Path: utzoo!attcan!uunet!wuarchive!zaphod.mps.ohio-state.edu!sdd.hp.com!decwrl!ucbvax!hplabs!hpcc01!hpccc!leinwand
From: leinwand@hpccc.HP.COM (Allan Leinwand)
Newsgroups: comp.dcom.sys.cisco
Subject: Re: Serial interfaces IP number question
Message-ID: <14050005@hpccc.HP.COM>
Date: 15 Aug 90 04:42:21 GMT
References: <2442@s3.ireq.hydro.qc.ca>
Organization: HP Corp Computing & Services
Lines: 61

>/ hpccc:comp.dcom.sys.cisco / vaillan@ireq.hydro.qc.ca (Clement Vaillancourt) /  7:26 am  Aug 14, 1990 /
> 
> We have the class B 131.195 address; we want to interconnect
> 3 different sites with 3 different subnets, what should I use for
> serial interfaces IP numbers? Each of my 3 Cisco have one ethernet and two 
> serial interfaces and we want to make a triangle with 3 Cisco's and 
> 3 serial links.
> 
> I don't want to waist subnet because we are planning to interconnect
> about 100 sites eventually.
> 
> We use a 9 bits subnet (max. of 128 subnets with 512 machines each);
> 	Site A = 131.195.2.x     We have about 400 machines per site
> 	Site B = 131.195.4.x
> 	Site C = 131.195.6.x

I know this is not the answer you want to hear, but let me tell you our rule 
of thumb concerning making a subnet mask and picking a network number:

1.  estimate the total number of subnets needed.

     You did this.  With serial links and all you potential sites this number
could exceed 150.  Find the closest power of 2 which exceeds this number.  For 
your setup I get 2^8 == 256.  So, you need 8 bits for the subnet number in your
subnet mask.

2.  estimate the total number of machines per subnet.  

   Again, you did this and came up with a number of 400.  This equates to
2^9 == 512.  So, allow 9 bits in the subnet mask for the host entries.  

3.  add the two powers expressed above and find the closest network class
needed.

   In your case I find you need 9+8 == 17 bits to express your subnet numbers
and host numbers.  I would have applied for a class A network number from the 
NIC.  Class A network give you 24 bits to play with, while class B only give you
16 bits of fun.  Is this painfully obvious? :-)

Realizing that this does not help much, I guess you will have to 
eliminate IP subnets on the serial links.  I think you can do this with an
interface sub-command as of 8.1.  Of course, this means network management 
could be a bear since you cannot ping the serial interfaces or query them 
via SNMP.

> -How should I configure the Cisco's to make shure the traffic can
>  reach each sites in the event of a single serial link failure?

Make sure the IGRP metrics match (by setting the bandwidth and delay parameters
the same) on all links in the triangle.  IGRP will provide load balancing
across the two links if the IGRP metrics are identical.

Take care,

Allan Leinwand
Hewlett-Packard
Network Solutions Engineering
allan@hpcta.corp.hp.com

"Remember, no matter where you go -- there you are."
   - Buckaroo Banzai