OSPF ABR as a DR or Drother?


Is there a recommendation to ensure that the ABR will be the DR?
The answer is NO. There is no relation between a DR and ABR. an ABR is just there to inject intra area routes with a Type-3 LSA into others areas. Keep in mind that the intra-area routes are carried either via Type-1 LSA or Type-2 LSA, since the DR is the only router allowed to generate a Type-2 LSA, we can formulate the question more specifically as follow: is it recommended that an ABR should be the DR that is responsible about the Type-2 LSA? the answer is no.

The Type-2 LSA carries the subnet of the shared segment between R1, R2, R3 and R4 (let’s say 10.1.1.0/24). This subnet is calculated by ANDing operation logic between the Link State ID (which is the IP address of the DR in this segment and the Mask field in the Type-2 LSA, if we look at your topology, regardless who is the DR, from R5 ‘s perspective, R5 sees this shared subnet (10.1.1.0/24 behind the ABR since OSPF INTER AREA design behaves as a distance protocol. Finally R5 will calculate the cost of this inter-area route 10.1.1.0/24 and its calculated cost is always the same regardless if this ABR who originates the Type-2 LSA or not (in other words it’s a DR or a drother), it does not matter. A DR has nothing to do with the area concept or inter-area routing, The DR applies only to a single segment, not the entire area.while the ABR is core component for inter-area routing. We can have many segments in one area so we can have many DR in one area.

Finally the DR and ABR are completely separate concepts.

Categories: OSPF Routing Protocol

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