Imported debug from /usr/lib/site-python/debug.pyc draft-li-hsrp-00 - Cisco Hot Standby Router Protocol (HSRP)
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Versions: 00 RFC 2281

Network Working Group                                              T. Li
INTERNET DRAFT                                          Juniper Networks
<draft-li-hsrp-00.txt>                                           B. Cole
                                                        Juniper Networks
                                                               P. Morton
                                                           Cisco Systems
                                                                   D. Li
                                                           Cisco Systems



                   Hot Standby Router Protocol (HSRP)


Status of this Memo

   This document is an Internet-Draft.  Internet-Drafts are working
   documents of the Internet Engineering Task Force (IETF), its areas,
   and its working groups.  Note that other groups may also distribute
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   Rim).  Distribution of this memo is unlimited.

Abstract

   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.  Multiple routers participate in this protocol and
   in concert create the illusion of a single virtual router.  The
   protocol insures that one and only one of the routers is forwarding
   packets on behalf of the virtual router.  End hosts forward their
   packets to the virtual router.

   The router forwarding packets is known as the active router.  A
   standby router is selected to replace the active router should it
   fail. The protocol provides a mechanism for determining active and
   standby routers, using the IP addresses on the participating routers.
   If an active router fails a standby router can take over without a
   major interruption in the host's connectivity.  This memo also
   discusses the ARP, MAC address, and security issues with this
   protocol.

TABLE OF CONTENTS

   1   Introduction
   2   Conditions of Use
   3   Scope
   3.1 Terminology
   4   Definitions
   5   Protocol
   5.1 Packet formats
   5.2 Operational parameters
   5.3 States
   5.4 Timers
   5.5 Events
   5.6 Actions
   5.7 State Transitions
   6   MAC address considerations
   6.1 General
   6.2 Address Filter
   6.3 ICMP Redirect
   6.4 Proxy ARP
   7   Security Considerations
   8   References
   9   Authors' Addresses


1. Introduction

   The Hot Standby Router Protocol, HSRP, provides a mechanism which is
   designed to support non-disruptive failover of IP traffic in certain
   circumstances.  In particular, the protocol protects against the
   failure of the first hop router when the source host cannot learn the
   IP address of the first hop router dynamically.  The protocol is
   designed for use over multi-access, multicast or broadcast capable
   LANs (e.g., Ethernet).  HSRP is not intended as a replacement for
   existing dynamic router discovery mechanisms and those protocols
   should be used instead whenever possible. [1]  A large class of
   legacy host implementations that do not support dynamic discovery are
   capable of configuring a default router.  HSRP provides failover
   services to those hosts.

   All of the routers participating in HSRP are assumed to be running
   appropriate IP routing protocols and have a consistent set of routes.
   The discussion of which protocols are appropriate and whether routing
   is consistent in any given situation is beyond the scope of this
   specification.

   Using HSRP, a set of routers work in concert to present the illusion
   of a single virtual router to the hosts on the LAN.  This set is
   known as an HSRP group or a standby group.  A single router elected
   from the group is responsible for forwarding the packets that hosts
   send to the virtual router.  This router is known as the active
   router.  Another router is elected as the standby router.  In the
   event that the active router fails, the standby assumes the packet
   forwarding duties of the active router.  Although an arbitrary number
   of routers may run HSRP, only the active router forwards the packets
   sent to the virtual router.

   To minimize network traffic, only the active and the standby routers
   send periodic HSRP messages once the protocol has completed the
   election process.  If the active router fails, the standby router
   takes over as the active router.  If the standby router fails or
   becomes the active router, another router is elected as the standby
   router.

   On a particular LAN, multiple hot standby groups may coexist and
   overlap.  Each standby group emulates a single virtual router.  For
   each standby group, a single well-known MAC address is allocated to
   the group, as well as an IP address.  The IP address SHOULD belong to
   the primary subnet in use on the LAN, but MUST differ from the
   addresses allocated as interface addresses on all routers and hosts
   on the LAN, including virtual IP addresses assigned to other HSRP
   groups.

   If multiple groups are used on a single LAN, load splitting can be
   achieved by distributing hosts among different standby groups.

   The remainder of this specification discusses the operation of a
   single standby group.  In the case of multiple groups, each group
   operates independently of other groups on the LAN and according to
   this specification.  Note that individual routers may participate in
   multiple groups.  In this case, the router maintains separate state
   and timers for each group.

2  Conditions of Use

   US Patent number 5,473,599 [2], assigned to Cisco Systems, Inc. may
   be applicable to HSRP.  If an implementation requires the use of any
   claims of patent no. 5,473,599, Cisco will license such claims on
   reasonable, nondiscriminatory terms for use in practicing the
   standard.  More specifically, such license will be available for a
   one-time, paid up fee.

3  Scope

   This document describes the packets, messages, states, and events
   used to implement the protocol.  It does not discuss network
   management or internal implementation issues.

3.1   Terminology

   The following language conventions are used in this document:

      MUST, SHALL, or MANDATORY -- the item is an absolute requirement
      of the specification.

      SHOULD or RECOMMENDED -- the item should generally be followed for
      all but exceptional circumstances.

      MAY or OPTIONAL -- the item is truly optional and may be followed
      or ignored according to the needs of the implementation.

4  Definitions

      Active Router           - the router that is currently forwarding packets
                                for the virtual router

      Standby Router          - the primary backup router

      Standby Group           - the set of routers participating in HSRP that
                                jointly emulate a virtual router

      Hello Time              - the interval between successive HSRP Hello
                                messages from a given router

      Hold Time               - the interval between the receipt of a Hello
                                message and the presumption that the sending
                                router has failed

5  Protocol

      Within a standby group, the routers periodically advertise state
      information using various messages.

5.1  Packet formats

      The standby protocol runs on top of UDP, and uses port number
      1985.  Packets are sent to multicast address 224.0.0.2 with TTL 1.

      Routers use their actual IP address as the source address for
      protocol packets, not the virtual IP address.  This is necessary
      so that the HSRP routers can identify each other.

      The format of the data portion of the UDP datagram is:

                          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
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |   Version     |   Op Code     |     State     |   Hellotime   |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |   Holdtime    |   Priority    |     Group     |   Reserved    |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                      Authentication  Data                     |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                      Authentication  Data                     |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                      Virtual IP Address                       |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

      Version:  1 octet

         The version of the HSRP messages.  This document describes
         version 0.

      Op Code:  1 octet

         The Op Code describes the type of message contained in this
         packet.  Possible values are:

            0 - Hello
            1 - Coup
            2 - Resign

         Hello messages are sent to indicate that a router is running
         and is capable of becoming the active or standby router.

         Coup messages are sent when a router wishes to become the
         active router.

         Resign messages are sent when a router no longer wishes to be
         the active router.

      State:  1 octet

         Internally, each router in the standby group implements a state
         machine.  The State field describes the current state of the
         router sending the message.  Details on the individual states
         are described below.  Possible values are:

             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.

      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 are valid.  For other media values
         between 0 and 255 are valid.

      Authentication Data:    8 octets

         This field is used to authenticate HSRP messages.  If the
         authentication data matches a corresponding value configured in
         the router, then the message is considered authentic. The
         authentication is only used to validate dynamic acquisition of
         the virtual IP address and timer values from an active router.

         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

      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.

   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.

      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.

      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.














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.



                                                   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     |
   +-----+----------+----------+----------+----------+----------+----------+
   |  f  |          |          |          |   B/3    |   B/3    |          |
   +-----+----------+----------+----------+----------+----------+----------+
   |  g  |          |   EAB/3  |    EA    |   EA     |   EA     |   AB/4   |
   +-----+----------+----------+----------+----------+----------+----------+
   |  h  |          |   EAB/3  | A|BGFI/6*| A|BGFI/6*| A|BGFI/6*|    G     |
   +-----+----------+----------+----------+----------+----------+----------+
   |  i  |          |          |   AB/4   |   A      |  CFI/6   |          |
   +-----+----------+----------+----------+----------+----------+----------+
   |  j  |          |          |          |          |          |   ABH/4  |
   +-----+----------+----------+----------+----------+----------+----------+
   |  k  |          |          |    B     |   B/3    |  B/3     |    B     |
   +-----+----------+----------+----------+----------+----------+----------+
   |  l  |          |          |    B/4   |   D/5    |          |    B     |
   +-----+----------+----------+----------+----------+----------+----------+

   Notes

   +   If the virtual IP address is configured, set state 3 (Listen) If
   the virtual IP address is not configured, set state 2 (Learn).  In
   either case do actions A and B.

   *   If the router is configured to preempt do actions B, G, F, and I
   and set state to 6 (Active).  If the router is not configured to
   preempt do actions A with no state change.

6  MAC Address Considerations

6.1 General

      Each HSRP group has an associated well known virtual MAC address.
      On token ring networks, these addresses are actually functional
      addresses.  The three addresses 0xC0 0x00 0x00 0x01 0x00 0x00,
      0xC0 0x00 0x00 0x02 0x00 0x00, and 0xC0 0x00 0x00 0x04 0x00 0x00
      correspond to groups 0, 1, and 2 respectively.

      On other media, the virtual MAC addresses are 0x00 0x00 0x0C 0x07
      0xAC XX where XX represents the HSRP group number.  Routers which
      implement HSRP SHOULD use well-known HSRP MAC addresses as the
      group's virtual MAC address whenever possible.

      The active router MUST accept and forward traffic that is destined
      for the group's virtual MAC address.  It MUST stop accepting or
      forwarding such traffic when the router leaves the Active state.

      If and only if the router is in the Active state, the router MUST
      use the group's virtual MAC address as the source MAC address for
      its Hello messages.  This is necessary in order to allow learning
      bridges to be able to determine which LAN segment the virtual MAC
      address currently belongs to.

      For each group, there is one virtual IP address and one virtual
      MAC address.  This is a desirable situation, since the ARP table
      entries in the end stations do not need to change over time as the
      HSRP active router moves from one router to another.

      Additionally, for HSRP to work in bridging environments, the
      bridges must be able to quickly update themselves as the virtual
      MAC address "moves".  Although learning bridges typically are able
      to do this, some have been known to have problems with this.  It
      is RECOMMENDED that only true learning bridges be used with HSRP.

      The movement of the virtual MAC address can cause further
      undesirable side effects in environments where additional state is
      tied to the MAC address.  For example on Token Ring, if Source
      Route Bridging is in use, a RIF will be stored with the virtual
      MAC address in a host's RIF cache.  The RIF indicates the path and
      final ring used to reach the MAC address.  As routers transition
      into Active state, they will not be able to affect the RIF caches
      on the hosts on the bridged ring.  This may lead to packets being
      bridged to the ring for the previous active router.

      In such circumstances, a router MAY use its normal MAC addresses
      as the virtual MAC address.  This method of operation is strongly
      discouraged.  In this mode, the virtual IP address will map to a
      different MAC address over time.  This can create problems for end
      stations, since ARP tables assume a relatively static mapping
      between MAC address and IP address.  These ARP tables are normally
      updated when the end stations receive the gratuitous ARP responses
      generated by a router that enters the active state.

6.2 Address Filter

      As noted, routers currently emulating a virtual router adopt their
      group's MAC and IP addresses.  MAC addresses are typically
      provided in an address filter or 'list' of MAC addresses in a
      router's interface controller.  It is desirable for routers to be
      able to add one or more virtual MAC addresses to their
      controllers' MAC address filter while maintaining their primary
      MAC addresses.

      Unfortunately, some interface controllers support address
      filtering for only one unicast MAC address.  Or, in the case of
      Token Ring, the functional address which HSRP should use is
      already in use for some other protocol.  In these cases, such
      routers can still implement HSRP, but the protocol must change the
      interface's primary MAC address when assuming or relinquishing
      control as the active router.

      This is potentially problematic because some traffic may otherwise
      wish to use the router's primary MAC address.  However, the
      problem MAY be mitigated by having the router send out gratuitous
      ARP packets regarding its non-HSRP IP addresses.  Through this,
      other network entities using IP should update their ARP tables to
      reflect that the router is now using a group virtual MAC address
      rather than its primary MAC address.

      Some protocols may not be able to run simultaneously with the
      standby protocol due to the interface primary MAC address change.
      For example, DECnet phase IV and HSRP will not be able to run at
      the same time on some equipment.

6.3 ICMP Redirect

      While running HSRP, it is important to prevent the host from
      discovering the primary MAC addresses of the routers in its
      standby group.  Thus, any protocol that informs a host of a
      router's primary address should be disabled.  Thus, routers
      participating in HSRP on an interface MUST NOT send ICMP redirects
      on that interface.


6.4 Proxy ARP

      Typically, hosts learn the HSRP virtual IP address through the
      configuration of their default router.  These hosts then send
      packets for destinations outside of the LAN to the virtual IP
      address.  In some environments, hosts may instead make use of
      proxy ARP in order to route off of the LAN.  In this case, the
      hosts use the MAC address that is supplied in proxy ARP responses.
      HSRP functionality is maintained if the proxy ARP responses
      specify the HSRP virtual MAC address.

      If an HSRP router is configured to support proxy ARP with HSRP,
      then the router MUST specify the HSRP virtual MAC address in any
      proxy ARP responses it generates.  These proxy ARP responses MUST
      not be suppressed based upon HSRP state.  Suppression based upon
      state could result in lack of any proxy ARP response being
      generated, since these proxy ARP responses may be suppressed due
      to other reasons, such as split-horizon rules.

7. Security Considerations

      This protocol does not provide security.  The authentication field
      found within the message is useful for preventing
      misconfiguration.  The protocol is easily subverted by an active
      intruder on the LAN.  This can result in a packet black hole and a
      denial-of-service attack.  It is difficult to subvert the protocol
      from outside the LAN as most routers will not forward packets
      addressed to the all-routers multicast address (224.0.0.2).


8. References

      [1] RFC 1256, S. Deering, "ICMP Router Discovery Messages"

      [2] United States Patent.  Patent Number : 5,473,599.  Standby
      Router Protocol.  Date of Patent: Dec. 5, 1995.

9. Authors' Addresses

      Tony Li
      Juniper Networks, Inc.
      3260 Jay St.
      Santa Clara, CA 95054
      Phone: (408) 327-1900
      Email: tli@juniper.net

      Bruce Cole
      Juniper Networks, Inc.
      3260 Jay St.
      Santa Clara, CA 95054
      Phone: (408) 327-1900
      Email: cole@juniper.net

      Phil Morton
      Cisco Systems
      170 Tasman Dr.
      San Jose, CA 95143
      Phone: (408) 526-7632
      Email: pmorton@cisco.com

      Dawn Li
      Cisco Systems
      170 Tasman Dr.
      San Jose, CA 95143
      Phone: (408) 527-2014
      Email: dawnli@cisco.com


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