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Label Distribution Protocol
A candidate upstream LSR node must be either an ECMP next hop or an LFA next
hop. Either option allows the downstream LSR to perform a fast switchover and to
source the traffic from another upstream LSR while IGP is converging due to a failure
of the LDP session of the upstream peer, which is the primary next hop of the root
LSR for the point-to-multipoint FEC. That is, the candidate upstream LSR node
provides upstream FRR node protection for the mLDP FEC packets.
Multicast LDP fast upstream switchover is illustrated in
primary next hop for the root LSR R of the point-to-multipoint FEC. LSR U' is an
ECMP or LFA backup next hop for the root LSR R of the same point-to-multipoint
FEC.
Figure 31
In
LSR nodes, and programs the primary ILM on the interface to LSR U and the backup
ILM on the interface to LSR U'. The labels for the primary and backup ILMs must be
different. Thus LSR Z attracts traffic from both ILMs. However, LSR Z blocks the ILM
on the interface to LSR U' and only accepts traffic from the ILM on the interface to
LSR U.
If the link to LSR U fails, or LSR U fails, causing the LDP session to LSR U to go
down, LSR Z will detect the failure and reverse the ILM blocking state. In addition,
LSR Z immediately starts receiving traffic from LSR U' until IGP converges and
provides a new primary next hop and a new ECMP or LFA backup next hop, which
may or may not be on the interface to LSR U'. When IGP convergence is complete,
LSR Z updates the primary and backup ILMs in the datapath.
Note: LDP uses the interface of either an ECMP next hop or an LFA next hop to the root
LSR prefix, whichever is available, to program the backup ILM. However, ECMP next hop
and LFA next hop are mutually exclusive for a given prefix. IGP installs the ECMP next hop
in preference to the LFA next hop as a prefix in the routing table manager (RTM).
408
Multicast LDP Fast Upstream Switchover
Figure
31, downstream LSR Z sends a label mapping message to both upstream
Use subject to Terms available at: www.nokia.com
R
U'
5
Z
10
Leaf
© 2022 Nokia.
Figure
31. LSR U is the
U
4
Leaf
3HE 18686 AAAB TQZZA
MPLS Guide
25937
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