From: Mas Kato (tealp729@xxxxxxxx)
Date: Thu Apr 26 2001 - 17:32:48 GMT-3
Michael,
I'm a little confused--hope you can help! I thought I had this down by
witnessing it in operation, but...
The docs state: "When this feature is disabled (i.e, 'no ip classless'
or IP classFUL operation), the software discards the packets when a
router receives packets for a subnet that numerically falls within its
subnetwork addressing scheme, if there is no such subnet number in the
routing table and there is no network default route." --implying that
the default route would be followed if it exists. In your example, you
say that a route to a subnet of the classful major network would be
followed. Slattery, in "Advanced IP Routing," pg. 283, only says the
default route would -not- be followed.
...What I -think- I've observed with 'no ip classless' operation is that
in the example you gave, packets destined for a unknown subnet of
10.x.x.x were dropped--even though a default route existed--agreeing
partially with, but ultimately contradicting what the docs imply
(Newsflash! Docs are ambiguous--say it isn't so!).
-With- 'IP classless,' the behavior I've observed is exactly as you (and
Slattery, in his book) stated--the packets destined for an unknown
subnet of an existing major net were sent along the default route (best
supernet).
In your experience with 'no ip classless,' do the packets really follow
a route to a subnet of the existing major net? What if there's more than
one route like that?
Thanks in advance,
Mas Kato
-----Original Message-----
From: nobody@groupstudy.com [mailto:nobody@groupstudy.com]On Behalf Of
Yurchenko, Michael
Sent: Thursday, April 26, 2001 11:17 AM
To: 'Jason1'; CCIE_Lab Groupstudy List
Subject: RE: Advance IP Routing - Terry Slattery - IP Classless
I am not sure how to explain it, but assuming you read whatever the book
says, i'll give you an example.
no ip classless
ip route 10.10.10.0 255.255.255.0 20.20.20.1
ip route 0.0.0.0 0.0.0.0 20.20.20.2
If ip classful is used, and you attempt to reach a subnet of a major
network
for which (a network) a route is present in the table, even though this
subnet may not be reachable via that destination, the router will send
the
traffic that way. So, if you
ping 10.20.10.1
which is a part of the network 10.0.0.0/8, the router will treat the
route
to 10.10.10.0/24 classfuly as a route to 10.0.0.0/8, and send the
traffic to
20.20.20.1, which will have no way to reach it. Now, if you were to
enter
ip classless
in the config, then the router would begin to correctly recognize the
subnets and if you were to
ping 10.20.10.1
then the router would realize that the network 10.0.0.0/8 is subnetted,
will
forward the traffic to the default gateway and reach the destination.
It can bite you mostly with static routes.
Hope that helps.
Michael Yurchenko
CCIE# 6695, CCDP, CCNP ATM Specialist, MCSE
Customer Support Engineer - 2
michael.yurchenko@verizon.com
610-407-2154
-----Original Message-----
From: Jason1 [mailto:jason1@v-labs.net]
Sent: Thursday, April 26, 2001 1:50 PM
To: CCIE_Lab Groupstudy List
Subject: Advance IP Routing - Terry Slattery - IP Classless
I'm looking at pg 99 of Advance IP Routing by Terry Slattery and I felt
that
it didn't give a good explaination (and in fact, I felt that it is
wrong) of
the "IP Classless" command in the Cisco IOS and also when you should be
using
the command. Any comments from anybody ?
Jason
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