[Cu-wireless] Re: Cu-wireless digest, Vol 1 #118 - 4 msgs

David Young dyoung at onthejob.net
Thu May 2 13:32:49 CDT 2002

(Response below)

On Thu, May 02, 2002 at 07:53:24AM -0500, The Morenz Family wrote:
> >From: David Young <dyoung at onthejob.net>
> >I can understand the desire to keep the routing tables compact, but subnet
> >routing seems unnatural for a wireless network. Will OSPF bog down if
> >we do not use subnets? Every host in a subnet needs to be individually
> >routed, anyway....
> The whole idea of subnet routing is that things in a subnet are not
> individually routed.  So, if you are individually routing to the
> elements of a subnet then you are not using subnet routing.  Am I
> missing something?

    I think that the way Zach conceives of the network, there are stations
    Q1, Q2, Q3, ..., Qn; each routes/forwards. At some instant T, Q1 can
    reach stations P11, P12, P13 with its radio, Q2 can reach stations
    P21, P22, P23 with its radio, station Q3 can reach stations P31,
    P32, P33, and so on. Stations P23 and P31 are the same station, and
    stations P12 and P33 are the same station. No matter, we *arbitrarily*
    lump the stations reachable by Q1 into a subnet, call it 10.0.5/24. We
    also lump the stations reachable by Q2 into subnet 10.0.7/24. Stations
    reachable by Q3 are in subnet 10.0.13/24.

    In this way, some station Q4 which is adjacent to Q1, Q2, and Q3
    knows that when it receives a packet addressed to P12 (whose IP
    number is, say), the next hop for that packet is over the
    link to station Q1.

    What happens to the routes if the link from Q1 to P12 is broken?
    That is not a rhetorical question, but I will try to answer it.
    It seems to me that Q1 will advertise that it cannot any longer reach
    all of the subnet 10.0.5/24, only hosts P11 and P13 on the subnet.
    The route through Q3 to P12 via P33 (P12 and P33 are the same station)
    takes effect.

    In the example I gave, a change in link state split a subnet in two,
    and we ended up with a host route---or, if you prefer, a route to
    a subnet of size 1, =) I don't see the use of subnet
    routing in a wireless network when a subnet falls apart with a change
    of link state.

    Certainly OSPF can deal with a route ?


David Young             OJC Technologies
dyoung at onthejob.net     Engineering from the Right Brain
                        Urbana, IL * (217) 278-3933

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