Can someone please explain what this command

does. I understand when to use it >> when

when using default routes << but don,t

understand why.

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This comes from "Enhaned IP Services for Cisco Networks" by Donald C. Lee, published by Cisco Press. This books rocks. I've found the chapters on managing routing protocols and the section on leveraging default routing to be invaluable. It's how I learned to address this very same question. Since he describes it much better than I can, I'll quote:
"As a classful router, RouterA checks for a direct connection to the major net befor it forwards a packet. If the packet is destined to a directly connected major net (called a local domain), the router ignores the default route and looks in its routing table for a route to the destination. If a route does not exist, the packet is dropped."

I can't recommend this book highly enough.

MadChef

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Pardon me, i still don't understand what that means. Can someone explain with an example.

Thanks.

jusccna,
with no IT exp.

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Your router has an interface on the 172.16.0.0 major network. It has routes to the following networks via IGRP:
172.16.1.0 /24
172.16.2.0 /24
172.16.3.0 /24
It has a default route to 172.16.1.1.
If the router gets a packet destined for 172.16.7.3, it will recognize that it has interfaces on that major net (172.16.0.0) and will start looking for the next hop in it's routing table; it will not consider the default route at all. This is the nature of classful routing. It will not find a route to this network so it will drop the packets.
'ip classless' makes the router consider the default gateway EVEN THOUGH it has an interface on the 172.16.0.0 major net. The router won't find a route to 172.16.7.0 /24 network so it will send the traffic on the its default gateway and hopefully the traffic will eventually get to the host.

Is that any better?

MadChef

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Here's what www.cisco.com says about "ip classless":

quote:

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This command allows the software to forward packets that are destined for unrecognized subnets of directly connected networks. The packets are forwarded to the best supernet route.

When this feature is disabled, 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.

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When they say network default route, I think they mean a route configured like
ip route 172.16.0.0 255.255.0.0 172.16.1.1
That's a way to get around using ip classless.
1) Only if that network happens be in the direction of the default gateway. Otherwise what would happen if you had multiple interfaces belonging to the same major net?
2) With "no ip classless", the packets would be dropped UNLESS you had also configured a route pointing to the major net. This would give you a network default route and would forward packets correctly.
3) Yes, it would definately be dropped unless you used the configuration above.

I _think_ I'm right about this. Someone could test it out if they felt particularly intersted.

MadChef

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A couple of other threads made me think that you could create a candidate default route with the ip default-network command that would allow a router to forward packets to another network in its major net without the ip classless command. Perhaps that's more of what they meant by network default route.
Sorry, I very rarely deal with classful routing protocols so I'm probably more than a bit rusty.

MadChef

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Everytime I visit this site I find a new term. What is a "major network" vs "minor network?". What, can't buy an address or hardware address. Some humor.

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quote:

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Originally posted by haubest:
Everytime I visit this site I find a new term. What is a "major network" vs "minor network?". What, can't buy an address or hardware address. Some humor.

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So the "major network" for a particular class is the same thing as the network portion of that particular class keeping in mind that I'm thinking an IP address is,in its simplist, network.host? So why change the terminolgy? Could I substitute the term "network" for the term "major network" in the question? Still don't understand the concept of a "major network". I've never heard of a "minor network" either and but maybe it could refer to a subnet and I've just coined a new buzzword.

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