Juniper ACX1000 Configuration Manual page 623

Copyright © 2017, Juniper Networks, Inc.
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:
Calculates the reverse shortest path first from the adjacent router across the protected
1.
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.
Calculates the shortest path first (SPF) on the remaining adjacent routers to find the
2.
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.
Determines the common nodes from the above results, These nodes are the remote
3.
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.
NOTE: 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
Chapter 18: Configuring Routing Protocols
565