Configuring Loop-Free Alternate Routes for OSPF

Per-Prefix Loop Free Alternates (LFA)—Loop Free Alternates (LFA) is a technology by which a neighbor can be used as a backup next hop to provide a local repair path for the traffic to flow temporarily in case of failures in the primary next hop (node or link). For this, the basic requirement is that the selected backup neighbor provides a loop free path with respect to primary next hop towards a destination, originating a set of interior gateway protocol (IGP) prefixes.

The following topology explains the deployment case where per prefix LFA feature is applicable.

Figure 1: Per-Prefix LFA Usage Scenario

ABR1 and ABR2 are area boundary routers (ABRs), dual homed to an IPv6 core network, which advertises the summary LSA for the prefix 10.0.1.0/24 with a metric of 10. Also, from PE router’s perspective, ABR1 is the best prefix originator for 10.0.1.0/24. In this case, P2 is not a valid LFA for ABR1 because of the equal cost multi paths (ECMP) {P2, PE, P1, ABR1} and {P2, ABR2, ABR1} causing some of the traffic to be looped back through the router PE (no valid LFA). However for ABR2, which is also a prefix originator for 10.0.1.0/24, P2 is a valid LFA because the only path is {P2, ABR2}.

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.)

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 three 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.

• Per-prefix loop-free alternates (LFAs)—It is a technology by which a neighbor can be used as a backup next hop to provide a local repair path for the traffic to flow temporarily in case of failures in the primary next hop (node or link). For this, the basic requirement is that the selected backup neighbor provides a loop-free path with respect to a primary next hop towards a destination, originating a set of interior gateway protocol (IGP) prefixes.

  In certain topologies and usage scenarios, it may be possible that multiple destinations are originating the same prefix and there is no viable LFA to the best prefix originator, while a non-best prefix originator has a viable LFA. Per-prefix LFA is a mechanism by which LFA to a non-best prefix originator can be used in lieu of the LFA to the best prefix originator to provide local repair. In such cases, per prefix LFA can be used to increase the local repair coverage for the OSPF protocol.

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.

This example demonstrates the use of link protection for interfaces that have OSPF enabled.

When you enable link protection, Junos OS creates an alternate path to the primary next hop for all destination routes that traverse a protected interface. Use link protection when you assume that only a single link might become unavailable but that the neighboring node would still be available through another interface.

To calculate the remote LFA backup path, the OSPF protocol determines the remote LFA node in the following manner:

1. Calculates the reverse shortest path first from the adjacent router across the protected link of a PLR. The reverse shortest path first uses the incoming link metric instead of the outgoing link metric to reach a neighboring node.

   The result is a set of links and nodes, which is the shortest path from each leaf node to the root node.

2. Calculates the shortest path first (SPF) on the remaining adjacent routers to find the list of nodes that can be reached without traversing the link being protected.

   The result is another set of links and nodes on the shortest path from the root node to all leaf nodes.

3. Determines the common nodes from the above results. These nodes are the remote LFAs.

OSPF listens to the advertised labels for the LDP routes. For each advertised LDP route, OSPF checks whether it contains an LDP supplied next hop. If the corresponding OSPF route does have a backup next hop, then OSPF runs the backup policy and adds an additional tracking route with the corresponding LDP label-switched path next hop as the backup next hop. If there are no backup next hops, LDP builds a dynamic LDP tunnel to the remote LFA, and LDP establishes a targeted adjacency between the remote LFA node and the PLR node. This backup route has two LDP labels. The top label is the OSPF route, which denotes the backup path from the PLR to the remote LFA route. The bottom label is the LDP MPLS label-switched path that denotes the route for reaching the ultimate destination from the remote LFA. When an LDP session goes down and a remote tunnel is no longer available, OSPF changes all the routes that have been using this backup LDP tunnel.

LDP might be vulnerable by an automatically targeted adjacency, and these threats can be mitigated using all or some of the following mechanisms:

• Remote LFAs that are several hops away use extended hello messages to indicate willingness to establish a targeted LDP session. A remote LFA can reduce the threat of spoofed extended hello messages by filtering them and accepting only those originating at sources permitted by an access or filter list.

• There is a need to authenticate with TCP-MD5 all auto-targeted LDP sessions in the given IGP/LDP domain using apply groups or LDP global-level authentication.

• As an added security measure, the repair or remote tunnel endpoint routers should be assigned from a set of addresses that are not reachable from outside of the routing domain.

In an OSPF network, a loop free alternate(LFA) is a directly connected neighbor that provides precomputed backup paths to the destinations reachable via the protected link on the point of local repair (PLR). A remote LFA is not directly connected to the PLR and provides precomputed backup paths using dynamically created LDP tunnels to the remote LFA node. The PLR uses this remote LFA backup path when the primary link fails. The primary goal of the remote LFA is to increase backup coverage for the OSPF networks and provide protection for Layer 1 metro-rings. This example shows how to configure remote LFA for LDP tunnels in an OSPF network for extending backup protection.