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Forum Replies

  1. andrew says:

    Hoan,
    DR/BDR is chosen by the following criteria

    1. Highest OSPF Priority (0-255, 0 is exclusion from election, 1 is default)
    2. Highest Router ID
    3. Highest Loopback Address
    4. Highest IP Address (must be up/up)
      In most cases, there is no DR/BDR preemption - if a better DR/BDR candidate comes online after an election, it will not take over the role without clearing the ospf process.
  2. Hi,

    I have a question, In the process of election DR/ BDR, who will elect first? DR or BDR ?
    I have only one router in my setup. I configured OSPF on that router and I started debug message. After the end of wait period ( dead interval 40 sec), router considers himself as BDR and then moved to DR .
    please explain me, in the election process, will BDR is elected first or DR ?

    ===========================
    Below is the debug message:

     R1#
        *Mar  1 00:01:12.207: %SYS-5-CONFIG_I: Configured from console by console
        R1#
        *Mar  1 00:01:18.215: OSPF: Send hello to 224.0.0.5 area 0 on FastEthernet0/0 from 100.100.100.1
        R1#
        *Mar  1 00:01:28.215: OSPF: Send hello to 224.0.0.5 area 0 on FastEthernet0/0 from 100.100.100.1
        R1#
        *Mar  1 00:01:38.215: OSPF: end of Wait on interface FastEthernet0/0
        *Mar  1 00:01:38.215: OSPF: DR/BDR election on FastEthernet0/0
        *Mar  1 00:01:38.215: OSPF: Elect BDR 13.1.1.1
        *Mar  1 00:01:38.215: OSPF: Elect DR 13.1.1.1
        *Mar  1 00:01:38.215: OSPF: Elect BDR 0.0.0.0
        *Mar  1 00:01:38.215: OSPF: Elect DR 13.1.1.1
        *Mar  1 00:01:38.215:        DR: 13.1.1.1 (Id)   BDR: none
        *Mar  1 00:01:38.215: OSPF: Send hello to 224.0.0.5 area 0 on FastEthernet0/0 from 100.100.100.1
        R1#
        *Mar  1 00:01:48.215: OSPF: Send hello to 224.0.0.5 area 0 on FastEthernet0/0 from 100.100.100.1
        R1#
  3. andrew says:

    Azeem,
    The BDR is elected first (see step #7 below). Here is a break down of what happens during the election:

    1. Once an OSPF becomes active on a multi-access network, it sets the DR and BDR values to 0.0.0.0 which indicates these are unknown. It also starts a wait timer of the value of the dead interval.
    2. The router starts neighbor discovery. It sends the 0.0.0.0 values for DR/BDR
    3. If a received Hello includes values for DR/BDR, these are accepted and the wait timer is stopped
    4. If the wait timer expires, the DR election starts
    5. A list of neighbors on the multiaccess network is established that are eligible for being DR/BDR (their priority is higher than zero)
    6. A subset of routers from this list is created. This subset excludes routers listing their own address as the DR (so routers claiming to be the DR are excluded)
    7. From this subset, select the router with the highest priority/RID/Loopback/Address to be the BDR
    8. From the original (not subset list), select the Router with the highest priority/RID/loopback/address to be the DR
    9. If there are no routers originally claiming to be the DR, then promote the chosen BDR to a DR, and elect another BDR via the same method
  4. Hello Americo

    When we have a BMA situation, and all routers boot up together, ALL routers declare that they are the DR. So at the beginning, the subset of all routers not claiming to be the DR is an empty set. There are NO routers in this subset. So initially, this list of eligible routers will be empty. As hello packets are exchanged however, routers will receive the priority and router IDs and any routers ineligible to be a DR will CHANGE that information in the next hellos.

    Now if a router creates a subset of all routers not claiming to be the DR, then this subset is non zero. It all depends on WHEN that subset will be created. Don’t forget that the DR and BDR election process is an iterative process as the network converges and even when network changes occur. At some point, routers some will stop declaring themselves DRs and the process can continue as described.

    A clearer albeit dryer explanation of the election process can be found at RFC 2328 for OSPFv2 in seciton 9.4 Electing the Designated Router. It is true that the BDR is elected first and then the DR. The RFC explains why:

    The reason behind the election algorithm’s complexity is the
    desire for an orderly transition from Backup Designated Router
    to Designated Router, when the current Designated Router fails.
    This orderly transition is ensured through the introduction of
    hysteresis: no new Backup Designated Router can be chosen until
    the old Backup accepts its new Designated Router
    responsibilities.

    This can be clearly seen in the case where the current DR fails and the election process is repeated (per step (4) in section 9.4 of RFC) the router it elected to BDR and DR is no longer eligible for BDR (because it is DR), and so the “correct” BDR is elected.

    I hope this has been helpful!

    Laz

  5. Thanks Laz! Good to know I understand this correctly. :grinning:

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