[OLSR-users] olsrd with 2 networks

Kosta Welke (spam-protected)
Wed Feb 8 01:10:32 CET 2006

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 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.

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

>> 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 
option. Thats why its there :)


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