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Enabling Inter-AS Traffic Engineering for LSPs

Generally, traffic engineering is possible for LSPs that meet the following conditions:

Both ends of the LSP are in the same OSPF area or at the same IS-IS level.
The two ends of the LSP are in different OSPF areas within the same autonomous system (AS). LSPs that end in different IS-IS levels are not supported.
The two ends of an explicit-path LSP are in different OSPF ASs and the autonomous system border routers (ASBRs) are configured statically as the loose hops supported on the explicit-path LSP. For more information about explicit-path LSPs, see Static and Explicit-Path LSP Configuration Guidelines.

Without statically defined ASBRs on LSPs, traffic engineering is not possible between one routing domain, or AS, and another. However, when the ASs are under the control of single service provider, it is possible in some cases to have traffic engineered LSPs span the ASs and dynamically discover the OSPF ASBRs linking them (IS-IS is not supported with this feature).

Inter-AS traffic engineered LSPs are possible as long as certain network requirements are met, none of the limiting conditions apply, and OSPF passive mode is configured with EBGP. Details are provided in the following sections:

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