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The amount of configuration the SP needs to talk to a new IdP depends on how much special treatment that IdP will require compared to your defaults. The base case is very simple: add the IdP's metadata to a metadata file referenced by a <MetadataProvider> from shibboleth2.xml, and you're done. If you're using the Login style of <SessionInitiator, you should change the entityID to the IdP's entityID.
If you do need to treat an IdP specially in one of the following ways, read that section:
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