Alcatel-Lucent 7750 SR OS Service Manual page 1377

Consider the following network topology:
PE1 ---- A ----- B ----- C ------ PE2
Assume LDP label distribution operates in the independent control mode and there is no active
LDP session between nodes B and C. Node B will have an unlabeled entry for FEC PE2 and will
forward the packet using the IGP path. The LDP ECMP OAM tool can detect the discontinuity of
the LSP from PE1 to PE2.
Persistent loops are the other types of faults the LDP ECMP OAM can detect. Assume that link B-
C failed and was subsequently restored. Node A can now forward to node B packets destined to
PE2. A failure of the IGP/LDP re-convergence on link B-C will cause node B to forward those
packets back to A since node B's NLHFE still indicates that the next hop for PE2 FEC is node A.
The LDP ECMP OAM feature consists of the two main capabilities:
1. An LDP ECMP tree building capability each ingress PE node will run to discover all possible
paths to each egress PE FEC. This capability is based on having the ingress PE probe the
network with LSP trace messages to collect the multipath information for a FEC from all
nodes, including the egress LER.
a. This tool can run periodically, anywhere between 15 minutes to an hour, on an ingress
PE. There are parameters defined to stop the tool searching after a number of paths have been
discovered for a FEC. Hence, the periodic mode of operation of the tool is not intended to
cover 100% of the tree.
b. Users can run the tool in a single run mode without bounds in order to discover the entire
c. When running in the periodic mode, the following are the user-configurable parameters
d. There is no requirement to run this tool after a network event such as IGP re-
e. The ingress PE automatically add an egress PE which FEC it learned to the list of probed
f.
7750 SR OS Services Guide
\ /
--- D ----
tree.
to halt the probing process for a given egress FEC:
→ Maximum number of ECMP paths discovered per FEC.
→ Maximum number of hops into the network to probe for ECMP paths.
convergence.
destinations.
Users can configure a policy to prevent specific FECs from being probed by the ingress
PE.
OAM and SAA
Page 1379