Fiber cut situation analysis
During Fiber cut, time can be divided into three parts.
There is no defined time for degrade to be converted to SF condition. It
varies depending on fiber cut. If the fiber cut method, it may be almost 0 or
few milliseconds.
Router response ( as LAN PHY)
lLAN PHY normally declares port down depending on BFD timer values.
l@ time T1 to T3, router can loose BFD packets or get errors BFD packets, which are discarded.
lFrom time T-3 to T5, router will not get any BFD packets.
lIf the time from T1 to T3 is more than hold off time, Router will declare Port down event.
Router response ( as WAN PHY)
lWAN PHY Starts SD condition from time T1 or little later depending on SD detection mechanism.
lHold off time is started as soon as SD/SF condition is declared.
lAfter completion of switching, T5, SD condition is cleared depending on method of SD clearance. It may take fewer milliseconds for declaring SD clearance.
lDue to SD condition presence , If T1-T5 time is more than hold off timer, Port will declared as down.
lWhen SF condition is declared, RDI is sent to report port. RDI cleared after SD condition is cleared. If RDI duration is not managed , Peer router port will be down due to RDI condition.
Few Simulation Test Results.
Case 1: Hold off 100ms
Condition: Fast Fiber break
Result : When the hold-off time is set to 100 ms for router ports, a switchover is performed on the WDM device and no transient service interruption occurs on the routers.
Result analysis: The switchover triggered by a fiber cut functions properly.
Case 2: Hold off 100ms
Condition: Simulate Degradation on WDM Line
Test result: Service interruptions occur on the routers. Service interruption occurs on the router each time of the test (the test is repeated five times).
Result analysis: Signals on the WDM link deteriorate gradually, but not to the extent to trigger an SD switchover yet. However, the routers start the hold-off timer. When the timer expires (100 ms), the state of the router ports becomes Down.
Few Simulation Test Results.
Case 3: Hold off 150- 175ms
Condition: Simulate Degradation on WDM Line
Test result: This test is repeated 20 times for each setting. When the hold-off time is set to 175 ms, LINK_DOWN does not occur.
When the hold-off time is set to 150 ms, LINK_DOWN occurs in 8 of the 20 tests.
Result analysis: Setting the hold-off time to a larger value can prevent the Down state of router ports caused by excessive bit errors on the live network.
Recommendation
LAN PHY Links
1.Set hold off time of 200ms for LAN PHY links using BFD timers.
WAN PHY Links.
1.Set hold off time of 200ms for WAN PHY links for SF condition.
2.Set hold off time of few milliseconds for SD condition for WAN links. ( depending on network conditions this value can be changed)
3.Remove RDI condition for link down hold – off
