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Network Working Group                                           Liming Wei
INTERNET-DRAFT                                          cisco Systems, Inc
                                                            Dino Farinacci
March 4, 1998                                           cisco Systems, Inc
Expires: in 6 months

                 PIM Version 2 DR Election Priority Option

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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  This draft specifies the DR Election Priority Option in PIM version 2
  Hello messages.

1. Introduction

  The current PIM specification uses an address-based designated router
  (DR) election algorithm. The router with the largest IP address is
  always elected as the DR.

  The DR Election Priority option is used when people want to have
  control over which router is elected as the DR, irrespective of the
  address of routers on the same LAN. This is needed on LANs where new
  routers can be added and configured by different operators.

2. DR Election Priority Option

  The DR election priority is a 32-bit unsigned number. The numerically
  larger priority is always preferred. The DR election priority is used
  only when all routers on the LAN include this option in their Hellos.

  If no DR election priority option is specified in a Hello
  message, the Hello sender is deemed not capable of handling the DR
  election priority option. When such a hello message is received, the
  neighbor with the highest IP address is elected the DR. This way new
  systems can interoperate with older systems in the old way.

  The DR election priority received in a Hello is kept until the
  next Hello from the same system arrives. The newly received priority
  replaces the cached priority for the same neighbor.

  An implementation capable of doing this option should always include
  it in the Hellos even if no DR election priority is explicitly
  configured. The default priority is 1.

  The following is the format of this option.

  OptionType:   19
  OptionLength: 4

    |    OptionType = 19            |      OptionLength = 4         |
    |                            Priority                           |

  Priority: 32-bit
        Priority value. This should be treated as the higher order bits
        to the address during DR election.

3. Acknowledgmentssss

  Pavlin Radoslavov commented on a previous version of this draft.

4. References

[PIM] draft-ietf-idmr-pim-sm-specv2-00.txt

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