[MLB-WIRELESS] Revisiting OSPF

[David Ashburner]

>If you connect your subnet or subnets to another network, such as the 
>Melbourne Wireless
>network, you’ll probably want to install a router in the subnet that 
>connects to the “other” outside network.  By running an OSPF service (or 
>daemon) on your router you will be able to
>advertise to the rest of the network that you exist and (presumably) 
>everyone on the rest of
>the network will be able to find your node(s). Likewise, the OSPF service 
>on your router will
>receive advertisements from other OSPF routers so your router will know the 
>direction (route)
>to traverse to find other nodes and subnets.

When you are connected to a internetwork that has many possible routes to 
the destination this is _really_ _really_ important but in the case of a 
fledgling internetwork ( such as melbourne wireless)  you are going to be 
connected to one or two other nodes. You routing decision is a lot simpler 
and you can defer the complicated stuff to the upstream node anyway.

If you have one link then it is your default route.
If you have two links, you set routes for each one and pick which one is to 
be the default. Traffic for either of the connected nodew will go to the 
right interface while unknown stuff will go to the default.

>If you do not have a router (or PC running a routing protocol), then you
>will have to enter the routes manually (statically) so that your PC
>(router) knows how to find the various parts of the rest of the network.
>Routing tables can get quite large - you could end up with a table that
>has directions to get to fifty or more subnets.  It would not be much
>fun to keep that up to date.

You only have to know how to route to directly connected subnets and set 
your default to one that can do the rest.

>
>If you do not have a PC or router at all then you are pretty much stuck
>with running all your own APs (or AP clients) and your AP peers in the
>same subnet.  You'll have to hope that your AP peers are doing the
>routing for you.  If there are lots of nodes involved this can get quite
>ugly - bandwidth will be used inefficiently - which is why the Melbourne
>Wireless network favours routing between lots of small Ethernet subnets.

Yep, this sucks big time. If only the AP came with a kernel that had IP 
forwarding on and allowed you to route between the wired and wireless 
interfaces then you have the equivalent of a "remote interface" in the AP 
mounted up the pole.


>We are hoping that small, cheap wireless routers such as the Linksys
>WRT54G are the answer to the problem of needing a chunky PC at each node
>location.

The WRT54G actually solves a lot of PC-less node issues. Running something 
like OpenWRT the bridging between wired and wireless is turrned off, you 
have vlans that allow you to use other (even cheaper) AP as remote 
interfaces and you  can set up the firewall so the WAN port connects to your 
"private" space.

Routing within the node is static, the WRT ic configured to route traffic 
between the vlan interfaces and let the upstream nodes decide what to do 
with your traffic.

I'm not saying there is no need for dynamic routing just that within the 
space of a single node you can get away without it. When you end up having 
multipe connections to other nodes then you need something a bit smarter 
than static routes but initially they should do.

dna



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