OSPF and SPF Algorithm

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I have been unable to find an answer for this question...when OSPF runs the
SPF algorithm, is the routing table wiped and started from scratch or is it
just updated with changes?  Most of what I've found would suggest it is
started over from scratch, but nothing I've read really says for sure.

Thanks in advance,


Re: OSPF and SPF Algorithm

Look at the timers in the route table.

Purposely cause a route to fail and grab another route.

Compare the timer for the molested route with the rest of the entries in the
route table.

If all the timers are the same, I suspect the table is startet from scratch.
If they are not, then I suspect just the new (molested) route was re-set as
far as 'show ip route' is concerened.....

I suspect the actual answer is "implementation"

Interesting question by the way. Good for the "routing" category in


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Re: OSPF and SPF Algorithm

On Mon, 25 Apr 2005, John Agosta wrote:

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OSPF is a link-state routing protocol. It uses incremental
updates (LSU) to notify other routers, not the entire routing table
like RIP, SPF will only be run on the affected routes.


Re: OSPF and SPF Algorithm

Yeah, I know about link state, etc...
Is there not a timer in the route table?
I forget.....

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Re: OSPF and SPF Algorithm

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Yea I believe there is a saperate timer for each entry in routing table. I
believe default time is 30 mins.

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Re: OSPF and SPF Algorithm

On Tue, 26 Apr 2005, Saad Ahmed wrote:

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I think you are talking about the default LSA MaxAge in the
link-state database, not routing entry. LSAs are flooded through out an
are when there are changes or when the LSAs have reached its MaxAge.

Anyway, I looked it up in RFC2328 and found the following:

16.  Calculation of the routing table

    This section details the OSPF routing table calculation.  Using its
    attached areas' link state databases as input, a router runs the
    following algorithm, building its routing table step by step.  At
    each step, the router must access individual pieces of the link
    state databases (e.g., a router-LSA originated by a certain router).
    This access is performed by the lookup function discussed in Section
    12.2.  The lookup process may return an LSA whose LS age is equal to
    MaxAge.  Such an LSA should not be used in the routing table
    calculation, and is treated just as if the lookup process had

    The OSPF routing table's organization is explained in Section 11.
    Two examples of the routing table build process are presented in
    Sections 11.2 and 11.3.  This process can be broken into the
    following steps:

    (1) The present routing table is invalidated.  The routing table is
        built again from scratch.  The old routing table is saved so
        that changes in routing table entries can be identified.
<end include>

So the answer is yes, the routing table is built from scratch but only
new information is updated.


Re: OSPF and SPF Algorithm

CiscoKid napisał(a):
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You have right. If there is a change in network topology first the
topology database is updated, then SPF algorithm generate new routing
table. Old routing table is replaced with new when SPF finish building
new routing table and hold timer expires. It is described in CCNP
curriculum, chapter 6.2.6.


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