Why do you want to separate the traffic physically? You can use OSPF load balance to aggregate bandwidth and you can use a QoS feature (like custom-queue) to guarantee 50% for each type of traffic. In this case, if one type of traffic is not present, the other will use two E1 links.
Ive understand that you want that the traffic between LAN1 and LAN3 uses an E1 circuit and the traffic between LAN2 and LAN4 it uses the other circuit. If "circuit down" then both traffic use the active circuit.
In this case, only the OSPF should not be the best option to isolate the traffic. If you use two autonomous systems maybe you get to isolate the traffic with OSPF, but I don't recommend.
You can use PBR (Policy-based Routing) to define for which interface the traffic should be routed. For example:
access-list 101 permit
access-list 102 permit
route-map circ1 permit 10
match ip address 101
set next-interface serial0
route-map circ2 permit 10
match ip address 102
set next-interface serial1
I never tried this before and Im not right about what it happens when a serial goes down, but I think this is a good clue.
Other way to define the path for different types of traffic is MPLS-TE, but this is not possible in all router models.
It is a high availability solution, if the circuits are provided by different ISP. But I havent been needing to separate the traffic in circuits. Ive been needing to guarantee 40% of the band for a certain type of traffic, 20% for another and 40% for the others. Ive applied custom-queue.
Summarizing: there are some ways of implementing a solution for the problem that you propose, but I think only the OSPF won't be a satisfactory solution.
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