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I am running a design where 2 WAN routers are connected to the same CORE switch (see drawing attached).

OSPF advertismeents are done using SVI for vlan 103 as follows :

WAN01#
interface Vlan103
 description OSPF
 ip address 192.168.1.1 255.255.255.0
 ip ospf dead-interval minimal hello-multiplier 5
 ip ospf 1 area 0


WAN02#
interface Vlan103
 description OSPF
 ip address 192.168.1.2 255.255.255.0
 ip ospf dead-interval minimal hello-multiplier 5
 ip ospf 1 area 0
 ip ospf cost 5

CORE01#
interface Vlan103
 description OSPF
 ip address 192.168.1.254 255.255.255.0
 ip ospf dead-interval minimal hello-multiplier 5
 ip ospf 1 area 0

As of now, WAN01 is advertising OSPF routes (internal & redistributed bgp) with a cost of 1 to my CORE switch. It's receiving routes from CORE with a cost of 2.

Here is its OSPF routing table :

O        192.168.3.0/27 [110/2] via 192.168.1.254, 7w0d, Vlan103
O        192.168.7.0/24 [110/2] via 192.168.1.254, 7w0d, Vlan103
O        192.168.9.0/24 [110/2] via 192.168.1.254, 7w0d, Vlan103
O        192.168.10.0/24 [110/2] via 192.168.1.254, 7w0d, Vlan103
O        192.168.11.0/24 [110/2] via 192.168.1.254, 7w0d, Vlan103
O        192.168.12.0/24 [110/2] via 192.168.1.254, 7w0d, Vlan103
O        192.168.14.0/24 [110/2] via 192.168.1.254, 7w0d, Vlan103
O        192.168.100.3/32 [110/2] via 192.168.1.254, 7w0d, Vlan103
O        192.168.1.32/28 [110/2] via 192.168.1.254, 7w0d, Vlan103
O        192.168.1.56/29 [110/2] via 192.168.1.254, 7w0d, Vlan103

WAN02 is also advertising OSPF routes (internal & redistributed bgp backup routes) with a cost of 5 to my CORE switch. It's receiving routes from CORE with a cost of 6.

Here is its OSPF routing table :

O        192.168.3.0/27 [110/6] via 192.168.1.254, 7w0d, Vlan103
O        192.168.7.0/24 [110/6] via 192.168.1.254, 7w0d, Vlan103
O        192.168.9.0/24 [110/6] via 192.168.1.254, 7w0d, Vlan103
O        192.168.10.0/24 [110/6] via 192.168.1.254, 7w0d, Vlan103
O        192.168.11.0/24 [110/6] via 192.168.1.254, 7w0d, Vlan103
O        192.168.12.0/24 [110/6] via 192.168.1.254, 7w0d, Vlan103
O        192.168.14.0/24 [110/6] via 192.168.1.254, 7w0d, Vlan103
O        192.168.100.3/32 [110/6] via 192.168.1.254, 7w0d, Vlan103
O        192.168.1.32/28 [110/6] via 192.168.1.254, 7w0d, Vlan103
O        192.168.1.56/29 [110/6] via 192.168.1.254, 7w0d, Vlan103

VLAN 103 is not extended between WAN01 & WAN02, so they see each other as OSPF neighbors through CORE01 on VLAN 103.

My question is :

When I add VLAN 103 to my trunk link between WAN01 and WAN02, what will happen?

To my knowledge, WAN02 will advertise its routes with a cost of 5 or 6 and will receive advertised routes from WAN01 with a cost of 1 or 2, essentially replacing all its OSPF routes shown above.

Is there a mechanism like split horizon (not used in OSPF) that will prevent this design to work ? I am thinking especially of not being able to advertise routes on a link where my router has received routes for the same networks - but using a SVI must surely prevent this kind of failure?

OSPF Design

4

If you enable VLAN 103 between WAN 01 and WAN 02, you will create a layer 2 loop, which presumably will be broken by Spanning Tree protocol. So depending on the STP parameters, one of those links will be in the blocking state. You will end up in a very similar state where you are now.

  • Indeed it does... sorry for the dumb question... – Jeremy G. Feb 2 '16 at 19:48

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