[OLSR-users] olsrd with 2 networks

[Bernd Petrovitsch]

On Wed, 2006-02-08 at 01:10 +0100, Kosta Welke wrote:
> Bernd Petrovitsch wrote:
> > On Tue, 2006-02-07 at 12:47 +0100, Kosta Welke wrote:
> 
> > The interface will have exactly one IP address.
> > And actually I'm thinking on configuring /32 on the interfaces IP
> > address because:
> > - I don't have to explicitly delete the automatically added route
> >   afterwards.
> 
> I do not completely understand what you mean, but please note that olsr 
> automatically deletes routes it created after process termination. (At 
> least, thats what I observed. Please correct me if I'm wrong or if this 
> is not 100% reliable)

This is correct.
I meant the route added with the eth<x>-interface creation at boot time.

> >   This may be a Linux-only problem and it doesn't hurt really, but it is
> >   far from elegant (and error prone during debugging).
> >   Think of hosts with several OLSR interfaces (e.g. on antennas,
> >   LANs, OpenVPN endpoints).
> 
> Do you mean that Linux automatically assigns a e.g. 10/8 netmask when 
> you only need a /24? Yes, the easiest solution is to adjust the netmask, 
> so that no networks that may be around interfere.

No, the kernel adds a route if you set the interface up. And *this*
route must be immediately removed - since olsrd is managing the routes
for that interface(s).

> Depending on the size of your "one-hop limit", your netmask could be 
> narrower, of course.
> 
> > - IMHO this is the conceptually *only* correct interface address
> >   configuration since OLSRD routes (conceptually) only hosts and not
> >   subnets (with != /32 netmask).
>  >   So relying on some "artificial" subnet declaration looks somewhat
> >   "fishy" to me.
> 
> I beg to differ. The subnet declaration is not artificial at all, it is 
> how IP works. The subnet tells the IP stack if it should send the packet 

Of course. But in the cabled world, the subnet has actually a technical
relevance (which is almost completely gone with host-only routing).

> to a router or directly on the interface. So it makes sense to advertise 

No, you tell that via the IP route (e.g. the `ip add route` call with or
without `via`). The netmasks simply defines the subnet (together with
the subnet address/IP address).

> that you have a route to your subnets. Of course, if you have an 
> uncommon network layout, using netmask /32 and setting olsrd's broadcast 
> address to 255.255.255.255 *can* be the best (or only) solution.
> 
> >   "artificial" because it has nothing to do with routing (except that it
> >   defines a broadcast address. But I can configure it also in the config
> >   file) or broadcast or collision domains but more a pure administrative
> >   given value.
> 
> As I mentioned earlier, I did not completely understand what you mean, 
> but note that the netmasks almost only purpose *is* routing decisions. 

Yes. But in the cable world, you probably won't configure IP addresses
from the *same* subnet on different interfaces on different broadcast
domains on the very same host *and* expect that it works.
Yes, there may be pathologically situations where this acually works but
I won't consider it by any means as "normal" (and for sure I won't
recommend it).

And that is the technical meaning of a "subnet" in the cable world: In
the vast majority of cases one subnet lives in one broadcast domain (at
least in theory without hacks).

On the other hand (read: host-based routing), the core box of my node
has several OLSRD managed interfaces with (almost) *all* IP addresses
from the same subnet on different physical interfaces.

> Defining a broadcast address is almost negligible :)
> 
> >> The broadcast option is only needed when you want to connect to nodes 
> >> that are in your one-hop radius, but have a different network address.
> > Yes, but the next hop has potentially an address from a complete
> > different subnet.
> 
> That why you use olsrd in the first place. Consider this simple example:
> 
> 10.0/16 ---box1--- 1.2.3/24 --- box2 ---- 5/8
> 
> Thanks to olsr you can route from 10.0/16 to 5/8, out of the box, no 
> configuration needed (Except telling olsrd which interface to use...)

He, this works (without 255.255.255.255 as configured broadcast
address)?
I think I must try this out.

> >> A (bad) example would be an ethernet where 10.0/16 and 192.168/16 
> > See it as an example setup for this discussion - there are several
> > reasons why such setups may make sense and they are IMHO absolutely
> > unavoidable in the mid-term.
> 
> If you have good reasons for such a layout, use the -b 255.255.255.255 
> option. Thats why its there :)

One (if not the) good reason is IMHO: You get officail IP addresses from
RIPE and use them. Then you get more (because you need more) or you use
private networks for not-to-be-seen outside hosts.
Voila.

	Bernd
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