It is not necessary to modify the cost of OSPF (Open Shortest Path First) links on all the routers in your MikroTik network every time you make a change to the cost of a specific link on a router.

OSPF is a link state protocol that uses Dijkstra's algorithm to calculate the shortest path between nodes. The cost of a link, which is a metric assigned to define the “difficulty” or “cost” of passing that link, is a crucial part of this calculation.

The cost of an OSPF link can reflect several factors, such as bandwidth, load, or even administrative considerations.

Points to consider:

• Updating the Link State Database (LSDB): When you change the cost of a link on an OSPF router, this information is propagated to all routers in the OSPF area through LSAs (Link State Advertisements). This means that all routers in the OSPF area will update their link state databases with this new information and recalculate routes as necessary. You do not need to manually adjust costs on other routers to reflect this change.
• Consistency in the OSPF area: As long as the cost change is made on a specific link and the information is propagated correctly across the LSAs, the routers in the OSPF area will work with the same view of the network and calculate their routes according to the updated cost. This maintains consistency in routing decisions within the area.
• Design considerations: Although it is not necessary to adjust costs on all routers after a change, it is important to consider the impact of these changes on the overall network design and preferred routes. Changing the cost of a link can alter preferred routes, which could have implications for network load, connection latency, and network resilience to failures.
• Management and planning: Any changes to the OSPF configuration, including link costs, must be carefully planned. It is advisable to document these changes and understand how they will affect network behavior. Using simulation or testing tools in a laboratory environment can help predict the impact of these changes.

In summary, while changes to OSPF link costs are part of network management and may be necessary to optimize traffic or respond to infrastructure changes, these adjustments are handled autonomously by the OSPF protocol without need to make manual cost changes on each of the routers in the network.