Remote LFA over LDP Tunnels in OSPF Networks Overview
In an OSPF network, a loop free alternate (LFA) is a directly connected neighbor that
provides precomputed backup paths to the destinations reachable through 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.
LFAs do not provide full backup coverage for OSPF networks. This is a major setback for
metro Ethernet networks that are often shaped as ring topologies. To overcome this
setback, Resource Reservation Protocol - Traffic Engineering (RSVP-TE) backup tunnels
are commonly used to extend the backup coverage. However, a majority of network
providers have already implemented LDP as the MPLS tunnel setup protocol and do not
want to implement the RSVP-TE protocol merely for backup coverage. LDP automatically
brings up transport tunnels to all potential destinations in an OSPF network and hence
is the preferred protocol. The existing LDP implemented for the MPLS tunnel setup can
be reused for protection of OSPF networks and subsequent LDP destinations, thereby
eliminating the need for RSVP-TE backup tunnels for backup coverage.
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
547Copyright © 2017, Juniper Networks, Inc.
Chapter 18: Configuring Routing Protocols
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