When the LDP tree trace feature is enabled, the ingress LER builds the ECMP tree for a specific FEC (egress LER) by sending LSP trace messages and including the LDP IPv4 Prefix FEC TLV as well as the downstream mapping TLV. To build the ECMP tree, the router LER inserts an IP address range drawn from the 127/8 space. When received by the downstream LSR, it uses this range to determine which ECMP path is exercised by any IP address or a sub-range of addresses within that range based on its internal hash routine. When the MPLS echo reply is received by the router LER, it records this information and proceed with the next echo request message targeted for a node downstream of the first LSR node along one of the ECMP paths. The sub-range of IP addresses indicated in the initial reply are used because the objective is to have the LSR downstream of the router LER pass this message to its downstream node along the first ECMP path.
The following figure illustrates the behavior through the following example adapted from RFC 8029:
PE1 ---- A ----- B ----- C ------ G ----- H ---- PE2
\ \---- D ------/ /
\ \--- E------/ /
-- F --------------------/
LSR A has two downstream LSRs, B and F, for PE2 FEC. PE1 receives an echo reply from A with the Multipath Type set to 4, with low/high IP addresses of 127.1.1.1->127.1.1.255 for downstream LSR B and 127.2.1.1->127.2.1.255 for downstream LSR F. PE1 reflects this information to LSR B. B, which has three downstream LSRs, C, D, and E, computes that 127.1.1.1->127.1.1.127 would go to C and 127.1.1.128-> 127.1.1.255 would go to D. B would then respond with 3 Downstream Mappings: to C, with Multipath Type 4 (127.1.1.1->127.1.1.127); to D, with Multipath Type 4 (127.1.1.127->127.1.1.255); and to E, with Multipath Type 0.
The router supports multipath type 0 and 8, and up to a maximum of 36 bytes for the multipath length and supports the LER part of the LDP ECMP tree building feature.
A user configurable parameter sets the frequency of running the tree trace capability. The minimum and default value is 60 minutes and the increment is 1 hour.
The router LER gets the list of FECs from the LDP FEC database. New FECs are added to the discovery list at the next tree trace and not when they are learned and added into the FEC database. The maximum number of FECs to be discovered with the tree building feature is limited to 500. The user can configure FECs to exclude the use of a policy profile.