[c-nsp] OSPF clarification

[Phil Mayers]

Brian Spade wrote:
> Hi Steinar,
> 
> 
> On Mon, Nov 30, 2009 at 8:11 AM, <sthaug at nethelp.no> wrote:
> 
>> Or put them in IBGP.
>>
>> Steinar Haug, Nethelp consulting, sthaug at nethelp.no
>>
>>
> Can you elaborate?  Why would one want to put edge VLANs into IBGP?  Thanks
> for clarifying.

The general advice is:

  * Put only the very bare minimum into OSPF; this means the p2p network 
between your routers, and their loopbacks i.e. anything which is needed 
to resolve next-hops

  * Put everything else (including edge vlans) into iBGP


The reasoning is that changes to p2p/loopbacks usually means a 
convergence event, and you want this to propagate as fast as possible, 
so smaller OSPF database is the aim.

There are other reasons; BGP has far superior (not to mention safer) 
filtering abilities, better policy controls (e.g. communities), arguably 
superior incremental updates, and so forth.

It's a very common model, which people often move to after finding their 
network doesn't work well with thousands or tens of thousands of LSAs. 
We ourselves moved from having edge networks in OSPF Extern's to iBGP, 
and I'm very happy with the results.

There are reasons to ignore this model, but it's a sensible starting 
point for advice, and fits a large number of people.