What is the purpose of a domain ID

OSPF domain ID is used in Virtual Private Network (VPN) scenarios.

When the domain ID carried by the packet received from the peer is the same as the local one, Type 3 link-state advertisements (LSAs) are generated for Type 1, 2, and 3 LSAs, and Type 5 and 7 LSAs are generated for Type 5 and 7 LSAs (depending on area type).

When the Domain ID is the different from the local one, Type 5 or 7 LSAs (depending on area type) are generated for Type 1, 2, and 3 LSAs, and Type 5 and 7 LSAs are generated for Type 5 and 7 LSAs (depending on area type).

Before sending routes to a remote CE switch, a PE switch sends Type-3 LSAs or Type-5 LSAs to the CE based on domain ID. If local domain IDs are the same as or compatible with remote domain IDs in BGP routes, the PE advertises Type 3 routes. If local domain IDs are different from or incompatible with remote domain IDs in BGP routes, the PE advertises Type 5 routes.

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