Questions stemming from the unidirectional nature of MPLS LSP

Unanswered Question
Mar 29th, 2010

After reading the following excerpt, I have some questions that I hope someone could clear up for me.

"An MPLS connection (LSP) is unidirectional—allowing data to flow in only one direction between two endpoints. Establishing two-way communications between endpoints requires a pair of LSPs to be established. Because 2 LSPs are required for connectivity, data flowing in the forward direction may use a different path from data flowing in the reverse direction."

Suppose I have a Network Management Station at each of the two geographically dispersed Data Centers, carrying out identical SNMP/ICMP-based monitoring against the same remote router at a third Data Center that's connected to the first two DCs via MPLS sold by the same ISP. There've been occasions when only one NMS (at say DC_A) reported the router at the third DC "down" (unreachable). Upon seeing the alert, the network operators sitting in DC_B pinged the remote router at DC_C successfully, therefore concluding the NMS was "crying wolf". I had thought it meant there's a problem with the particular MPLS circuit over there. But given the above paragraph about MPLS' unidirectionality, and the fact multiple interruptions in either forward or reverse direction could cause the NMS software to eventually conclude that the remote router has become unreachable, was my original conclusion wrong? What correct conclusions could be drawn from the aforementioned scenario of a single NMS reporting an outage at the other end of the MPLS? Furthermore, is it possible that actual business application traffic flowing through the same DC_A <=> DC_C MPLS could be unimpacted while my NMS there was alerting, due to the former taking a "different path"?

I have this problem too.
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Giuseppe Larosa Mon, 03/29/2010 - 08:24

Hello Yjdabear,

your understanding is correct in line of principle one LSP can be broken and the NMS of site A complains of connectivity failure.

testing on path site A to site C can provide a different result then test from site B to site C

To be noted that there are some tests that could be run on routers to check the health state of LSPs


so there are ways to understand if LSPs between site A and site C and between site B and site C are operational

Hope to help



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