[c-nsp] DR and BDR election

[Bruce Pinsky]

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Brian Johnson wrote:
> I don't think this really answers this question, which I would like to
> know also. ;-)
> 
> The Cisco document in the link mentions the following about a priority
> of 0:
> 
> 	A priority value of zero indicates an interface which is not to
> be elected as DR or BDR.
> 	The state of the interface with priority zero will be DROTHER.
> 
> This does not tell us what will happen if all interfaces have a priority
> of 0.
> 
> Any ideas?
> 


A quick test shows:

Router 1
- -----------
Neighbor ID     Pri   State           Dead Time   Address         Interface
210.150.248.69    1   FULL/DR         00:00:38    192.168.0.10    Ethernet1/0
210.150.248.69    0   2WAY/DROTHER    00:00:35    192.168.0.2     Ethernet0/0

R1#sh ip ospf da

            OSPF Router with ID (210.150.248.68) (Process ID 1)

                Router Link States (Area 0)

Link ID         ADV Router      Age         Seq#       Checksum Link count
210.150.248.68  210.150.248.68  122         0x80000133 0x00A2E9 4
210.150.248.69  210.150.248.69  122         0x8000013A 0x00DCA5 4

                Net Link States (Area 0)

Link ID         ADV Router      Age         Seq#       Checksum
192.168.0.10    210.150.248.69  123         0x80000001 0x00577C

Router 2
- ------------
Neighbor ID     Pri   State           Dead Time   Address         Interface
210.150.248.68    1   FULL/BDR        00:00:34    192.168.0.9     Ethernet1/0
210.150.248.68    0   2WAY/DROTHER    00:00:35    192.168.0.1     Ethernet0/0

R2#sh ip ospf da

            OSPF Router with ID (210.150.248.69) (Process ID 1)

                Router Link States (Area 0)

Link ID         ADV Router      Age         Seq#       Checksum Link count
210.150.248.68  210.150.248.68  83          0x80000133 0x00A2E9 4
210.150.248.69  210.150.248.69  83          0x8000013A 0x00DCA5 4

                Net Link States (Area 0)

Link ID         ADV Router      Age         Seq#       Checksum
192.168.0.10    210.150.248.69  83          0x80000001 0x00577C


E0/0 on both these routers have priority 0.  Note that only the adjacency
in Full state has its network LSA announced.


And the debug:

6d20h: OSPF: 2 Way Communication to 210.150.248.68 on Ethernet0/0, state 2WAY
6d20h: OSPF: end of Wait on interface Ethernet0/0
6d20h: OSPF: DR/BDR election on Ethernet0/0
6d20h: OSPF: Elect BDR 0.0.0.0
6d20h: OSPF: Elect DR 0.0.0.0
6d20h:        DR: none    BDR: none

- --
=========
bep

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