Hunt, With "no ip classless" configured, the router assumes classful behavior. If it has any route at all to a network it assumes that it has routing to all of that major network (this is why RIP has such problems with discontiginous networks). If it receives a packet destined for a major network for which it has routing but for a subnet that it does not have a route to it presumes that there is no route available to the subnet and black holes it.
If a classful router receives a packet for a major network that it does not have a route to, it sensibly forwards it to the default route. This type of behavior only makes sense from the point of view of a classful routing protocol (RIP, IGRP, etc) where there is an underlying presumption of knowledge of an entire major network if the router has any routes to that major network. "No ip classless" can cause some very weird behavior and should be configured in modern networks only very cautiously. Dave -----Original Message----- From: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED]]On Behalf Of Hunt Lee Sent: Tuesday, December 25, 2001 5:45 PM To: [EMAIL PROTECTED] Subject: Re: IP classless command [7:30056] Hello Mike. thanks so much for your detailed explanation. However, I'm still confused about how the ip classless works. I understand that "classful" rules (if no ip classless is configured), with the 10.1.1.0/24 static route, it would forward anything from 10.1.1.1 to 10.1.1.254, but why would it worked for 11.x.x.x? Also, with the "ip classless", will it just forward anything like 10.x.x.x? Thanks again. Hunt ""Mike"" wrote in message [EMAIL PROTECTED]">news:[EMAIL PROTECTED]... > The best way to explain IP classless is to explain how a router works with > "no ip classless". Suppose you have a router with a static route configured > to 10.1.1.0/24 out some interface and you also have a default gateway > configured. Again, ip classless is disabled, "no ip classless". > > A packet comes into the router destined for 10.1.1.2, the router looks at > the routing table, sees the static route and forwards as expected. > > A packet comes into the router destined for 11.x.x.x, the router looks at > the routing table, sees the default gateway and forwards as expected. > > A packet comes into the router destined for 10.10.10.1, you would expect the > router to forward the packet via the default gateway. However, because the > router is operting in "classful mode", the router drops the packet rather > then using the default gateway. This is because the router is in the same > classful network as the 10.1.1.0/24 network used for the static route. > > If the "ip classless" command was used, the dropped packet would actually be > forwarded via the default gateway. There really is no reason not to use "ip > classless" on all router configurations. > > Mike > > > > ""Hunt Lee"" wrote in message > [EMAIL PROTECTED]">news:[EMAIL PROTECTED]... > > Can anyone please explain to me what is "ip classless" used for? I looked > > it up on the Caslow book, and it states that by enabling IP classless, it > > allows one to override the contiguous subnet rule and allow the router to > > look for the longest match beyond the listed subnets. > > > > But I still don't understand what it means? Can anyone give me some > > examples? > > > > Thanks in advance. > > > > Best Regards, > > Hunt Lee > > IP Solution Analyst > > Cable & Wireless Message Posted at: http://www.groupstudy.com/form/read.php?f=7&i=30090&t=30056 -------------------------------------------------- FAQ, list archives, and subscription info: http://www.groupstudy.com/list/cisco.html Report misconduct and Nondisclosure violations to [EMAIL PROTECTED]
