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Railander
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OSPF avoid asymmetric routing (add cost at both exit and entry)

Thu Jul 16, 2020 9:29 pm

We have complexity issues in our network when trying to make sure OSPF routing remains symmetrical.
This is not a problem when there are only 2 nodes in the same broadcast domain (typical router-router connection), but OSPF starts trying to route asymmetrically if there are more than 2 nodes and paths have different costs.

For instance, router A, B and C are all interconnected through the same switch in the same broadcast domain. If you configure the interface on C with a higher cost, although traffic will no longer come out from C to the others, it will still receive traffic from A and B through that interface.
This is because OSPF by default only takes into account the cost for outgoing routes, and never for incoming ones.

The solution is using separate (virtual) interfaces in order to have 1 interface for each 1 other router (so VLAN for A-B, VLAN for A-C, and VLAN for B-C).

This works fine in this example, but complexity blows exponentially the more nodes you add to that central switch.

This behaviour could be avoided if OSPF added cost at both outgoing and incoming routes. For instance, if A-switch and B-switch have cost 10 and C-switch has cost 100, any route going from either A or B to C should have a cost of 110 instead of 10.

Is this functionality possible at all with OSPF?
 
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sri2007
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Re: OSPF avoid asymmetric routing (add cost at both exit and entry)

Fri Jul 17, 2020 9:34 pm

as far as I understand, OSPF doesn't work in that way, the OSPF interface cost is only added at the routes advertised (out); usually a broadcast or a PTMP network-type is configured between several routers at the same node, and you'll need to consider that in a broadcast OSPF will chose a Designated Router & a Backup Designated Router, those are the only routers allowed to advertise the LSA type 2 (pointing to the same subnet).

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