Downstream On Demand - Cisco ASR 9000 Series Configuration Manual

Implementing MPLS Label Distribution Protocol
• Prefix-based (per-prefix)--Prefix-based LFAs allow computing backup information per prefix. It
The per-prefix LFA approach is preferred for LDP IP Fast Reroute LFA for these reasons:
• Better node failure resistance
• Better capacity planning and coverage
Features Not Supported
These interfaces and features are not supported for the IP LDP Fast Reroute Loop Free Alternate feature:
• BVI interface (IRB) is not supported either as primary or backup path.
• GRE tunnel is not supported either as primary or backup path.
• Cisco ASR 9000 Series SPA Interface Processor-700 POS line card on Cisco ASR 9000 Series Router
• In a multi-topology scenerio, the route in topology T can only use LFA within topology T. Hence, the
For more information about configuring the IP Fast Reroute Loop-free alternate , see Implementing IS-IS on
Cisco IOS XR Software module of the Cisco ASR 9000 Series Aggregation Services Router Routing
Configuration Guide.
Related Topics
Configure IP LDP Fast Reroute Loop Free Alternate: Examples, on page 57
Verify IP LDP Fast Reroute Loop Free Alternate: Example, on page 58

Downstream on Demand

This Downstream on demand feature adds support for downstream-on-demand mode, where the label is not
advertised to a peer, unless the peer explicitly requests it. At the same time, since the peer does not automatically
advertise labels, the label request is sent whenever the next-hop points out to a peer that no remote label has
been assigned.
To enable downstream-on-demand mode, this configuration must be applied at mpls ldp configuration mode:
mpls ldp downstream-on-demand with ACL
The ACL contains a list of peer IDs that are configured for downstream-on-demand mode. When the ACL is
changed or configured, the list of established neighbors is traversed. If a session's downstream-on-demand
configuration has changed, the session is reset in order that the new down-stream-on-demand mode can be
configured. The reason for resetting the session is to ensure that the labels are properly advertised between
the peers. When a new session is established, the ACL is verified to determine whether the session should
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protects the destination address. The per-prefix approach is the preferred approach due to its greater
applicability, and the greater protection and better bandwidth utilization that it offers.
The repair or backup information computed for a given prefix using prefix-based LFA
Note
may be different from the computed by link-based LFA.
is not supported as primary link. It can be used as LFA backup only on main interface.
availability of a backup path depends on the topology.
Cisco ASR 9000 Series Aggregation Services Router MPLS Configuration Guide, Release 4.3.x
Downstream on Demand
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