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?