From: Yurchenko, Michael (michael.yurchenko@xxxxxxxxxxx)
Date: Thu Apr 26 2001 - 15:16:47 GMT-3
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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