Why configure OSPF route tag on S series switches

For S series switches supporting OSPF, the OSPF router tag is applied to VPNs and prevents loops of Type 5 LSAs in CE dual-homing networking. If the tag of a received Type 5 LSA is the same as the router tag of OSPF on a PE, the PE neglects this LSA when calculating routes. When a CE is connected to two PEs, PE1 generates a Type 5 LSA based on the imported BGP route and sends the LSA to the CE, and the CE forwards the LSA to PE2. The OSPF route takes precedence over the BGP route, so PE2 replaces the BGP route with the OSPF route. As a result, a loop occurs. If the route tag is configured on a PE, when the PE receives an LSA with the same route tag as its own route tag, it neglects this LSA, avoiding loops. By default, the route tag is calculated using the BGP AS number. If BGP is not configured, the route tag is 0.
In OSPF public network instances, router tags cannot be used to prevent loops but can be used as a filtering condition in a policy.

When setting the router tag of ASE-LSAs and NSSA-LSAs, not the following:
The route-tag command is used in the OSPF multi-instance scenario.
The default tag command is used in the OSPF public network instance scenario.
The import-route tag command can be used in either of the preceding scenarios.

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