IS-IS and OSPF Support for Loop-Free Alternate Calculation
Page 558 7450 ESS MPLS Guide
tunneled next-hop as the primary next-hop for a given prefix. LDP is also able of resolving a FEC
to a tunneled next-hop when the IGP shortcut feature is enabled.
When both IGP shortcut and LFA are enabled in IS-IS or OSPF, and LDP FRR is also enabled,
then the following additional LDP FRR capabilities are supported:
1. A FEC which is resolved to a direct primary next-hop can be backed up by a LFA tunneled
next-hop.
2. A FEC which is resolved to a tunneled primary next-hop will not have an LFA next-hop. It
will rely on RSVP FRR for protection.
The LFA SPF is extended to use IGP shortcuts as LFA next-hops as explained in Loop-Free
Alternate Calculation in the Presence of IGP shortcuts on page 562.
IS-IS and OSPF Support for Loop-Free Alternate Calculation
SPF computation in IS-IS and OSPF is enhanced to compute LFA alternate routes for each learned
prefix and populate it in RTM.
Figure 52 illustrates a simple network topology with point-to-point (P2P) interfaces and highlights
three routes to reach router R5 from router R1.
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