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

[Jon Dugan]

  I apologize for missing the Tuesday meeting, I had plans the predated the
  change to Tuesdays.  I will be there not the next Tuesday (out of town on
  business) but the following Tuesday.

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....

  Why does every host in the subnet need to be routed individually?  I am
  trying to understand what problem you are trying to solve/what the
  architecture you are building toward.

> 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?

  No that is absolutely correct.  Routing works in the following manner, there
  are three components to a route table entry:

    1. the network address
    2. the mask
    3. the next hop

  The network address specifys the high bits of interest.  The mask specifies
  how many of those bits are interesting and the next hop determines what the
  next hop router is (that is the next Layer 3 device).  

  So for example, NCSA has the 141.142.0.0/16 block of address space.  The /16
  notation means that the higest 16 bits make up the legitimate prefix.  This
  is equivalent to a 255.255.0.0 netmask.  A network address with a mask is
  often referred to as a prefix.  A /24 is the most typical length for a
  subnet, it has a netmask of 255.255.255.0.

  Routes are always matches in longest prefix order, that is if you are trying
  to forward a packet to 141.142.2.2 and you have the following two routes:

    Prefix          Next Hop
    --------------  ----------------
    141.142.0.0/16  141.142.x.y
    141.142.2.0/24  141.142.a.b

  You will select the second route because it has a longer prefix.  However,
  for example outside of NCSA we only announce the /16.

  I'm not sure what the current proposed architecture is, however it seems
  reasonable that you would want to use subnet routing.  If we use the RFC
  1918 address space 10.0.0.0/8 and dividie that up amongts the various nodes,
  we should have plenty of room for growth.  We could allocate a /16's to major
  aggregation point which would hand out /24's to their downstreams.  This is
  essential the same as the BxNode and CxNode from the Seattle Wireless stuff.

  In this sort of a model, the CxNodes would advertise their connected subnets
  into OSPF.  Each BxNode would collect all the routes from their downstream
  Cx nodes and exchange routes with other Bx nodes.

  I have given some thought to a routing architecture, but I'm afraid I'm out
  of time right now (the day job calls).  

  One last question which routing protocol implementation did you have in
  mind?  GateD?  Zebra?

> My understanding is that a few thousand entries in the routing table
> is no big deal.  Millions are a problem.  Are you talking about just
> Champaign-Urbana?  If so, what is the point of subnet routing?

  Correct.  We really don't need to worry about the size of the routing table
  until it gets into the 100,000 range or so.  A P133 can handle stuff under
  100,000 routes with no major problem.

> ####(THERE SEEMS TO BE SOME CONFUSION REGARDING ROUTING; "SUBNET 
> ROUTING" IS A REDUNDANT TERM. ALL LAYER 3 PACKET SWITCHING IS DONE BY 
> NETWORK (OR "SUBNET") NUMBER. ROUTERS DO NOT, IN FACT *CAN NOT* CARE 
> ABOUT HOSTS...THEY ONLY ROUTE TO NETWORKS.)

  A router can in fact provide a route to a single host.  You can specify a
  route with a mask that is 32 bits long.  This route will cover a single
  host.  This is known as a host route or a /32.

Jon
-- 
Jon Dugan             |  Senior Network Engineer, NCSA Network Development
jdugan at ncsa.uiuc.edu  |  269 CAB, 605 E Springfield, Champaign, IL 61820
217-244-7715          |  http://www.ncsa.uiuc.edu/~jdugan/