Path: utzoo!utgpu!news-server.csri.toronto.edu!rutgers!apple!hercules!frisbee.cisco.com!allan
From: allan@frisbee.cisco.com (Allan Leinwand)
Newsgroups: comp.dcom.sys.cisco
Subject: Re: Adding static routes
Keywords: subnets
Message-ID: <21285@hercules.csl.sri.com>
Date: 27 Sep 90 18:51:48 GMT
References: <1990Sep24.234745.15897@cbnewsi.att.com>
Sender: usenet@csl.sri.com
Reply-To: allan@frisbee.cisco.com (Allan Leinwand)
Distribution: usa
Organization: cisco Systems, Inc. - Menlo Park, CA
Lines: 57

>Perhaps a picture would help:
>
>                                                  \
>            \      -------             -------     |    
>             |    |       | #1        |       |    |
>134.40.64.0  |----| cisco |-----------| cisco |----| 134.38.192.0
>             |    |   A   |           |   B   |    |
>             \     -------             -------     \
>                  #2 |                    |
>                     |   /                |   /
>                   ------               ------   
>                  /    |               /    |
>                       |                    |     \
>            \      -------             -------     |
>             |    |       |           |       |    |
>134.40.32.0  |----| cisco |-----------| cisco |----| 134.38.128.0
>             |    |   C   |           |   D   |    |
>             \     -------             -------     \
>                                
>
>I have two IP networks, using class B addresses and a subnet mask of
>255.255.255.192.  Can I tell cisco A to route traffic from a host on
>134.40.64.0 to a host on 134.38.192.0 using interface #1 and to route
>traffic from a host on 134.40.64.0 to a host on 134.38.128.0 using 
>interface #2, (i.e., through cisco C rather than cisco B)?

First, assuming that the link from A --> B is not significantly lower
than all the other links in the picture, you will automatically route
from A --> B using interface #1.  If you want to be assured that this is
the primary path taken, lower the 'bandwidth' on this link to something
a little greater than the minimum bandwidth on C --> D.

By looking at your picture, it seems to me that you do not want any
traffic to traverse the link between cisco B and cisco D.  Thus, set the
'bandwidth' metric on the interfaces on B and D (which go to the common
link) to something lower than normal.  Thus, the A --> C --> D route
would have a lower metric.  This scheme also has the advantage of not
using static routes.  Thus, if you link from C --> D fails, you can
always use the A --> B --> D link as backup.

Maybe something like this:
          
    
                    64
           A  --------------- B
         56|                  |32
           |                  |
           C ---------------- D
                    56

Even if your real bandwidth does not match these 'bandwidth' settings, this 
should produce the desired routing.

Thanks,

Allan Leinwand
cisco Systems