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OSPF - Convergence & Downtime

Posted on 2017-02-19
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Last Modified: 2017-03-20
I have the attached diagram.
ALL The L3 switches are configured with OSPF with hello timers as 10 s & Dead timers as 40 s
If the L3 switch as shown in the diagram fails or it gets rebooted (assuming the traffic from PC1 & PC2 is flowing through that switch) how much will be the downtime approximately.
OSPF_Diagram.png
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Question by:SrikantRajeev
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9 Comments
 
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Expert Comment

by:Predrag
ID: 42015082
Downtime length would depend on actual OSPF, interfaces configuration and type of error that caused downtime, but should not be longer than 40 seconds. The shortest downtime period could be 50 - 200ms (with BFD configured).
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by:SrikantRajeev
ID: 42015101
What is BFD ?
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Expert Comment

by:Predrag
ID: 42015105
Bidirectional Forwarding Detection
BFD is a detection protocol designed to provide fast forwarding path failure detection times for all media types, encapsulations, topologies, and routing protocols. In addition to fast forwarding path failure detection, BFD provides a consistent failure detection method for network administrators. Because the network administrator can use BFD to detect forwarding path failures at a uniform rate, rather than the variable rates for different routing protocol hello mechanisms, network profiling and planning will be easier, and reconvergence time will be consistent and predictable.
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by:SrikantRajeev
ID: 42015619
I have a basic question. Assuming I have not enabled BFD.

As shown in the Diagram the L3 swtich fails.
The switches which are connected to the failed switch will first detect physical layer down.
Once the physical layer down is detected will the switches will stop sending the traffic to the failed switch & choose the available active redundant path or it will wait till the hello & dead timers gets converged.
In this scenario what will be the down time.
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Author Comment

by:SrikantRajeev
ID: 42015647
I have made the clear diagram in the attachment. OSPF is enabled in all the L3 switches & have equal cost.
OSPF is having active - active path since the OSPF Cost is same.

Assuming R3 switch fails & the traffic between the PCs are flowing through this R3 switch.
R3 switch fails the R5 & R6 detects that the physical layer L2 is down.
Q1 - Will R5 & R6 immediately stops sending the traffic to R3  on detecting physical L2 layer down & send all the traffic to R4
Q2 - Or will it wait for the Hello Packets & Hold down timer to expire before forwarding all the packets to R4
Q3 - When will the routing table entry from R5 & R6 will remove pointing to R3

Basically would like to understand how many packet drop is experienced in this scenario.
Diagram.png
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Expert Comment

by:gheist
ID: 42015818
OSPF manages Layer 3 i.e multiple paths to IP network with default gateway. It has zero impact on L2 switching
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Predrag earned 2000 total points
ID: 42016172
I have made the clear diagram in the attachment.
The first or the second one (not the same diagrams)? The first one can actually have up to 50 seconds of convergence time (STP convergence can be longer that OSPF convergence time).
:)
Q1 - Will R5 & R6 immediately stops sending the traffic to R3  on detecting physical L2 layer down & send all the traffic to R4
Typically if the L2 or L1 link is down protocol will reroute around failure right away (few hundred milliseconds).
Q2 - Or will it wait for the Hello Packets & Hold down timer to expire before forwarding all the packets to R4
However, configuration dependent, actually can lead up till 40 seconds of downtime (and traffic can be blackholed). Simple example can be if links between switches are created as trunks and on one side VLAN used for OSPF neighbor is removed from trunk - in that case - it will wait until timer times out (on one side everything is OK - that one will send hello, but other side is not functioning properly). Links are up, but there are no hello messages. Or similarly devices have some problem (e.g memory leak) and does not function properly so links are up, but hello messages are not sent or not processed.
Q3 - When will the routing table entry from R5 & R6 will remove pointing to R3
That depends on actual scenario, as described above. If device is considered present until dead timer expires routes to non existing device will not be removed from neighboring devices and in the case of load balanced traffic will most likely lead to half of the traffic is not reaching target device (due to black hole) until routes from neighbor devices are removed.
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Author Closing Comment

by:SrikantRajeev
ID: 42056926
Thanx
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Expert Comment

by:Predrag
ID: 42056999
You're welcome.
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