Open Shortest Path First
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This value is key when electing the DR and BDR. Let's go through the
steps that occur when the DR and BDR are elected.
1.
A list of eligible routers is created. The criteria for eligible routers are:
Priority
1.
OSPF state of 2Way.
DR or BDR IP address is the same as the participating interface's
IP address.
2.
A list of all routers not claming to be the DR (the DR IP address is the
same as the participating interface's IP address) is compiled from the
list of eligible routers.
3.
The BDR is chosen from the list in Step 2 based on the following
criteria:
The BDR IP address is the same as the participating interface's IP
address.
The router with the highest Router Priority becomes the BDR.
If all Router Priorities are equal, the router with the highest
Router ID becomes the BDR.
or
If none of the above criteria hold true, the router with the highest
Router Priority is chosen, and in case of a tie, the router with the
highest Router ID is chosen as BDR.
4.
The DR is chosen from the remaining eligible routers based on the fol-
lowing criteria:
The DR field is set with the router's interface IP address.
The router with the highest Router Priority is chosen DR. If all
Router Priorities are equal, the router with the highest Router ID
is chosen.
or
If none of the remaining eligible routers claim to be the DR, the
BDR that was chosen in Step 3 becomes the DR. Step 3 would
then be repeated to choose another BDR.
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