OSPF convergence

[cisco8887]

Hi Guys,

I am working on something and need a fairly quick answer and cannot run a lab and test.

How does OSPF convergence work when on link fails? it doens't store any backup path like eigrp does it ?

it will recalculate the path.

Is there a way to have a redandunt route in place to quickly failover ?

if not does anyone know how long the convergence take using SPF algorithm ? is my question like how long is a peice of string ?

Thanks

[David_Che]

If you configure LFA feature, OSPF will be immediate to switch-over to the backup route.

[cisco8887]

many thannks, does bgp support a multiple backup route like eigrp ?

[Mark Malone]

BGP is more flexible than OSPF it can have multiple paths installed in the BGP table waiting to take over from the primary , depending on your design and what your trying to do there are a few ways to achieve it , one way below is install command

bgp additional-paths install

http://www.cisco.com/c/en/us/td/docs/ios/iproute_bgp/command/reference/irg_book/irg_bgp1.html#wp1117217

With BGP though its best not to have a redundant path as it will be wan and costly to have sitting there so usually you would load share the links so there all used at once and at the same time use bgp path attributes so there backed up between each wan link

[cisco8887]

many thanks, so what you are saying is there will be no convergence ?

if you use ospf and tune the metric to load balance across three links, if one dies, does it trigger spf algorithm ? would you loose connectivity ?

[Joseph W. Doherty]

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Posting

How does OSPF convergence work when on link fails?

Normally, it recomputes a new best path.

it doens't store any backup path like eigrp does it ?

Normally, no.

Is there a way to have a redandunt route in place to quickly failover ?

Normally, no, but if you have two equal cost paths, besides using both normally, it will just use the remaining one.

if not does anyone know how long the convergence take using SPF algorithm ? is my question like how long is a peice of string ?

Laugh, yes much like how long is a piece of string.

Many variables are involved.  Convergence time is impacted by design and configuration settings.  The latter, when properly set, can lead to pretty quick OSPF convergence with a link failure.  See http://www.cisco.com/c/en/us/td/docs/solutions/Enterprise/Campus/routed-ex.html#wp998679