Ldp Ecmp Uniform Failover - Alcatel-Lucent 7450 Manual

LDP ECMP Uniform Failover

LDP ECMP uniform failover allows the fast re-distribution by the ingress data path of packets
forwarded over an LDP FEC next-hop to other next-hops of the same FEC when the currently
used next-hop fails. The switchover is performed within a bounded time, which does not depend
on the number of impacted LDP ILMs (LSR role) or service records (ingress LER role). The
uniform failover time is only supported for a single LDP interface or LDP next-hop failure event.
This feature complements the coverage provided by the LDP Fast-ReRoute (FRR) feature, which
provides a Loop-Free Alternate (LFA) backup next-hop with uniform failover time. Prefixes that
have one or more ECMP next-hop protection are not programmed with a LFA back-up next-hop,
and vice-versa.
The LDP ECMP uniform failover feature builds on the concept of Protect Group ID (PG-ID)
introduced in LDP FRR. LDP assigns a unique PG-ID to all FECs that have their primary Next-
Hop Label Forwarding Entry (NHLFE) resolved to the same outgoing interface and next-hop.
When an ILM record (LSR role) or LSPid-to-NHLFE (LTN) record (LER role) is created on the
IOM, it has the PG-ID of each ECMP NHLFE the FEC is using.
When a packet is received on this ILM/LTN, the hash routine selects one of the up to 32, or the
ECMP value configured on the system, whichever is less, ECMP NHLFEs for the FEC based on a
hash of the packet's header. If the selected NHLFE has its PG-ID in DOWN state, the hash routine
re-computes the hash to select a backup NHLFE among the first 16, or the ECMP value
configured on the system, whichever is less, NHLFEs of the FEC, excluding the one that is in
DOWN state. Packets of the subset of flows that resolved to the failed NHLFE are thus sprayed
among a maximum of 16 NHLFEs.
LDP then re-computes the new ECMP set to exclude the failed path and downloads it into the
IOM. At that point, the hash routine will update the computation and begin spraying over the
updated set of NHLFEs.
LDP sends the DOWN state update of the PG-ID to the IOM when the outgoing interface or a
specific LDP next-hop goes down. This can be the result of any of the following events:
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In addition, PIP will send an interface down event to the IOM if the interface failure is detected by
other means than the LDP control plane or BFD. In that case, all PG-IDs associated with this
interface will have their state updated by the IOM.
When tunneling LDP packets over an RSVP LSP, it is the detection of the T-LDP session going
down, via BFD or Keep-Alive, which triggers the LDP ECMP uniform failover procedures. If the
7450 ESS MPLS Guide
Interface failure detected directly.
Failure of the LDP session detected via T-LDP BFD or LDP Keep-Alive.
Failure of LDP Hello adjacency detected via link LDP BFD or LDP Hello.
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