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Understanding Remote LFA over LDP Tunnels in IS-IS Networks

In an IS-IS 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 IS-IS networks and provide protection for Layer 1 metro-rings.

However, LFAs do not provide full backup coverage for IS-IS based Metro Ethernet networks, which are often deployed in a ring topology. To overcome this limitation, the Resource Reservation Protocol 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 IS-IS network and hence is the preferred protocol. The existing LDP implemented for the MPLS tunnel setup can be reused for protection of IS-IS 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 IS-IS 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.

IS-IS listens to the advertised labels for the LDP routes. For each advertised LDP route, IS-IS checks whether it contains an LDP supplied next hop. If the corresponding IS-IS route does have a backup next hop, then IS-IS 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 IS-IS 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, IS-IS changes all the routes that have been using this backup LDP tunnel.


Currently, Junos OS supports only IPv4 transport LSPs. If you need to reuse IPv4 transport LSPs for IPv6 IGP networks, add an IPv6 explicit NULL label to the label stack of the tracking route. The system automatically converts the IPv4 LSP to an IPv6 LSP.

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 hellos 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.