OSPF routing

[hmc250000]

We will be migrating to SDWAN and OSPF routing between our global offices. We have about 40 sites (1 or 2 hub sites, the rest spoke sites) and hope to keep it all in one area 0. Should we be worried about the OSPF database becoming too large and possibly slowing down traffic? 

Is there a way to summarize routes within an area (area 0)? It looks to me that OSPF advertises subnets in the long /24 prefixes even if I add summarized network statements. 

[David Ruess]

Hello,

We would need more information such as how many routes do you have now, what routing protocol are you running and how much distance between sites, how are the sites connected, how are the devices within each site connected, among other things.

Some things to consider:

1. Geographical location. If sites are separated by a substantial amount and everything is in Area 0, when a routing update occurs everyone has to get it so it may take a bit to propagate through the entire network. Plus every 20 minutes OSPF refloods out the network topology by default.

2. You can only summarize at ABRs and ABSRs not within an area itself.

3. Depending on the connection type you would need a DR/BDR for every link to each router causing more LSA traffic.

4. If you are using something like DMVPN you would need the HUB interface pointing to the spokes and the spokes in the same area weather it be Area 0 or another Non-0 Area

An idea could possibly be your 2 main sites put in Area 0 and the other remote sites in its own Non-Area 0. That way you can summarize at every site towards Area 0, send a default route into each spoke site form the HUB sites, and reduce network convergence (every non-0 area has to connect to Area 0). 

Again all this is assuming alot of your network, but hopefully its a good start.

-David

[MHM Cisco World]

https://journey2theccie.wordpress.com/2020/12/18/ospf-over-dmvpn-phase-3-filtering-lsas/

please find link how OSPF deal with DB in DMVPN network. 
I think you can apply same.

[Joseph W. Doherty]

". . . hope to keep it all in one area 0."

BTW, if only using a single area, it doesn't need to be area zero.

"Should we be worried about the OSPF database becoming too large and possibly slowing down traffic?"

Maybe.  This is a very, very much a "it depends" answer.

Often, the issue with large, single area, OSPF topologies, with modern "high-speed" WAN link (which I assume, in your case, is more likely because of SD-WAN) often isn't the issue of sloshing around the topology data, but the impact of recomputing routes with any topology change.

Also, a vendor's implementation of OSPF can make a huge difference.

"Is there a way to summarize routes within an area (area 0)?"

Not that I recall (?).  You can summarize on ABR sending routes into area zero and you can block many routes, from area zero, by the kind of non-zero area it is.  (I recall [?] you can summarize on ASBRs, as to what they inject into their connected area.)

"It looks to me that OSPF advertises subnets in the long /24 prefixes even if I add summarized network statements."

What do you interface and network statements look like?

Is no auto-summary defined in the OSPF process?