Want to win a PS4? Go Premium and enter to win our High-Tech Treats giveaway. Enter to Win

x
?
Solved

OSPF - Convergence & Downtime

Posted on 2017-02-19
9
Medium Priority
?
145 Views
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
0
Comment
Question by:SrikantRajeev
[X]
Welcome to Experts Exchange

Add your voice to the tech community where 5M+ people just like you are talking about what matters.

  • Help others & share knowledge
  • Earn cash & points
  • Learn & ask questions
  • 4
  • 4
9 Comments
 
LVL 31

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).
0
 
LVL 1

Author Comment

by:SrikantRajeev
ID: 42015101
What is BFD ?
0
 
LVL 31

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.
0
 [eBook] Windows Nano Server

Download this FREE eBook and learn all you need to get started with Windows Nano Server, including deployment options, remote management
and troubleshooting tips and tricks

 
LVL 1

Author Comment

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.
0
 
LVL 1

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
0
 
LVL 62

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
0
 
LVL 31

Accepted Solution

by:
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.
0
 
LVL 1

Author Closing Comment

by:SrikantRajeev
ID: 42056926
Thanx
0
 
LVL 31

Expert Comment

by:Predrag
ID: 42056999
You're welcome.
0

Featured Post

Independent Software Vendors: We Want Your Opinion

We value your feedback.

Take our survey and automatically be enter to win anyone of the following:
Yeti Cooler, Amazon eGift Card, and Movie eGift Card!

Question has a verified solution.

If you are experiencing a similar issue, please ask a related question

In this article, I am going to show you how to simulate a multi-site Lab environment on a single Hyper-V host. I use this method successfully in my own lab to simulate three fully routed global AD Sites on a Windows 10 Hyper-V host.
I had an issue with InstallShield not being able to use Computer Browser service on Windows Server 2012. Here is the solution I found.
Internet Business Fax to Email Made Easy - With  eFax Corporate (http://www.enterprise.efax.com), you'll receive a dedicated online fax number, which is used the same way as a typical analog fax number. You'll receive secure faxes in your email, f…
This video gives you a great overview about bandwidth monitoring with SNMP and WMI with our network monitoring solution PRTG Network Monitor (https://www.paessler.com/prtg). If you're looking for how to monitor bandwidth using netflow or packet s…

610 members asked questions and received personalized solutions in the past 7 days.

Join the community of 500,000 technology professionals and ask your questions.

Join & Ask a Question