OSPF Issues

Unanswered Question
Jul 24th, 2008

I just want to know what can be the Issues with running OSPF in a Network.

If anybody has faced any issues in a Network due to OSPF please share ur experiences regarding OSPF Issues.

I have this problem too.
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Giuseppe Larosa Thu, 07/24/2008 - 00:17

Hello Sushil,

may you describe your network environment (using NBMA etc.) it is your net single area or multiple area ?

If you use OSPF single area you should see if it is time to move to a multi-area design.

Multi-Area provides a lot of benefits: it reduces the probabilty of ospf recalculations because topology changes are confined to a single area.

Are you trying to redistribute BGP into OSPF ?

Be aware that BGP can handle much more prefixes then OSPF.

Hope to help

Giuseppe

guruprasadr Thu, 07/24/2008 - 00:54

HI, [Pls RATE if HELPS]

Noticable point in OSPF is E2 Routes. The MAX amount of E2 Routes in the Network will impact the CPU Performance. Beacause each E2 is CPU intensive.

When you redistribute something into OSPF the same will be converted into E2 Route.

By Controlling the Amount of E2 Routes will show better performance. The same can be arrested by carrying the Routes with BGP.

Hope i am Informative.

Pls RATE if HELPS

Best Regards,

Guru Prasad R

Joseph W. Doherty Thu, 07/24/2008 - 03:09

Perhaps the major issue with OSPF is its usage of the Dijkstra algorithm. This algorithm doesn't scale well, which is why there's often so much emphasis on area design with OSPF. Often not fully understood is how much network topology can be safely contained within an area, which can be more involved then just having X number of OSPF routers within an area. When these factors are ignored, I have seen OSPF meltdown from a link flap.

Another issue with OSPF is its costing capabilities. If you have a large spread between the fastest links and slowest links, OSPF might be unable to indicate different path costs if the accumulated path cost "overflows", so for instance a T1 link might be costed the same as a fractional T1. This is not the issue of what Cisco's OSPF is using for a base cost metric, although it can be related to it. (On that subject, believe Cisco sets a base cost of 1 for 100 Mbps, so without adjustment or override, 100 Mbps and faster links all appear the same cost.)

[edit]

PS:

My understanding of the OSPF standard, some issues are not fully defined and left to the implementation. (Sort of like how big an INT is in C.) My experience has been Cisco's implemention has been modified over the years to better handle some corner cases. Other vendors implementations are not always as robust. In other words, I've seen a condition crash brand X but not crash Cisco.

AJAZ NAWAZ Thu, 07/24/2008 - 05:22

From my experience one of the biggest drawbacks with OSPF is the ability to control routes e.g filtering. EIGRP is better than OSPF for this. i/eBGP are in their own class when for route control.

That said OSPF is very powerful and good candidate for interior dynamic routing. Implemented correctly in terms of sizing and areas it offers fairly reasonable convergence times. The key thing about OPSF is IP addressing design eg. try to avoid discontiguous subnets. John T. Moy & Thomas M. Thomas are amongst the most popular writers on this topic. And there would be no justice without mentioning Jeff Doyle.

hth,

Ajaz

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