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OSPF network size and IP address managment

I would like to know what is the recommended size of the number of prefixes injected into and OSPF backbone area? i am asking this cause we are an ISP network we have several registered Ip address blocks assigned, and most of these address space is permanently assigned to our leased line customers and dynamically assigned to our ADSL and dial-up customers, currently all these prefixes are injected in ospf via network command or redistribution and i would like to know if it is better if i treat this prefixes as external and announced it via bgp in order of gaining more stability in my interior gateway protocol, at least i would like to do that in the case of my leased line customers that currently are, in most of the cases, statically learned networks redistributed into OSPF. Thanks ahead,

Aisha Sanes

  • Other Network Infrastructure Subjects
Cisco Employee

Re: OSPF network size and IP address managment

Without knowing the details of the network (and the possible impact that the current setup has), the answer is "it depends".

However, I'll try to point at some numbers I've seen in real networks (and some lab tests).

Number of prefixes injected into area 0?? In general, numbers in the 100's are safe. The more instability you have (see below) the lower the number should be, with as many as 1000s in a very stable network (not too many changes).

Dynamic Address Management?? What we see many people do is assign pools of addresses to specific servers...that way, OSPF can just import the pool and avoid carrying specific /32 changes. An alternate mechanism is to summarize at an ABR so that the specific changes are kept w/in an area.

Should you carry these prefixes in BGP?? I would look into increasing the stability of the prefixes first (see above). BGP has the advantage that it can carry a lot more prefixes...but it mat converge a lot slower than OSPF. Looking at the stability and convergence times will help you make the right decision. Static load (no changes) doesn't add stability risks to OSPF...but the changes may.

Hope some of this helps.