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 Jeapardy.

-ja

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

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,

Mikey

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

CiscoKid napisa³(a):

Hi, 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.

Peter,

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:

1. 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 failed.

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. ....

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

Doan