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how do loop occur in ospf and how to prevent

Hi All,

Kindly let me know this answer

How do loop occur in OSPF and how to prevent ?

WISH U ALL VERY HAPPY NEW AND PROSPEROUS

Regards,

Sanjay

4 REPLIES

Re: how do loop occur in ospf and how to prevent

Hi Sanjay,

within an Area, all OSPF routers synchronize their link state databases (LSDB) and then run the shortest path first (SPF) algorithm (all the routers have the very same topology information but set themself as root of the tree), so the calculations result in loop-free shortest paths to the destinations. This is a key property of link-state routing protocols.

Unfortunately things are much more complicated between Areas, because here OSPF makes use of distance-vector mechanisms, so different loop prevention mechanisms have to be used. There is a great document about this topic with many details written by Petr Lapukhov: Inter-Area Loop Prevention in OSPF

HTH

Rolf

P.S.: A happy and healthy new year to you as well, we're still waiting here in good old 2013 ;-)

Super Bronze

Re: how do loop occur in ospf and how to prevent

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Within a single OSPF domain, without any redistribution with other routing protocols, the only two situations I can think of where you might see a routing loop if is OSPF is "broke" (e.g. some kind of bug) or perhaps during OSPF convergence (when not every router's topology database agrees).  The latter, though, is often very brief.  Not much you can do to totally preclude loops during convergence, but on Cisco platforms you can tune some parameters that will support faster convergence.

Regarding dealing with broken OSPF, avoiding bugs is sometimes aided by avoiding special code releases and/or new "new" feature code releases.  Also, Cisco devices generally don't deal well with low memory, so you want to insure you have adequate memory.

how do loop occur in ospf and how to prevent

Thanks Joseph, I completely forgot the "microloops" ...

how do loop occur in ospf and how to prevent

Having OSPF routers that use different path selection procedures within an OSPF domain can, under certain conditions, also result in loops.

Most router operating systems still use the original procedure (defined in RFC1583) by default (e.g. Cisco IOS).

The "new" procedure, defined in RFC 2328 (1998!), works very different; this is the default e.g. on NX-OS.

Linked document: Nexus 7000 and IOS OSPF Loop Prevention and Interoperability

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