LSP trace behavior when S2L path traverses a re-merge node

When a 7450 ESS, 7750 SR or 7950 XRS LSR performs a re-merge of one or more ILMs of the P2MP LSP to which the traced S2L sub-LSP belongs, it may block the ILM over which the traced S2L resides. This causes the trace to either fail or to succeed with a missing hop.

The following is an example of this behavior.

S2L1 and S2L2 use ILMs which re-merge at node B. Depending on which ILM is blocked at B, the TTL=2 probe either yields two responses or times out.

S2L1 = ACBDF (to leaf F)
S2L2 = ABDE (to leaf E)
 
   A
 /  \
B -- C
|  
D
| \
F  E

• tracing S2L1 when ILM on interface C-B blocked at node B

  For TTL=1, A gets a response from C only as B does not have S2L1 on the ILM on interface A-B.

  For TTL=2, assume A gets first the response from B which indicates a success. It then builds the next probe with TTL=3. B only passes the copy of the message arriving on interface A-B and drops the one arriving on interface C-B (treats it like a data packet because it does not expire at node B). This copy expires at F. However, F returns a "DSMappingMismatched" error message because the DDMAP TLV was the one provided by node B in TTL=2 step. The trace aborts at this point in time. However, A knows it received a second response from Node D for TTL=2 with a "DSMappingMismatched" error message.

  If A gets the response from D first with the error code, it waits to see if it gets a response from B or it times out. In either case, it logs this status as multiple replies received per probe in the last probe history and aborts the trace.

• tracing S2L2 when ILM on interface A-B blocked at node B

  For TTL=1, B responds with a success. C does not respond as it does not have an ILM for S2L2.

  For TTL=2, B drops the copy coming on interface A-B. It receives a copy coming on interface B-C but drops it as the ILM does not contain S2L2. Node A times out. Next, node A generates a probe with TTL=3 without a DDMAP TLV. This time node D responds with a success and includes its downstream DDMAP TLV to node E. The rest of the path is discovered correctly. The traced path for S2L2 looks like: A-B-(*)-D-E.

The router ingress LER detects a re-merge condition when it receives two or more replies to the same probe, such as the same TTL value. It displays the following message to the user regardless if the trace operation successfully reached the egress LER or was aborted earlier:

Probe returned multiple responses. Result may be inconsistent.

This warning message indicates to the user the potential of a re-merge scenario and that a p2mp-lsp-ping command for this S2L should be used to verify that the S2L path is not defective.

The router ingress LER behavior is to always proceed to the next ttl probe when it receives an OK response to a probe or when it times out on a probe. If however it receives replies with an error return code, it must wait until it receives an OK response or it times out. If it times out without receiving an OK reply, the LSP trace must be aborted.

Possible echo reply messages and corresponding ingress LER behaviors are described in Table: Echo reply messages and ingress LER behavior.