Why OSPF on an S series switch does not preferentially select a route with a small cost value and through the backbone area

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OSPF defined by RFC 2328 must be compatible with RFC 1583. As defined in RFC 2328, if "RFC1583 Compatibility" is disabled, the routes learned through common areas are preferred regardless of their costs when routes can be learned through common areas and the backbone area. This helps reduce the burden of the backbone area.

Other related questions:
Why are the routes with smaller costs passing through the backbone area not preferred
OSPF defined by RFC 2328 must be backward compatible with RFC 1583. As specified in RFC 2328, if "RFC1583 Compatibility" is disabled, when routes can be learnt through common areas and the backbone area, the routes learnt through common areas are preferred regardless of their costs. This is to reduce the burden on the backbone area.

Can an S series switch correctly calculate routes when OSPF backbone areas are not continuous
For S series switches supporting OSPF, if OSPF backbone areas are not continuous, some areas may be unreachable and loops may occur on inter-area and external routes.

Can I configure only an OSPF non-backbone area on S series switches
OSPF partitions an AS into different areas to resolve the problem of frequent LSDB updates and improve network efficiency. An area is regarded as a logical group, and each group is identified by an area ID. A switch, not a link, resides at the border of an area. A network segment or link belongs only to one area. The area to which each OSPF-enabled interface belongs must be specified. On an OSPF network, the backbone area connects to all other OSPF areas and transmits inter-area routes. A single non-backbone area (for example, Area 3) can be configured to implement intra-area communication. However, if multiple non-backbone areas are configured, a backbone area is required to enable communication between the non-backbone areas. Therefore, all devices on a small network can be added to Area 0 (the backbone area). You are advised not to configure a single non-backbone area.

OSPF cost of S series switches
The ospf cost command can be executed on an S series switch that supports OSPF to configure the cost required for running the OSPF protocol on an interface. Application scenario: The interface cost can be automatically calculated by OSPF based on the interface bandwidth or manually configured by using the ospf cost command. If the costs and destination IP addresses of multiple routes discovered by one routing protocol are the same, load balancing can be implemented among the routes. Modify interface costs to implement load balancing as required. Configuration impact: If no interface cost is configured by using the ospf cost command, OSPF automatically calculates the interface cost based on the interface bandwidth. The calculation formula is as follows: Cost of the interface = Bandwidth reference value/Interface bandwidth. The integer of the calculated result is the cost of the interface. If the calculated result is smaller than 1, the cost value is 1. Changing the bandwidth reference value can change the cost of an interface. By default, the bandwidth reference value is 100 Mbit/s. Therefore, the default cost value of an Ethernet (100 Mbit/s) interface is 1 (100,000,000/the interface bandwidth). Example: Set the cost required by VLANIF 100 for running OSPF to 65. [HUAWEI] interface vlanif 100 [HUAWEI-Vlanif100] ospf cost 65 Set the cost required by GE0/0/1 for running OSPF to 65. [HUAWEI] interface gigabitethernet 0/0/1 [HUAWEI-GigabitEthernet0/0/1] undo portswitch [HUAWEI-GigabitEthernet0/0/1] ospf cost 65 Note: The cost calculated by OSPF is the cost of the route to the outbound interface of the destination network.

On S series switches, why cannot inter-area OSPF route summarization take effect
Question: Why cannot inter-area OSPF route summarization take effect? Answer: As defined in RFC 2328, when an ABR summarizes inter-area routes, it directly generates a Type 3 LSA regardless of the configured range value. When the ABR summarizes intra-area routes, it generates a Type 3 LSA based on the configured range value. However, route summarization can cause more LSAs being transmitted into the backbone area, making the network unstable.

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