The P2MP LSP trace complies to RFC 6425. An LSP trace can be generated by entering the following OAM command:
- oam p2mp-lsp-trace lsp-name p2mp-instance instance-name s2l-dest-address ip-address [fc fc-name [profile {in | out}]] [size octets] [max-fail no-response-count] [probe-count probes-per-hop] [min-ttl min-label-ttl] [max-ttl max-label-ttl] [timeout timeout] [interval interval] [detail]The LSP trace capability allows the user to trace the path of a single S2L path of a P2MP LSP. Its operation is similar to that of the p2mp-lsp-ping command but the sender of the echo reply request message includes the downstream mapping TLV to request the downstream branch information from a branch LSR or bud LSR. The branch LSR or bud LSR then also includes the downstream mapping TLV to report the information about the downstream branches of the P2MP LSP. An egress LER does not include this TLV in the echo response message.
The probe-count parameter operates in the same way as in LSP trace on a P2P LSP. It represents the maximum number of probes sent per TTL value before giving up on receiving the echo reply message. If a response is received from the traced node before reaching maximum number of probes, then no more probes are sent for the same TTL. The sender of the echo request then increments the TTL and uses the information it received in the downstream mapping TLV to start sending probes to the node downstream of the last node which replied. This continues until the egress LER for the traced S2L path replied.
Because the command traces a single S2L path, the timeout and interval parameters keep the same value range as in LSP trace for a P2P LSP.
The P2MP LSP Trace makes use of the Downstream Detailed Mapping (DDMAP) TLV. The following excerpt from RFC 6424 details the format of the new DDMAP TLV entered in the path-destination belongs to one of the possible outgoing interface of the FEC.
0 1 2
3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0
1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| MTU | Address Type | DS Flags
|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Downstream Address (4 or 16 octets)
|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Downstream Interface Address (4 or 16 octets)
|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Return Code | Return Subcode | Subtlv Length
|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
.
.
. List of SubTLVs
.
.
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The Downstream Detailed Mapping TLV format is derived from the Downstream Mapping (DSMAP) TLV format. The key change is that variable length and optional fields have been converted into sub-TLVs. The fields have the same use and meaning as in RFC 8029.
Similar to p2mp-lsp-ping, an LSP trace probe results on all egress LER nodes eventually receiving the echo request message but only the traced egress LER node replies to the last probe.
Also any branch LSR node or bud LSR node in the P2MP LSP tree may receive a copy of the echo request message with the TTL in the outer label expiring at this node. However, only a branch LSR or bud LSR which has a downstream branch over which the traced egress LER is reachable must respond.
When a branch LSR or BUD LSR node responds to the sender of the echo request message, it sets the global return code in the echo response message to RC=14 - "See DDMAP TLV for Return Code and Return Sub-Code" and the return code in the DDMAP TLV corresponding to the outgoing interface of the branch used by the traced S2L path to RC=8 - "Label switched at stack-depth <RSC>".
Because a single egress LER address, for example an S2L path, can be traced, the branch LSR or bud LSR node sets the multipath type of zero in the downstream mapping TLV in the echo response message as no egress LER address need to be included.