ITEM: G8208L

TCPIP problems with dynamic routing

Question:

There are two class C networks, using ethernet (thinnet) with a Unix PC
acting as a router between the two nets.  One of the networks is 
subnetted and has a couple of slip connections.  The other contains a 
Cisco router, to route into internet.  When the network first comes up,
everything appears fine.  Shortly afterwards, the systems in the net
with the Cisco router loose the route to the the other net via the PC.

Both AIX systems are running AIX 3.2.5.

This is what the network looks like:

{Slip subnet}
   |            193.1.1.1
  _|_______________________
                
                |
               {PC}
                |   192.1.1.2
------------------------------
        |                    |
    {RS/6000}           {Cisco router} 192.1.1.1
                             |
                             |
                         {Internet}

After the system is rebooted the route points to the PC for only 
six minutes and then it points to the Cisco router (192.1.1.1).

The PC is broadcasting a metric of 2 for network 193.1.1
while the Cisco is broadcasting a metric of 0.  The lower metric 
determines the fastest route for dynamic routing. Therefore the Cisco 
is always being written into the routing table.  

The RIP protocol (both routed and gated daemons) routing is based on the
metric parameter. There is no time measurement involved. A lower metric
usually implies a faster route, but it is not guaranteed.

If gated is running, it will convert metrics from RIP packets to times 
for internal use by gated itself because gated can talk/listen several 
different dynamic routing protocols, some which are time based. But, 
when gated talks RIP, it will send metrics, not times.

In this situation, routed on the PC is broadcasting a metric of 2 for the 
193.1.1 network and the Cisco is broadcasting a route to the same 
network with a metric of 0. 

The Cisco should not broadcast routing information for the 193.1.1
network on the network containing the RISC System/6000, PC, and Cicso. 
In addition, routed on the PC should broadcast the best route (metric 0)
for the 193.1.1 network, since it is the router to this network. This is
a routing configuration issue on the PC and Cisco router.  



Support Line: TCPIP problems with dynamic routing ITEM: G8208L

Dated: May 1994 Category: N/A

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