a) Can I confirm inter-area traffic will be routed towards Area 0 / Area 0 will be the transit network for all inter-area traffic?

b) Consider the diagram below

enter image description here q1) Does R1 receive Type 3 LSA for Area 3 via both Area 1 and Area 0 from R2 ?

q2) Going by the rule above that all inter-area traffic must transit Area 0, isn't it totally not efficient for traffic from Area 2 <-> Area 3 to pass through Area 0 via a 100 Mbs link, when Area 2 can reach Area 3 through Area 1 on 1 Gbps link?

q3) How does/will traffic from Area1 go to Area2 ? All inter-area traffic must go through Area0... but.. how ?

1 Answer 1


The goal of the area 0 backbone is not efficiency, it's loop prevention. By requiring that all inter-area traffic traverse the backbone, OSPF prevents routing loops. OSPF uses the SPF algorithm inside an area, but it uses a distance vector algorithm for inter-area routing.

  • thanks for your reply - do you really mean that traffic from area2 to area3 will actually transit through area0 instead of area1 rendering the 1Gb link unused ?
    – Noob
    Nov 27, 2018 at 19:36
  • That is correct.
    – Ron Trunk
    Nov 27, 2018 at 19:43
  • even if R1 receives both Type3 LSA (area3) from R2 through area0 and area1 - it will still send traffic via Area0 to reach Area3 ??
    – Noob
    Nov 27, 2018 at 19:49
  • R2 won’t send type 3s to R1 because they are only sent to the backbone.
    – Ron Trunk
    Nov 28, 2018 at 2:38
  • but R1 has an interface in the backbone - do you mean R2 wont send type3s to R1 via area 1 ?
    – Noob
    Nov 28, 2018 at 5:49

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