A diagnostic code specifying the local system's reason for
the last session state change.
Options:
1. 0 -no diagnostic
2. 1 -control detection time expired
3. 2 -echo function failed
4. 3 -neighbor signaled session down
5. 4 -forwarding plane reset
6. 5 -path down
7. 6 -concatenated path down
8. 7 -administratively down
9. 8 -reverse concatenated path down
10. 9 -neighbor signaled session down (receive admindown)
11. 10-31 -reserved for future use