What Is the Purpose of an OSPF Route Tag?

Created: Jun 30, 2016 14:58:17Latest reply: Nov 23, 2018 11:06:44 1213 3 0 0
What Is the Purpose of an OSPF Route Tag?
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.

The default route tag is calculated based on the AS numbers in the BGP. If BGP is not configured, the default value of route tag is 0.
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debugger     Created Jun 30, 2016 15:50:09 Helpful(0) Helpful(0)

thanks
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user_235153     Created Jun 30, 2016 15:57:04 Helpful(0) Helpful(0)

THNKS SHARE
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Mark.hu  Visitor   Created Nov 23, 2018 11:06:44 Helpful(0) Helpful(0)

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.
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