Why does the USG OSPF not be able to import multiple routes with the same mask for the same network address?

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In the USG, you can not import multiple routes with the same mask as the same mask.
Because products based on the VRP3.0 platform do not support the features defined in RFC2328 Appendix E. VRP3.0 uses the network address as the LS. When multiple routes with the same network address need to be imported, only the first route can be imported, and the rest is duplicated.

Other related questions:
Problem and solution when routes with the same network address but different masks cannot be imported in OSPF on the USG
The reason that routes with the same network address but different masks cannot be imported in OSPF on the USG is as follows; VRP3.0-based products do not support the features defined in RFC2328 Appendix E. Because VRP3.0 uses a network address as an LS identifier, only the first route among the routes with the same network address can be imported to OSPF.

What are route selection rules if OSPF imports external routes with the same prefix
OSPF can import Type 1 and Type 2 external routes. By default, OSPF imports Type 2 external routes. If a Type 1 external route is imported, the cost contained in the corresponding LSA and the cost of the Type 1 external route are used for route calculation. If a Type 2 external route is imported, the cost of the Type 2 external route is much larger than the cost of an OSPF internal route. Therefore, only the cost of the corresponding LSA is used for route calculation. Route selection rules are described below if OSPF imports Type 1 external routes with the same prefix: Costs (cost of a route to the ASBR+cost contained in the corresponding LSA) of the imported routes are compared. The route with a smaller cost is preferred. If the route costs are the same, the corresponding LSA types are compared. The route that is imported using a Type 5 LSA is preferred. Route selection rules are described below if OSPF imports Type 2 external routes with the same prefix: The costs contained in the LSAs that are used to import the routes are compared. The route that is imported using the LSA with a smaller cost is preferred. If the costs in LSAs are the same, the costs of the routes to the ASBR are compared. The route with a smaller cost is preferred. If the costs of the routes to the ASBR are the same, the corresponding LSA types are compared. The route that is imported using a Type 5 LSA is preferred.

Why cannot OSPF import BGP routes
As defined in RFC 1364, OSPF cannot import IBGP routes. Routes learned via IBGP must not be imported into OSPF OSPF, however, can import IBGP routes on a PE. BGP routes exist in a VPNv4 routing table on a device functioning as a PE but cannot be imported to OSPF. This is because the role of the device changes from a PE to an MCE after the vpn-instance-capability simple command in run in the OSPF process. Solution: If the device needs to function as a PE, you need to run the undo vpn-instance-capability command on the device.

Why routes cannot be imported when AS numbers on the BGP/MPLS IP VPN are the same
When the AS number in the Update message to be received by the EBGP-enabled device is the same as the AS number on the device, the device does not receive the Update message. This prevents routing loops. In some scenarios, the device needs to receive a Update message that carries the same AS number as the AS number on the device. In Hub and Spoke networking, when the Hub-PE and Hub-CE use EBGP, the Update message received by the Hub-CE contains the AS number of the Hub-PE. To prevent the Hub-PE from discarding such Update message, run the peer allow-as-loop command to set the number of times for the repeated AS number.

What route selection rules should be followed when external routes with the same prefix are imported into OSPF on an S series switch
Question: What route selection rules should be followed when external routes with the same prefix are imported into an S series switch supporting OSPF? Answer: Imported external OSPF routes include Type 1 and Type 2 routes. By default, Type 2 routes are imported. When Type 1 external routes are imported, in route calculation, the cost of the route to be advertised is the sum of the LSA cost and the cost of an external route. When Type 2 external routes are imported, OSPF considers that the cost of an external route is much higher than that of an intra-area route. Therefore, only the LSA cost value is used in route calculation. When Type 1 external routes with the same prefix are imported, the route selection rules are as follows: The route with the lowest cost is selected. The cost is the sum of the cost of the route to the ASBR and the LSA cost. If the costs of the routes are the same, the route in a Type 5 LSA is selected. When Type 2 external routes with the same prefix are imported, the route selection rules are as follows: The imported route with the smallest LSA cost value is selected. If the LSA cost values are the same, the route with the lowest cost of the route to the ASBR is selected. If the costs of the routes are the same, the route in a Type 5 LSA is selected.

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