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Forum Replies

  1. andrew says:

    DRs and BDRs are not necessarily one-to-one with Areas. I think this is the key point to answer your question. DRs and BDRs are the result of the OSPF network type defined on a Router's interface, not because of some requirement of an Area to have them. OSPF recognizes the following network types:

    • Broadcast

    • Non-Broadcast

    • Point to Multipoint (Broadcast)

    • Point to Multipoint (Non-Broadcast)

    • Point to Point

    Out of all of those possibilities, only Broadcast and Non-Broadcast form DRs and BDRs. The Broadcast and Non-Broadcast network types describe a multi-access network media, such as Ethernet. In this case OSPF requires that all routers on the same network segment have direct reachability both to the DR and BDR, otherwise the network will break. While reachability to the DR and BDR is required, reachability between DROthers is not.

    An OSPF Area might have zero, one, or more than one DR--it all comes down to what type of networks there are and how many in your Area. Below is the output on a router that connects together two different Ethernet segments, all in the same Area:

    Neighbor ID     Pri   State           Dead Time   Address         Interface           1   FULL/DROTHER    00:00:35        FastEthernet0/1           1   FULL/DR         00:00:34        FastEthernet0/1           1   FULL/BDR        00:00:35        FastEthernet0/0           1   FULL/DR         00:00:39        FastEthernet0/0

    See how there are multiple DRs? Each Ethernet segment would have its own DR and BDR election. Therefore, there is no need to pass Type 2 LSAs through another router. In fact, that couldn't happen anyway, because the multicast addresses, both (all routers) and (all DR/BDR) have a "link-local" scope, where their TTL is set to 1, so the packet would be discarded beyond the local segment. I have attached a Wireshark capture as an example.

  2. andrew says:

    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.

  3. 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:

        *Mar  1 00:01:12.207: %SYS-5-CONFIG_I: Configured from console by console
        *Mar  1 00:01:18.215: OSPF: Send hello to area 0 on FastEthernet0/0 from
        *Mar  1 00:01:28.215: OSPF: Send hello to area 0 on FastEthernet0/0 from
        *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
        *Mar  1 00:01:38.215: OSPF: Elect DR
        *Mar  1 00:01:38.215: OSPF: Elect BDR
        *Mar  1 00:01:38.215: OSPF: Elect DR
        *Mar  1 00:01:38.215:        DR: (Id)   BDR: none
        *Mar  1 00:01:38.215: OSPF: Send hello to area 0 on FastEthernet0/0 from
        *Mar  1 00:01:48.215: OSPF: Send hello to area 0 on FastEthernet0/0 from
  4. andrew says:

    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 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 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
  5. 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

    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!


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