rfc2281.txt

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      the active router.  The Holdtime SHOULD only be learned if the
      Hello message is authenticated.  A router that sends a Hello
      message MUST insert the Holdtime that it is using in the Holdtime
      field in the Hello message.

      A router which is in active state MUST NOT learn new values for
      the Hellotime and the Holdtime from other routers, although it may
      continue to use values which it learned from the previous active
      router.  It MAY also use the Hellotime and Holdtime values learned
      through manual configuration.  The active router MUST NOT use one
      configured time and one learned time.  If the Holdtime is not
      learned and it is not manually configured, a default value of 10
      seconds is RECOMMENDED.





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RFC 2281                       Cisco HSRP                     March 1998


   Priority:  1 octet

      This field is used to elect the active and standby routers.  When
      comparing priorities of two different routers, the router with the
      numerically higher priority wins.  In the case of routers with
      equal priority the router with the higher IP address wins.

   Group:   1 octet

      This field identifies the standby group.  For Token Ring, values
      between 0 and 2 inclusive are valid.  For other media values
      between 0 and 255 inclusive are valid.

   Authentication Data:    8 octets

      This field contains a clear-text 8 character reused password.

      If no authentication data is configured, the RECOMMENDED default
      value is 0x63 0x69 0x73 0x63 0x6F 0x00 0x00 0x00.

   Virtual IP Address:     4 octets

      The virtual IP address used by this group.

      If the virtual IP address is not configured on a router, then it
      MAY be learned from the Hello message from the active router.  An
      address SHOULD only be learned if no address was configured and
      the Hello message is authenticated.

5.2  Operational parameters

   The following information MUST be known to each router in the standby
   group.  The mechanisms used to determine this information are outside
   of the scope of this document.

      Standby group number

      Virtual MAC address

      Priority

      Authentication Data

      Hellotime

      Holdtime





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RFC 2281                       Cisco HSRP                     March 1998


   The following information MUST be known to at least one router in
   each standby group and MAY be known by any of the other routers in
   the group.

      Virtual IP Address

   The following information MAY be configured on any router:

      Preemption capability

         If a router has higher priority than the active router and
         preemption is configured, it MAY take over as the active router
         using a Coup message.

5.3 States

   Each router in the group participates in the protocol by implementing
   a simple state machine.  This specification describes the externally
   visible behavior of this state machine.  Implementations MAY vary
   their internal implementations within the functional description of
   the state machine.

   All routers begin in the Initial state.  This section discusses the
   intent of each state.  For specific details on the actions taken in
   each state, please see the state transition table in section 5.7.

   1. Initial

      This is the starting state and indicates that HSRP is not running.
      This state is entered via a configuration change or when an
      interface first comes up.

   2. Learn

      The router has not determined the virtual IP address, and not yet
      seen an authenticated Hello message from the active router.  In
      this state the router is still waiting to hear from the active
      router.

   3. Listen

      The router knows the virtual IP address, but is neither the active
      router nor the standby router.  It listens for Hello messages from
      those routers.







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RFC 2281                       Cisco HSRP                     March 1998


   4. Speak

      The router sends periodic Hello messages and is actively
      participating in the election of the active and/or standby router.
      A router cannot enter Speak state unless it has the virtual IP
      address.

   5. Standby

      The router is a candidate to become the next active router and
      sends periodic Hello messages.  Excluding transient conditions,
      there MUST be at most one router in the group in Standby state.

   6. Active

      The router is currently forwarding packets that are sent to the
      group's virtual MAC address.  The router sends periodic Hello
      messages.  Excluding transient conditions, there MUST be at most
      one router in Active state in the group.

5.4 Timers

   Each router maintains three timers, an Active timer, a Standby timer,
   and a Hello timer.

   The Active timer is used to monitor the active router.  The active
   timer is started anytime an authenticated Hello message is seen from
   the active router.  It is set to expire in the Holdtime seen in the
   Hello message.

   The Standby timer is used to monitor the standby router The Standby
   timer is started anytime an authenticated Hello message is seen from
   the standby router.  It is set to expire in the Holdtime seen in the
   Hello message.

   The Hello timer expires once per Hellotime period.  If the router is
   in Speak, Standby, or Active states, it should generate a Hello
   message upon Hello timer expiry.  The Hello timer MUST be jittered.

5.5 Events

   These are the events in the HSRP finite state machine.

      a - HSRP is configured on an enabled interface.

      b - HSRP is disabled on an interface or the interface is disabled.





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RFC 2281                       Cisco HSRP                     March 1998


      c - Active timer expiry.  The Active timer was set to the Holdtime
      when the last Hello message was seen from the active router.

      d - Standby timer expiry.  The Standby timer was set to the
      Holdtime when the last Hello message was seen from the standby
      router.

      e - Hello timer expiry.  The periodic timer for sending Hello
      messages has expired.

      f - Receipt of a Hello message of higher priority from a router in
      Speak state.

      g - Receipt of a Hello message of higher priority from the active
      router.

      h - Receipt of a Hello message of lower priority from the active
      router.

      i - Receipt of a Resign message from the active router.

      j - Receipt of a Coup message from a higher priority router.

      k - Receipt of a Hello message of higher priority from the standby
      router.

      l - Receipt of a Hello message of lower priority from the standby
      router.

5.6 Actions

   This section specifies the actions to be taken as part of the state
   machine.

      A  Start Active Timer
         If this action occurred as the result of the receipt of a an
         authenticated Hello message from the active router, the Active
         timer is set to the Holdtime field in the Hello message.
         Otherwise the Active timer is set to the current Holdtime value
         in use by this router.  The Active timer is then started.

      B  Start Standby Timer
         If this action occurred as the result of the receipt of an
         authenticated Hello message from the standby router, the
         Standby timer is set to the Holdtime field in the Hello
         message.  Otherwise the Standby timer is set to the current
         hold time value in use by this router.  The Standby timer is
         then started.



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RFC 2281                       Cisco HSRP                     March 1998


      C  Stop Active Timer
         The Active timer is stopped.

      D  Stop Standby Timer
         The Standby timer is stopped.

      E  Learn Parameters
         This action is taken when an authenticated message is received
         from the active router.  If the virtual IP address for this
         group was not manually configured, the virtual IP address MAY
         be learned from the message.  The router MAY learn Hellotime
         and Holdtime values from the message.

      F  Send Hello Message
         The router sends a Hello message with its current State,
         Hellotime and Holdtime.

      G  Send Coup Message
         The router sends a Coup message to inform the active router
         that there is a higher priority router available.

      H  Send Resign Message
         The router sends a Resign message to allow another router to
         become the active router.

      I  Send Gratuitous ARP Message
         The router broadcasts an ARP response packet advertising the
         group's virtual IP address and virtual MAC address.  The packet
         is sent using the virtual MAC address as the source MAC address
         in the link layer header, as well as within the ARP packet.

5.7 State Transitions

   This table describes the state transitions of the state machine.  For
   each event and current state of the router, the router MUST perform
   the set of actions specified and transition to the designated state.
   If no action is specified, no action should be taken.  If no state
   change is specified, no state change should be performed.

   The notation used in this table has the specified set of actions
   listed as letters corresponding to the actions listed in section 5.6.
   The next state is listed as a number as specified in section 5.3.  A
   slash ('/') separates the actions and states.  Certain state
   transitions have alternatives which depend on external state.
   Alternatives are separated by a '|'.  See the attached notes for
   details on these transitions.





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RFC 2281                       Cisco HSRP                     March 1998


                                                 States
+-----+----------+----------+----------+----------+----------+----------+
|     |    1     |     2    |    3     |    4     |    5     |     6    |
|     |  Initial |  Learn   |  Listen  |  Speak   |  Standby |   Active |
+-----+----------+----------+----------+----------+----------+----------+
|Event|                                                                 |
+-----+----------+----------+----------+----------+----------+----------+
|  a  |  AB/2|3+ |          |          |          |          |          |
+-----+----------+----------+----------+----------+----------+----------+
|  b  |          |   CD/1   |   CD/1   |   CD/1   |   CD/1   |   CDH/1  |
+-----+----------+----------+----------+----------+----------+----------+
|  c  |          |          |   AB/4   |          |  CDFI/6  |          |
+-----+----------+----------+----------+----------+----------+----------+
|  d  |          |          |   B/4    |   D/5    |          |          |
+-----+----------+----------+----------+----------+----------+----------+
|  e  |          |          |          |    F     |    F     |    F     |