Loop-Free Alternate Calculation In The Presence Of Igp Shortcuts - Alcatel-Lucent 7450 Manual

IS-IS and OSPF Support for Loop-Free Alternate Calculation

Loop-Free Alternate Calculation in the Presence of IGP shortcuts

In order to expand the coverage of the LFA backup protection in a network, RSVP LSP based IGP
shortcuts can be placed selectively in parts of the network and be used as an LFA backup next-hop.
When IGP shortcut is enabled in IS-IS or OSPF on a given node, all RSVP LSP originating on this
node and with a destination address matching the router-id of any other node in the network are
included in the main SPF by default.
In order to limit the time it takes to compute the LFA SPF, the user must explicitly enable the use
of an IGP shortcut as LFA backup next-hop using one of a couple of new optional argument for the
existing LSP level IGP shortcut command:
config>router>mpls>lsp>igp-shortcut [lfa-protect | lfa-only]
The lfa-protect option allows an LSP to be included in both the main SPF and the LFA SPFs. For
a given prefix, the LSP can be used either as a primary next-hop or as an LFA next-hop but not
both. If the main SPF computation selected a tunneled primary next-hop for a prefix, the LFA SPF
will not select an LFA next-hop for this prefix and the protection of this prefix will rely on the
RSVP LSP FRR protection. If the main SPF computation selected a direct primary next-hop, then
the LFA SPF will select an LFA next-hop for this prefix but will prefer a direct LFA next-hop over
a tunneled LFA next-hop.
The lfa-only option allows an LSP to be included in the LFA SPFs only such that the introduction
of IGP shortcuts does not impact the main SPF decision. For a given prefix, the main SPF always
selects a direct primary next-hop. The LFA SPF will select a an LFA next-hop for this prefix but
will prefer a direct LFA next-hop over a tunneled LFA next-hop.
Thus the selection algorithm in Section 1.3 when SPF finds multiple LFA next-hops for a given
primary next-hop is modified as follows:
A) The algorithm splits the LFA next-hops into two sets:
B) The algorithms continues with first set if not empty, otherwise it continues with second
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→ the first set consists of direct LFA next-hops
→ the second set consists of tunneled LFA next-hops. after excluding the LSPs which
use the same outgoing interface as the primary next-hop.
set.
7450 ESS MPLS Guide