OSPF route tags are used in only Virtual Private Network (VPN) scenarios to prevent Type 5 link-state advertisement (LSA) loops in Customer Edge (CE) dual-homing networks.

When OSPF detects that the route tag of a Type 5 LSA is the same as a route tag on the Provider Edge (PE) router, this route is ignored.

When a CE router is connected to two PEs, PE1 sends the Type 5 LSA generated based on the redistributed Border Gateway Protocol (BGP) route. The CE then forwards this LSA to PE 2. Because an OSPF route has a higher priority than a BGP route to the CE, PE 2 replaces the BGP route with the OSPF route. Thus, a routing loop occurs. With a route tag configured, when the PE detects that the route tag of the LSA is the same as that of its route tag, the PE ignores the LSA, thereby avoiding routing loops.

good knowledge sharing

nice sharing

nice job

nice post

thanks for share

splendid know-how contributed  it

useful document, thanks