

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

2.2.2 All Links Treated as Point-to-Point

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

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





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

2.2.3 Routing Path Information

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

2.2.4 Configurable Parameters

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

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

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

   o  Authentication is not supported.

2.2.5 Features Not supported

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

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

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

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



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2.3 Functional Summary

   There are essentially four operational stages of the VLS protocol.

   o  Discovery Process The discovery process involves two steps:

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

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

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

   o  Synchronizing the Databases

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

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

   o  Maintaining the Databases

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

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










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   o  Calculating the Best Paths

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

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

2.4 Protocol Packets

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

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

      Type   Packet Name            Protocol Function

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

                  Table 1: VLSP Packet Types

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

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











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         LS     Advertisement    Advertisement Description
         Type   Name

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

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

                  Table 2: VLSP Link State Advertisements

2.5 Protocol Data Structures

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

         Structure Name                        Description

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

                     Table 3: VLSP Data Structures

2.6 Basic Implementation Requirements

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

   Timers

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

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



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

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