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RFC2281

Cisco Hot Standby Router Protocol (HSRP)

The memo specifies the Hot Standby Router Protocol (HSRP). The goal of the protocol is to allow hosts to appear to use a single router and to maintain connectivity even if the actual first hop router they are using fails. This memo provides information for the Internet community. It does not specify an Internet standard of any kind.

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RFC 2281                       Cisco HSRP                     March 1998


         0 - Initial
         1 - Learn
         2 - Listen
         4 - Speak
         8 - Standby
        16 - Active

   Hellotime:  1 octet

      This field is only meaningful in Hello messages.  It contains the
      approximate period between the Hello messages that the router
      sends.  The time is given in seconds.

      If the Hellotime is not configured on a router, then it MAY be
      learned from the Hello message from the active router.  The
      Hellotime SHOULD only be learned if no Hellotime is configured and
      the Hello message is authenticated.  A router that sends a Hello
      message MUST insert the Hellotime that it is using in the
      Hellotime field in the Hello message.  If the Hellotime is not
      learned from a Hello message from the active router and it is not
      manually configured, a default value of 3 seconds is RECOMMENDED.

   Holdtime:  1 octet

      This field is only meaningful in Hello messages.  It contains the
      amount of time that the current Hello message should be considered
      valid.  The time is given in seconds.

      If a router sends a Hello message, then receivers should consider
      that Hello message to be valid for one Holdtime.  The Holdtime
      SHOULD be at least three times the value of the Hellotime and MUST
      be greater than the Hellotime.  If the Holdtime is not configured
      on a router, then it MAY be learned from the Hello message from
      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.

Li, et. al.                  Informational                      [Page 6]
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