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Forum Replies

  1. Jason,
    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
    0.0.0.2           1   FULL/DROTHER    00:00:35    10.1.1.2        FastEthernet0/1
    1.1.1.1           1   FULL/DR         00:00:34    10.1.1.1        FastEthernet0/1
    0.0.0.1           1   FULL/BDR        00:00:35    10.0.0.1        FastEthernet0/0
    0.0.0.2           1   FULL/DR         00:00:39    10.0.0.2        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 224.0.0.5 (all routers) and 224.0.0.6 (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. I think it would be a good idea to specify clearly the algorithm in the beginning (higher priority is better, if priority is a tie, highest RID wins).

  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:

     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#
  4. 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
  5. Thanks Laz! Good to know I understand this correctly. :grinning:

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