Feature Behavior
Page 574 7450 ESS MPLS Guide
switchover and 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 P2MP FEC. In a sense it provides an upstream Fast-Reroute (FRR) node-protection capability
for the mLDP FEC packets.
Figure 54: mLDP LSP with Backup Upstream LSR Nodes
Upstream LSR U in Figure 54 is the primary next-hop for the root LSR R of the P2MP FEC. This
is also referred to as primary upstream LSR. Upstream LSR U’ is an ECMP or LFA backup next-
hop for the root LSR R of the same P2MP FEC. This is referred to as backup upstream LSR.
Downstream LSR Z sends a label mapping message to both upstream LSR nodes and programs the
primary ILM on the interface to LSR U and a backup ILM on the interface to LSR U’. The labels
for the primary and backup ILMs must be different. LSR Z thus will attract traffic from both of
them. However, LSR Z will block the ILM on the interface to LSR U’ and will only accept traffic
from the ILM on the interface to LSR U.
In case of a failure of the link to LSR U or of the LSR U itself causing the LDP session to LSR U
to go down, LSR Z will detect it and reverse the ILM blocking state and will immediately start
receiving traffic from LSR U’ until IGP converges and provides a new primary next-hop, and
ECMP or LFA backup next-hop, which may or may not be on the interface to LSR U’. At that
point LSR Z will update the primary and backup ILMs in the data path.
Note that LDP will use the interface of either an ECMP next-hop or a LFA next-hop to the root
LSR prefix, whichever is available, to program the backup ILM. ECMP next-hop and LFA next-
hop are however mutually exclusive for a given prefix. IGP installs the ECMP next-hop in
preference to an LFA next-hop for a prefix in the Routing Table Manager (RTM).
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