If an MPLS traceroute is issued to an ACX5448 and there are ECMP paths tied to the egress of the traffic, the reply that the ACX sends does NOT contain all the next-hops.
This is a product limitation.
If the ACX has an MPLS ECMP egress path with 2 legs:
user@routerACX5448> show route 13.13.13.13 inet.0: 18 destinations, 18 routes (18 active, 0 holddown, 0 hidden) + = Active Route, - = Last Active, * = Both 13.13.13.13/32 *[IS-IS/15] 01:42:30, metric 20 > to 10.7.0.2 via xe-0/0/14.2225 to 10.8.0.2 via xe-0/0/14.2226 inet.3: 3 destinations, 3 routes (3 active, 0 holddown, 0 hidden) + = Active Route, - = Last Active, * = Both 13.13.13.13/32 *[LDP/9] 01:38:55, metric 20 > to 10.7.0.2 via xe-0/0/14.2225, Push 27 to 10.8.0.2 via xe-0/0/14.2226, Push 27
When the ACX processes the Echo Request and responds, the Downstream Mapping portion of the reply will only contain information for one leg:
Frame 8: 134 bytes on wire (1072 bits), 134 bytes captured (1072 bits) Juniper Ethernet Internet Protocol Version 4, Src: 100.1.1.1, Dst: 1.0.0.1 User Datagram Protocol, Src Port: 3503, Dst Port: 54022 Multiprotocol Label Switching Echo Version: 1 Global Flags: 0x0000 Message Type: MPLS Echo Reply (2) Reply Mode: Reply via an IPv4/IPv6 UDP packet (2) Return Code: Label switched at stack-depth RSC (8) Return Subcode: 1 Sender's Handle: 0xb0000000 Sequence Number: 1 Timestamp Sent: Jun 1, 2021 22:29:55.240345999 UTC Timestamp Received: Jun 1, 2021 18:17:12.758558999 UTC Detailed Downstream Mapping Type: Detailed Downstream Mapping (20) Length: 44 MTU: 9216 Address Type: IPv4 Numbered (1) DS Flags: 0x00 Downstream IP Address: 10.7.0.2 Downstream Interface Address: 10.7.0.2 Return Code: 0 Return Subcode: 0 Subtlv Length: 28 Multipath sub-TLV
This is in contrast to the behavior on the MX platform where the DM portion of the reply is populated with all available legs that are contained within the ECMP set:
Multiprotocol Label Switching Echo Version: 1 Global Flags: 0x0000 Message Type: MPLS Echo Reply (2) Reply Mode: Reply via an IPv4/IPv6 UDP packet (2) Return Code: Label switched at stack-depth RSC (8) Return Subcode: 1 Sender's Handle: 0xb0000000 Sequence Number: 1 Timestamp Sent: May 24, 2021 15:29:26.450299999 UTC Timestamp Received: May 24, 2021 19:42:15.218920999 UTC Detailed Downstream Mapping Type: Detailed Downstream Mapping (20) Length: 44 MTU: 1518 Address Type: IPv4 Numbered (1) DS Flags: 0x00 Downstream IP Address: 10.18.0.2 Downstream Interface Address: 10.18.0.2 Return Code: 0 Return Subcode: 0 Subtlv Length: 28 Multipath sub-TLV Detailed Downstream Mapping Type: Detailed Downstream Mapping (20) Length: 44 MTU: 1518 Address Type: IPv4 Numbered (1) DS Flags: 0x00 Downstream IP Address: 10.17.0.2 Downstream Interface Address: 10.17.0.2 Return Code: 0 Return Subcode: 0 Subtlv Length: 28 Multipath sub-TLV
Note that there is no loss of connectivity as at least one of the ECMP paths is returned in the Echo Reply.
Due to an ASIC limitation with the ACX5488, only information for one ECMP leg can be sent from PFE.
This is a product limitation. There is no service impact.