From: Scott Morris (swm@emanon.com)
Date: Tue Nov 07 2006 - 01:49:53 ART
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@ipexpert.com
http://www.ipexpert.com
-----Original Message-----
From: Michael Zuo [mailto:mzuo@ixiacom.com]
Sent: Monday, November 06, 2006 11:23 PM
To: Scott Morris; Heiko Liedtke; Lab Rat #109385382
Cc: cisco@groupstudy.com; ccielab@groupstudy.com
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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