A Link Fails in the Network
Let’s now say that one of the links (Link 1) in Figure 1 fails with the result that the LSP123 has to find another path, such as R2, R7, R6, R5. An important point about such path placement is that there may be additional constraints, such as, a requirement for 10Mbps bandwidth.
If no path can be found, the data carried across LSP123 might get dropped. Let’s assume that this is the case (some router vendors include technology called fast reroute to automatically attempt to reroute the LSP), so we now have what’s called a service-affecting problem.
The failure of Link 1 will be picked up by one or both of R2 and R3 with the result that a notification message is sent to the network management system. So we lost the link and the notification message has been passed into the NMS.