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Community Member

OSPF AREA 0

Hi,

 

I work for a company and noticed they having everything in area 0. They must have well over 350 routers in the core. As I was studying my CCNP I new there was a limitation to having some many routers in area 0. I have been told that they way this is designed is with loads of point to point interfaces so /30 so at max there is only 2 interface performing in area 0 at each time. Is this correct? is this a good OSPF design?

 

 

3 ACCEPTED SOLUTIONS

Accepted Solutions
Hall of Fame Super Gold

I think that I do not fully

I think that I do not fully understand your question, especially the part about /30 is only 2 interfaces performing in area 0 at each time. With all routers in area 0 when any of the routers sends an LSA representing a change in one of its networks then all of the routers will receive the update and all of the routers will have to process the shortest path algorithm and update their routing tables. That part is true whether interfaces are point to point or multipoint or whatever.

 

I do not think that any of us would say that this is an optimal design. But that does not necessarily mean that it is a bad design. We would need more information about the topology of the network and about how stable the network is to base an opinion about the design.

 

In general the advantages of designing OSPF with multiple areas is that it reduces the size of the link state data base and it reduces the frequency of running the SPF algorithm (you only need to run SPF when there is a change in your area but do not run SPF for changes in other areas). So multi area OSPF helps to insulate your routers from the effects of changes in remote areas.

 

HTH

 

Rick

Hall of Fame Super Gold

Using a /30 on an interface

Using a /30 on an interface does mean that there can be only a single neighbor on that connection. But it does not change the number of routers that must respond when any change occurs in the network.

 

When you configure priority zero on an interface it means only that this router will not become DR. The router will still become a neighbor and traffic will still be routed over that link. If you have OSPF neighbors and one of them is more powerful and one is less powerful you might prefer that the more powerful router becomes DR (since the DR does a bit more work than a DR-other router. Using priority 0 is a way to accomplish that.

 

HTH

 

Rick

VIP Purple

Hellowhen you say they have

Hello

when you say they have over 350 rtrs are all these running ospf and if so all connected to the backbone?

There's is a suggested rtr limitation for ospf regards areas etc but I would think its more to do with the amount of route being advertised within the area than the rtrs themselves regarding memory utilisation to process the link state db

 

Lots of rtrs with a small rib within one area I think wouldnt be much of a concern than a few rtrs with a  very large rib with one area.

I would say I lot would depend also on the processing CPU/memory specifications of the rtrs and if the rtrs are non transit within the area may be it would applicable to stub them in its own area

 

res

paul

 

 

 

 

Please don't forget to rate any posts that have been helpful. Thanks.
5 REPLIES
Hall of Fame Super Gold

I think that I do not fully

I think that I do not fully understand your question, especially the part about /30 is only 2 interfaces performing in area 0 at each time. With all routers in area 0 when any of the routers sends an LSA representing a change in one of its networks then all of the routers will receive the update and all of the routers will have to process the shortest path algorithm and update their routing tables. That part is true whether interfaces are point to point or multipoint or whatever.

 

I do not think that any of us would say that this is an optimal design. But that does not necessarily mean that it is a bad design. We would need more information about the topology of the network and about how stable the network is to base an opinion about the design.

 

In general the advantages of designing OSPF with multiple areas is that it reduces the size of the link state data base and it reduces the frequency of running the SPF algorithm (you only need to run SPF when there is a change in your area but do not run SPF for changes in other areas). So multi area OSPF helps to insulate your routers from the effects of changes in remote areas.

 

HTH

 

Rick

Community Member

Hi Richard,Thanks for the

Hi Richard,

Thanks for the response.

not fully sure on the topology but I know the following.

 

Router A - Connects to router B on there gig0/0 interface with a /30 IP.  So they become ospf neighbors

Then there will be another router hanging off router B on its gig0/1 int with a /30 with another ospf neighbor then this will connect to another router and so on.

Again I'm not sure on the full design. Once I find out ill update the post. I have seen on a few interfaces with ospf priority 0. Does this just mean do not route traffic down this path? as this has been put on some of the slow links.

 

 

Hall of Fame Super Gold

Using a /30 on an interface

Using a /30 on an interface does mean that there can be only a single neighbor on that connection. But it does not change the number of routers that must respond when any change occurs in the network.

 

When you configure priority zero on an interface it means only that this router will not become DR. The router will still become a neighbor and traffic will still be routed over that link. If you have OSPF neighbors and one of them is more powerful and one is less powerful you might prefer that the more powerful router becomes DR (since the DR does a bit more work than a DR-other router. Using priority 0 is a way to accomplish that.

 

HTH

 

Rick

Super Bronze

DisclaimerThe Author of this

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Posting

As both Paul and Rick have already noted, the real advantage of OSPF areas is containing how much information a router must (internally) manage for the OSPF topology.

An old rule-of-thumb, was no more than 50 routers per OSPF area, but many variables are involved beyond just the number of routers.

Newer "routers" are often much "faster", and on Cisco devices, there's an optional incremental SPF feature which might reduce some of the CPU cycles needed to (internally) manage the topology.  (NB: Cisco OSPF later OSPF implementations have other subtle ways of supporting OSPF that assist in supporting larger topologies.)

So, as Rick also noted, just knowing there's 350 routers doesn't mean it's actually a bad design.

 

PS:

BTW, for your /30 Ethernet links, is OSPF p2p defined on the interfaces?

VIP Purple

Hellowhen you say they have

Hello

when you say they have over 350 rtrs are all these running ospf and if so all connected to the backbone?

There's is a suggested rtr limitation for ospf regards areas etc but I would think its more to do with the amount of route being advertised within the area than the rtrs themselves regarding memory utilisation to process the link state db

 

Lots of rtrs with a small rib within one area I think wouldnt be much of a concern than a few rtrs with a  very large rib with one area.

I would say I lot would depend also on the processing CPU/memory specifications of the rtrs and if the rtrs are non transit within the area may be it would applicable to stub them in its own area

 

res

paul

 

 

 

 

Please don't forget to rate any posts that have been helpful. Thanks.
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