OSPF Domain-ID || Domain-Tag
OSPF Domain ID When OSPF is used as the routing protocol on a provider edge to customer edge (PE-CE) link in a multiprotocol label switching (MPLS) VPN. PE routers mark OSPF routes with the domain attribute derived from the OSPF process number to indicate whether the route originated within the same OSPF domain or from outside it. Importance of Domain-ID In MPLS-VPN network ISP cloud is treated as a super backbone area, and PEs are considered as ABRs or ASBRs depending on the domain-id value, then routes redistributed to CEs would be OSPF inter- area routes or would be OSPF external routes. Why would we care to have OIA or E or O routes in our OSPF database? The answer is simply that OSPF prefers intra-area routes then inter-area routes and finally external routes to be installed in routing table. And we don’t want to come in to the case that routes are leaked from a back door in then being redistributed to the ISP cloud creating a loop in the network and disrupting traffic. ...

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