How does OSPF on S series switches create ECMP routes

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Question: Two routes learned from two areas with the same cost are not ECMP routes. Why?
Answer: According to RFC 2328, OSPF can generate ECMP routes only when the following conditions are met:
The routes are learned from the same area.
The routes have the same priority and cost.
RFC 2328 does not define selection rules of intra-area and inter-area routes for switches. When conditions of multiple routes are the same, a switch may not select the last advertised route. If routes are Type 5 external routes, the route with the largest area ID is preferentially selected, as defined in RFC 2328.

Other related questions:
Configure the priority of an OSPF route on an S series switch
The routing protocols may share and select the routing information because a switch may run multiple dynamic routing protocols simultaneously. In this case, a priority should be set for each routing protocol. When different protocols find multiple routes to the same destination, the route discovered by the protocol with a higher priority is preferred. On an S series switch, you can run the preference command in the OSPF process view to configure the OSPF priority. Perform the following operations: 1. Run the ospf [ process-id ] command in the system view to enter the OSPF process view. 2. Run the preference [ ase ] { preference | route-policy route-policy-name } * command to configure the OSPF priority. The parameters are described as follows: ase: indicates the priority of the AS-External route. preference: indicates the OSPF route priority. A smaller value indicates a higher priority. route-policy route-policy-name: indicates the priority for specified routes in the routing policy. The default OSPF priority value is 10. When an ASE is specified, the default OSPF priority is 150. For example, set the priority of routes in OSPF process 100 to 150 as follows: [HUAWEI] ospf 100 [HUAWEI-ospf-100] preference 150

How does an S series switch hide an OSPF route so that the route is not advertised to any other area
Question: How to hide an OSPF route so that it is not advertised to any other area? Answer: A filter list on an S series switch supporting OSPF cannot prevent a route from being advertised. You can use either of the following methods: Configure route summarization on an ABR and specify the keyword no-advertise in the command. Configure an ABR to filter the routes carried in Type 3 LSAs. The ABR filters routes when advertising Type 3 LSAs between OSPF areas. Only the packets with prefixes meeting requirements can be transmitted from one area to another. In this way, the incoming and outgoing packets of an area are controlled.

Configure OSPF to filter received routes on S series switches
The Open Shortest Path First (OSPF) is a routing protocol based on the link status. Unlike the routing protocols using the distance-vector (D-V) algorithm, OSPF ensures topology consistency and provides loop-free routes. To configure OSPF to filter routes, run the filter-policy import command. The record about the LSA of a route filtered out exists in the OSPF database. The OSPF process does not add the route to the routing table but the LSA of the route is advertised. That is, the peer end can receive the route. The filter-policy export command can only be used to configure OSPF to filter imported external routes to be advertised. OSPF also supports the filtering of routes carried in Type 3 LSAs on ABRs. This feature enables ABRs to filter routes when advertising Type 3 LSAs between OSPF areas. Only the packets with prefixes meeting requirements can be transmitted from one area to another. In this way, the incoming and outgoing packets of an area are controlled.

Configure an S series switch to accelerate OSPF route convergence
The following network changes affect OSPF route convergence. A device detects that the peer link is down and the neighbor relationship becomes invalid. A new link-state advertisement (LSA) is generated and flooded. The Link State Database (LSDB) is updated. A new route is calculated through the Shortest Path First (SPF) routing algorithm and delivered to the Forwarding Information Base (FIB) table. To accelerate OSPF aggregation, you can perform the following operations on a switch: Set a Hello interval and a Dead interval on the peer device or configure the BFD to rapidly detect failures. Set spf-schedule-interval (of which the default value is 5, in seconds) to a smaller value to shorten the SPF calculation interval. Set the LSA generation interval to a smaller value. Set the LSA arrival check interval to a smaller value. Run the ospf trans-delay command in an interface view to speed up LSA flooding.

Precedence of external routes imported by OSPF on S series switches
When an S series switch runs OSPF, you can run the import-route command in the OSPF process view to import external routes. The routes imported using this method are OSPF ASE routes and default precedence is 150.

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