Looking for real world OSPF example...

Okay, I'm getting ready to test for the BSCI. I understand much of the OSPF stuff conceptually, but am having a hard time getting my hands around some details without a large lab network to try it on. I've set it up on a few pieces of equipment with no issues, but it still doesn't give me the bigger picture of a larger network.

What is the norm for larger networks that span multiple areas and are not hub and spoke. Do most people just use one large area 0 and no other areas? One large area 0 and some other areas where they make sense? Why?

In a larger area that spans multiple subnets, what is the best way to identify which routers are the BDR and DR. Is a BDR and DR elected once for the whole area, or once for each broadcast segment?

Thanks for any input...

Jim

Reply to
Scooby
Loading thread data ...

A (show ip ospf interface) will show this info. DR and BDR are elected from the router with the highest RID. The RID is the highest IP addresses on any active interface, or the highest IP on any loopback interface(s). So you can control DRs and BDRs with the IP addresses you assign to your loopback interfaces.

My understanding is that DR's and BDR's are formed from adjacences (routers that can talk directly to one another), areas have nothing to do with DRs and BDRs.

Reply to
Freeride

Hi,

Using a "flat" design is not very usefull in a large enterprise network. Some very large multinational companies use an IGP like OSPF as well as BGP to solve scalability issues.

More routes you have and more powerful your router has to be, so if you have one flat area every router has the same LSA database ( rember that when a change occur every router has to perform the Dijkstra algorithm ) and every router must be "powerful"... "segmenting" using areas let you have "big" routers only in the backbone.

Since the DR and the BDR has to make an adjacency with every other router on the LAN segment it is preferred to use a powerful router if there is a choice; you've also to consider what other rules the router has to perform ( ABR, ASBR, BGP, ecc. ) to make the right choise... if for example you have a segment with 30 low-performing routers and 2 "high-end" routers it's better to make those two router DR and BDR unless they are overburdened with other heavy duties.

another way is to set the OSPF router-id or use interface OSPF priority ( eg. ip ospf priority ) to force a router's interface to be DR/BDR for a LAN segment.

Regards, Gabriele

Reply to
Gabriele Beltrame

"my" valid reasons for split into different areas: scale - 100 routers in any area (including the backbone) - practical limit is less. resilience - split highly resilient bits of the network into different areas to low resilience reliability - bits with likely flapping links and other reliability issues should be in separate areas adr aggregation - only places you can aggregate in OSPF are at ABRs or where you import routes (eg blocks of statics for dialup). loops - scale limits of an area reduce as you increase the number of "loops" within it - because of the way LSAs circulate when a large topology change happens.

always try to minimise the size of the backbone - as it always tends to grow, and it has to keep to the area size limits as well.

having a neighbour in itself costs in terms of CPU, memory etc.

So run interfaces in passive where you dont need that subnet to carry transit traffic (or use external for the interface instead).

Classic case is where there are dual central routers in a campus - you only need a couple of the dozens of subnets to build a neighbour adj between the

2 switches. As a side effect this also controls which way traffic will go during a fault.

Not much in practice - because the election is sticky. so once something gets elected, you only have another election when you force that device out of being DR.

so all this does is alter the probability. if you tend to reboot your high priority device a lot, it probabyl wont end up as DR.

if you have to control DR / BDR, set priority to 0 on some devices so they cannot get elected.

this means you can spread the DR around when the box has lots of interfaces - but in practice this is just too much hassle to be worth it 99% of the time.....

Reply to
stephen

Cabling-Design.com Forums website is not affiliated with any of the manufacturers or service providers discussed here. All logos and trade names are the property of their respective owners.