Can an S series switch correctly calculate routes when OSPF backbone areas are not continuous

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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.

Other related questions:
Do problems occur in route calculation when the backbone area is discontinuous
If the backbone area is discontinuous, the networks in some areas may be unreachable and routing loops may occur in inter-area routes 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.

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

Configure OSPF special areas on S series switches
1. Configure a stub area. A stub area is a special area where ABRs do not flood received AS external routes, significantly reducing the routing table size and transmitted routing information of routers. A border area on an OSPF network is often configured as a stub area. For example, configure Area1 as a stub area. [SwitchA] ospf 1 [SwitchA-ospf-1] area 1 [SwitchA-ospf-1-area-0.0.0.1] stub [SwitchA-ospf-1-area-0.0.0.1] quit [SwitchA-ospf-1] quit Precautions: 1. To configure an area as a stub area, you must run the stub command on all the devices in this area. 2. To configure an area as a totally stub area, run the stub no-summary command on the ABR in this area and run the stub command on other devices in this area. This prevents the ABR from transmitting Type 3 LSAs to the stub area, making the area a totally stub area. 2. Configure an NSSA area. In an NSSA, an ABR does not flood AS external routes received from other areas, similar to the situation in a stub area. The difference is that an ABR can import and flood AS external routes to the entire OSPF domain. A border area connected to another AS on an OSPF network is often configured as an NSSA. For example, configure Area2 as an NSSA. [SwitchB] ospf 1 [SwitchB-ospf-1] area 2 [SwitchB-ospf-1-area-0.0.0.2] nssa [SwitchB-ospf-1-area-0.0.0.2] quit [SwitchB-ospf-1] quit Precautions: 1. To configure an area as an NSSA, you must run the nssa command on all the devices in this area. 2. To configure an area as a totally NSSA, run the nssa no-summary command on the ABR in this area and run the nssa command on other devices in this area. This prevents the ABR from transmitting Type 3 LSAs to the NSSA, making the area a totally NSSA.

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.

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