OSPF – DR and BDR

DR & BDR

DR and BDR are elected in Broadcast and NBMA (Non-Broadcast Multiaccess Network)

Pre-Conditions for electing DR adn BDR

  • Each multi-access interface of each router has a Router Priority, which is an 8-bit unsigned
    integer ranging from 0 to 255. The default priority on Cisco routers is 1 and can be changed on a
    per multi-access-interface basis with the command ip ospf priority. Routers with a priority of 0
    are ineligible to become the DR or BDR.
  • Hello packets include fields for the originating router to specify its Router Priority and for the IP
    addresses of the connected interfaces of the routers it considers the DR and BDR.
  • When an interface first becomes active on a multi-access network, it sets the DR and BDR to
    0.0.0.0. It also sets a wait timer with a value equal to the RouterDeadInterval.
  • Existing interfaces on a multi-access network record the addresses of the DR and the BDR in the
    interface data structure.

The election procedure of the DR and BDR is as follows:

  1. After 2-Way communication has been established with one or more neighbors, examine the
    Priority, DR, and BDR fields of each neighbor’s Hello. List all routers eligible for election (that is,
    routers with priority greater than 0 and whose neighbor state is at least 2-Way); all routers
    declaring themselves to be the DR (their own interface address is in the DR field of the Hello
    packet); and all routers declaring themselves to be the BDR (their own interface address is in the
    BDR field of the Hello packet). The calculating router will include itself on this list unless it is
    ineligible.
  2. From the list of eligible routers, create a subset of all routers not claiming to be the DR (routers
    declaring themselves to be the DR cannot be elected BDR).
  3. If one or more neighbors in this subset include its own interface address in the BDR field, the
    neighbor with the highest priority will be declared the BDR. In a tie, the neighbor with the highest
    Router ID will be chosen.
  4. If no router in the subset claims to be the BDR, the neighbor with the highest priority will become
    the BDR. In a tie, the neighbor with the highest Router ID will be chosen.
  5. If one or more of the eligible routers include their own address in the DR field, the neighbor with
    the highest priority will be declared the DR. In a tie, the neighbor with the highest Router ID will
    be chosen.
  6. If no router has declared itself the DR, the newly elected BDR will become the DR.
  7. If the router performing the calculation is the newly elected DR or BDR, or if it is no longer the
    DR or BDR, repeat steps 2 through 6.
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