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OSPF Question


we have a big network and all the routers are in area 0 .below is a part from show ip route

O E2 [110/20] via, 00:36:34, Serial0/0

O [110/55] via, 00:36:34, Serial0/0

O [110/145] via, 00:36:34, Seria0/0

O [110/55] via, 00:36:34, Serial0/0

as ypu notice above the timer is 00:36:34 .

What does that mean?Is this a big problem if the routes are refreshed every approximately 30 minutes?

7 Replies 7

Kevin Dorrell

What that means is that the last time the SPF calculation was done was 36:34 ago, which means that some link change took place 36:34 ago. Is this happening at regular intervals, or haphazardly?

One of the disadvantages of having all your routers in one big area is that every time a link changes state in the area, then every router in the area recalculates every route in the area. The advantage of dividing the network into areas is that a link change in an area has an impact only within that area.

Kevin Dorrell


this Is happening at haphazardly intervals

If it is happening haphazardly then the logical assumption is that it is caused because some link changed state (went from up to down or from down to up). As Kevin pointed out, one of the issue with an implementation of OSPF all in one area is that whenever any router needs to run the SPF then all routers must run the SPF. In smaller networks this is not much of an issue and as the size of the network increases the impact of this also increases. I am not clear what size network we are talking about here, so I am not sure how much impact it really has.

If it is the result of links changing state then in addition to considering multi area OSPF the thing that you can do is to try to make links more stable.





Area zero contains about 50 Routers

There is not any particular number of routers at which OSPF should be configured with multiple areas. There are a number of variables which influence whether multi-area should be adopted. These variables include how many interfaces on the routers, how often links are changing state and causing calculation of the SPF, and to some extent what the geographical relationships are. At 50 routers in area 0 it might be worthwhile considering changing your OSPF to use multiple areas.

The original post asked if it were a problem to refresh the routes (calculate the SPF) every 30 minutes. We are not in a good position to know how much of a problem. In general I would say that calculation every 30 minutes is not a big problem. But you would need to look at the performance of your network and determine if performance is suffering because of SPF calculation.






1 query related to this topic. I read that all router participating in ospf process within an area, synchronise their databases everytime there is a link change or complete databse synch every 30 min by default.

If that is the case then at any moment of time, the advertisements should be not older than 30 min. In another question on this forum, I read the databse was last synchronised weeks ago. how is that possible?

Regards SPF Calculations:

every 30 mins (default setting) a Link State Update is sent.

On the router that receives the LSU: If the entry already exists and the received LSU has the same info, the router ingnores the Link State Advertisement.

So if the network is the same, it will not re-calculate SPF.

Regards Routers per Area:

Studies and real-world implementations have led to the following OSPF design guidelines, as documented in the Cisco Press book OSPF Network Design Solutions:

Routers in Domain Min. 20 Mean 510 Max 1000

Routers per single area Min. 20 Mean 160 Max 350

Area per domain Min. 1 Mean 23 Max 60

if you get some snmp(cpu-memory)/netflow(network utilization) graphs set up on your network monitoring system you should be able to see and then determine if this is undue strain.

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