From: Pierre-Alex GUANEL (paguanel@hotmail.com)
Date: Thu Nov 09 2006 - 17:43:34 ART
Thank you for the reply Scott. I actually had not received your reply ...
I understand the need for advertising a mask other than /32.
What is not clear is the possible (real life) applications of using a
network advertised in /32
, why ospf defaults to a /32 , and in which sense it is a "desirable"
behaviour.
Pierre-Alex
--------------------------
The /32 mask is default for OSPF's handling of a loopback interface. When
you do "show ip ospf interface" and look at the loopbacks they are a "stub
host" and just a /32 in the table.
If you are holding a network (allocating things for a NAT pool, dialer pool
or VPN pool, it's always nice that people can reach your router (the /32)
but it's likely nicer that they can reach back to whatever people are
dialed/vpn'd/nat'd through your box! :)
For NAT specifically, your ISP may allocate a /24 (or whatever) for you to
use. That particular route is not used on the serial interface going to the
ISP (these are usually /30's). So you either REALLY use that network
internally, or you use it for NAT pool. If you are exchanging routes with
anyone (ISP is likely static'd to you), like a business partner or BGP peer
then you'll want to advertise that /24. IF you want to advertise the route,
you need to tie it to an interface.
I suppose an alternative to that would be to have a static to NULL0 and then
redistribute static. But simple enough to stick it on a loopback.
HTH,
Scott Morris, CCIE4 (R&S/ISP-Dial/Security/Service Provider) #4713, JNCIE
#153, CISSP, et al.
CCSI/JNCI-M/JNCI-J
IPExpert VP - Curriculum Development
IPExpert Sr. Technical Instructor
smorris@xxxxxxxxxxxx
http://www.ipexpert.com----- Original Message -----
From: "Scott Morris" <swm@emanon.com>
To: "'Pierre-Alex GUANEL'" <paguanel@hotmail.com>; "'Heiko Liedtke'"
<heiko.liedtke@gmx.net>; "'Lab Rat #109385382'" <techlist01@gmail.com>;
"'Michael Zuo'" <mzuo@ixiacom.com>
Cc: <cisco@groupstudy.com>; <ccielab@groupstudy.com>
Sent: Thursday, November 09, 2006 8:24 PM
Subject: RE: OSPF Question
> Not sure if this is a duplicate... Check the archives though. Already
> replied!
>
> http://www.groupstudy.com/archives/ccielab/200611/msg00348.html
>
>
> Scott Morris, CCIE4 (R&S/ISP-Dial/Security/Service Provider) #4713, JNCIE
> #153, CISSP, et al.
> CCSI/JNCI-M/JNCI-J
> IPExpert VP - Curriculum Development
> IPExpert Sr. Technical Instructor
> smorris@ipexpert.com
> http://www.ipexpert.com
>
>
>
> -----Original Message-----
> From: Pierre-Alex GUANEL [mailto:paguanel@hotmail.com]
> Sent: Thursday, November 09, 2006 1:15 PM
> To: Scott Morris; Heiko Liedtke; Lab Rat #109385382; Michael Zuo
> Cc: cisco@groupstudy.com; ccielab@groupstudy.com
> Subject: Re: OSPF Question
>
> I am interested in this scenario too.
>
> Would you mind giving us a brief explaination?
>
> Thanks
>
> Pierre-Alex
>
>
> ----- Original Message -----
> From: "Michael Zuo" <mzuo@ixiacom.com>
> To: "Scott Morris" <swm@emanon.com>; "Heiko Liedtke"
> <heiko.liedtke@gmx.net>; "Lab Rat #109385382" <techlist01@gmail.com>
> Cc: <cisco@groupstudy.com>; <ccielab@groupstudy.com>
> Sent: Tuesday, November 07, 2006 5:23 AM
> Subject: RE: OSPF Question
>
>
> > Hi Scott,
> >
> > Could you please explain the "holding" scenario with NAT? where does
> > the
> > /32 mask come in?
> >
> > Much appreciate it...
> >
> > -----Original Message-----
> > From: nobody@groupstudy.com [mailto:nobody@groupstudy.com] On Behalf
> > Of Scott Morris
> > Sent: Monday, November 06, 2006 10:03 AM
> > To: 'Heiko Liedtke'; 'Lab Rat #109385382'
> > Cc: cisco@groupstudy.com; ccielab@groupstudy.com
> > Subject: RE: OSPF Question
> >
> > Or if you are "holding" networks that are used for dialer pools, VPN
> > pools or NAT pools, they must exist on an interface in order to be
> > advertised.
> >
> > There are many real-world needs for masks other than /32 on a loopback
> > interface!
> >
> > HTH,
> >
> >
> > Scott Morris, CCIE4 (R&S/ISP-Dial/Security/Service Provider) #4713,
> > JNCIE #153, CISSP, et al.
> > CCSI/JNCI-M/JNCI-J
> > IPExpert VP - Curriculum Development
> > IPExpert Sr. Technical Instructor
> > smorris@ipexpert.com
> > http://www.ipexpert.com
> >
> >
> >
> > -----Original Message-----
> > From: nobody@groupstudy.com [mailto:nobody@groupstudy.com] On Behalf
> > Of Heiko Liedtke
> > Sent: Monday, November 06, 2006 11:34 AM
> > To: Lab Rat #109385382
> > Cc: cisco@groupstudy.com; ccielab@groupstudy.com
> > Subject: Re: OSPF Question
> >
> > I think the only purpose of letting us advertise networks as /32 or
> > not as
> > /32 (loopbacks) is that they want to know if we are familiar with the
> > different ospf network types...
> > :-)
> > I think there is no deeper background on this..
> >
> > heiko
> >
> >
> >
> > Lab Rat #109385382 schrieb:
> >
> > >What's the purpose of advertising a network in OSPF as a /32 if the
> > >subnet the interface is attached to is a /24?
> > >
> > >I see many lab scenarios like this and wonder why...
> > >
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