Subject: RE: IPCP IP address

From: Joe Frazier <[email-masked]>
Date: Thu, 18 Dec 1997 14:45:08 -0500

Hi Jim and Ohad,

     I have a couple of comments below.

-----Original Message-----
From:	James Carlson [SMTP:[email-masked]]
Sent:	Thursday, December 18, 1997 8:23 AM
To:	[email-masked]
Cc:	[email-masked]
Subject:	Re: IPCP IP address

> I'll thank anyone's help in the next issue:
>
> When assigning an IP address through the PPP IPCP IP address option, 
no
> information about the subnet mask exchanges.

This is quite intentional.  PPP links do not work on broadcast media,
so there's no need for a subnet definition.  A PPP link simply links
two individual IP hosts together.

> Is there a common solution to this problem? ( by using class A/B/C 
as
> default  \  using always  RIP1 on this link  \  learning the subnet 
from
> HELLO messages(OSPF), or RIP2 routing information exchange ?....)

Personally, I treat the remote address on a PPP link as a host route
("subnet" mask 255.255.255.255) and use any available routing 
protocol
(or static routes) to set up routes to any networks reachable over 
the
link.  The subnet mask isn't meaningful, so, for those administrators
who insist on having such things, I generate a separate static route
to represent it.

It definitely is a good idea to use host routes in this situation, and 
your explanation about the fundamental definition of a PPP link really 
drives the point home, Jim.

However, your last sentence does open the door for a little more 
discussion. I'm not sure that a static route is beneficial in a 
"generic" configuration, or an administrators need to insist it.

PPP links may be host specific, but the interfaces they use don't have 
to be. All PPP configuration packages do include the subnet mask as 
part of the configuration.

If an administrator wants to set up a network interface for a PPP 
link, (he,she) can configure to do so. Of course, the subnet mask is 
not communicated over the link in IPCP, so both sides of the link 
would need to be configured to use the same subnet masks.

However, since these PPP link "interfaces" are usually non-permanent, 
a host node configuration would permit a "generic" configuration. And 
if both sides of the PPP link are capable of routing, then all the 
routing info can be retrieved via the PPP interface, just like any 
other interface,

Thanks,

Joe Frazier

--
James Carlson, Consulting Engineer 
               <[email-masked]>
IronBridge Networks / 5 Corporate Drive            Vox:  +1 978 691 
4644
Andover MA  01810-2448 / USA                       Fax:  +1 978 691 
6300

---

From: James Carlson <[email-masked]>
Date: Thu, 18 Dec 1997 15:54:51 -0500

Hey, smokin' Joe, I'd wondered where you'd gone.

I had responded to Ohad's mail with this:
[...]

> "generic" configuration, or an administrators need to insist it.

Some administrators think of all interfaces of subnets, whether or not
there actually is a subnet in existence.  No, there's no need to
insist on it.  But, then, there's no need for carriers to insist on
ATM, either ...

> part of the configuration.

Wrongly, in my opinion.

> would need to be configured to use the same subnet masks.

Wrong.  The subnet mask on my end of the link represents my belief
about the size of your other attached broadcast medium.  The subnet
mask on your end of the link represents your belief about the size of
my attached broadcast medium.  They are the same if and only if we
both have attached broadcast media (such as Ethernet) with configured
subnet masks that are equal.  (Or if we're trying to aggregate to the
same level ... ugh.)

> routing info can be retrieved via the PPP interface, just like any 

I couldn't parse that paragraph.  If you mean that PPP with IP is
perfectly capable of running RIP and other routing protocols, then,
yes, that's the right way to do it.

-- 
James Carlson, Consulting Engineer                <[email-masked]>
IronBridge Networks / 5 Corporate Drive            Vox:  +1 978 691 4644
Andover MA  01810-2448 / USA                       Fax:  +1 978 691 6300