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, see Configuring Explicit-Path LSPs.

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:

• Inter-AS Traffic Engineering Requirements
• Inter-AS Traffic Engineering Limitations
• Configuring OSPF Passive TE Mode

Inter-AS Traffic Engineering Requirements

The proper establishment and functioning of inter-AS traffic engineered LSPs depend on the following network requirements, all of which must be met:

• All ASs are under control of a single service provider.
• OSPF is used as the routing protocol within each AS, and EBGP is used as the routing protocol between the ASs.
• ASBR information is available inside each AS.
• EBGP routing information is distributed by OSPF, and an IBGP full mesh is in place within each AS.
• Transit LSPs are not configured on the inter-AS links, but are configured between entry and exit point ASBRs on each AS.
• The EBGP link between ASBRs in different ASs is a direct link and must be configured as a passive traffic engineering link under OSPF. The remote link address itself, not the loopback or any other link address, is used as the remote node identifier for this passive link. For more information about OSPF passive traffic engineering mode configuration, see Enabling Inter-AS Traffic Engineering for LSPs.

In addition, the address used for the remote node of the OSPF passive traffic engineering link must be the same as the address used for the EBGP link. For more information about OSPF and BGP in general, see the Junos Routing Protocols Configuration Guide.

Inter-AS Traffic Engineering Limitations

Only LSP hierarchical, or nested, signaling is supported for inter-AS traffic engineered LSPs. Only point-to-point LSPs are supported (there is no point-to-multipoint support).

In addition, the following limitations apply. Any one of these conditions is sufficient to render inter-AS traffic engineered LSPs impossible, even if the above requirements are met.

• The use of multihop BGP is not supported.
• The use of policers or topologies that prevent BGP routes from being known inside the AS is not supported.
• Multiple ASBRs on a LAN between EBGP peers are not supported. Only one ASBR on a LAN between EBGP peers is supported (others ASBRs can exist on the LAN, but cannot be advertised).
• Route reflectors or policies that hide ASBR information or prevent ASBR information from being distributed inside the ASs are not supported.
• Bidirectional LSPs are not supported (LSPs are unidirectional from the traffic engineering perspective).
• Topologies with both inter-AS and intra-AS paths to the same destination are not supported.

In addition, several features that are routine with all LSPs are not supported with inter-AS traffic engineering:

• Admin group link colors are not supported.
• Secondary standby is not supported.
• Reoptimization is not supported.
• Crankback on transit routers is not supported.
• Diverse path calculation is not supported.
• Graceful restart is not supported.

These lists of limitations or unsupported features with inter-AS traffic engineered LSPs are not exhaustive.

Configuring OSPF Passive TE Mode

Ordinarily, interior routing protocols such as OSPF are not run on links between ASs. However, for inter-AS traffic engineering to function properly, information about the inter-AS link, in particular, the address on the remote interface, must be made available inside the AS. This information is not normally included either in EBGP reachability messages or in OSPF routing advertisements.

To flood this link address information within the AS and make it available for traffic engineering calculations, you must configure OSPF passive mode for traffic engineering on each inter-AS interface. You must also supply the remote address for OSPF to distribute and include in the traffic engineering database.

To configure OSPF passive mode for traffic engineering on an inter-AS interface, include the passive statement for the link at the [edit protocols ospf area area-id interface interface-name] hierarchy level:

OSPF must be properly configured on the router. The following example configures the inter-AS link so-1/1/0 to distribute traffic engineering information with OSPF within the AS. The local IP address on the link is 192.168.207.1 and the remote address is 192.168.207.2.