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Loop-Free Alternate Routes for OSPF Overview
Support for OSPF loop-free alternate routes essentially adds IP fast-reroute capability for OSPF. Junos OS precomputes loop-free backup routes for all OSPF routes. These backup routes are preinstalled in the Packet Forwarding Engine, which performs a local repair and implements the backup path when the link for a primary next hop for a particular route is no longer available. With local repair, the Packet Forwarding Engine can correct a path failure before it receives precomputed paths from the Routing Engine. Local repair reduces the amount of time needed to reroute traffic to less than 50 milliseconds. In contrast, global repair can take up to 800 milliseconds to compute a new route. Local repair enables traffic to continue to be routed using a backup path until global repair is able to calculate a new route.
A loop-free path is one that does not forward traffic back through the routing device to reach a given destination. That is, a neighbor whose shortest path first to the destination traverses the routing device that is not used as a backup route to that destination. To determine loop-free alternate paths for OSPF routes, Junos OS runs shortest-path-first (SPF) calculations on each one-hop neighbor. You can enable support for alternate loop-free routes on any OSPF interface. Because it is common practice to enable LDP on an interface for which OSPF is already enabled, this feature also provides support for LDP label-switched paths (LSPs.)
 | Note: If you enable support for alternate loop-free routes on an interface configured for both LDP and OSPF, you can use the traceroute command to trace the active path to the primary next hop. |
The level of backup coverage available through OSPF routes depends on the actual network topology and is typically less than 100 percent for all destinations on any given routing device. You can extend backup coverage to include RSVP LSP paths.
Junos OS provides two mechanisms for route redundancy for OSPF through alternate loop-free routes:
- Link protection—Offers per-link traffic protection. Use link protection when you assume that only a single link might become unavailable but that the neighboring node on the primary path would still be available through another interface.
- Node-link protection—Establishes an alternate path through a different routing device altogether. Use node-link protection when you assume that access to a node is lost when a link is no longer available. As a result, Junos OS calculates a backup path that avoids the primary next-hop routing device.
When you enable link protection or node-link protection on an OSPF interface, Junos OS creates an alternate path to the primary next hop for all destination routes that traverse a protected interface.
