Re: IP classless command [7:30056]
The history is like this: In the beginning it used to be that a classful IP address block (such as 170.15.0.0 /16 or 197.100.2.0 /24) was assigned to a particular organization for its exclusive use. If that organization chose to subnet that block, and the organization's routers did not have a route listed for a particular subnet of that block, then a packet for that block would be dropped (even if a gateway of last resort had been configured). The logic being that if your organization doesn't know where that subnet is then nobody does, and thus the packet is dropped. These days, ISPs have possession of various address blocks which they divvy up as needed to their customers. So its absolutely common for the various subnets of a particular classful address block to be split up amongst multiple organizations. So the ip classless command was used to get around this default behavior of the router. Recognizing the changed landscape of IP address allocation, ip classless became part of a router's default configuration starting with IOS version 11.3, permitting packets for any destination not explicitly listed in the router's routing table to utilize the router's configured gateway of last resort. Official information: http://www.cisco.com/univercd/cc/td/doc/product/software/ios120/12cgcr/np1_r /1rprt2/1ripadr.htm#1018036 Dan Garfield CCNP, CCDP, CCSI, MCSE, CNE, CTT, A+ 221 Oak Springs Drive San Anselmo, CA 94960 phone: (415) 453-5659 fax: (413) 691-6074 [EMAIL PROTECTED] wrote in message [EMAIL PROTECTED]">news:[EMAIL PROTECTED]... Brilliant! Pierre-Alex -Original Message- From: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED]]On Behalf Of Bernard Omrani Sent: Tuesday, December 25, 2001 7:21 PM To: [EMAIL PROTECTED] Subject: RE: IP classless command [7:30056] For a simple explanation if IP classless, see: http://www.networkking.net/out/ipclassless.htm Bernard -Original Message- From: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED]] Sent: Tuesday, December 25, 2001 12:41 PM To: [EMAIL PROTECTED] Subject: Re: IP classless command [7:30056] IP CLASSLESS is a little hard to understand. Even Cisco is very vague on this. Once a TAC engineer just told me that the command just makes routing better. Here is what I believe happens. If you don't have the IP CLASSLESS command defined in a Cisco router then the router will not forward any packets towards a default route for any subnets of a classfull network that the router thinks are local. For instance. Rtr A local net 10.1.0.0/24. 192.168.1.0 interconnects routers a and b RTR B (sends default network only to RTR A and RTR C) 192.168.2.0 interconnects routers b and C RTR C local net 10.2.0.0/24 Note how 10.0.0.0 is split by the 192.168.1.0 and 192.168.2.0 networks. In the above example if RTR A gets a packet destined for 10.2.0.1, but it does not have the IP CLASSLESS command defined then RTR A will drop the packets. If you include the IP CLASSLESS command then the packets will be forwarded to the default router b. I hope this helps Mike Paulson Network Architect Infrastructure Design Systems LLP Hunt Lee wrote: 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 [GroupStudy.com removed an attachment of type text/x-vcard which had a name of michael.paulson.vcf] Message Posted at: http://www.groupstudy.com/form/read.php?f=7i=30122t=30056 -- FAQ, list archives, and subscription info: http://www.groupstudy.com/list/cisco.html Report misconduct and Nondisclosure violations to [EMAIL PROTECTED]
Re: IP classless command [7:30056]
IP CLASSLESS is a little hard to understand. Even Cisco is very vague on this. Once a TAC engineer just told me that the command just makes routing better. Here is what I believe happens. If you don't have the IP CLASSLESS command defined in a Cisco router then the router will not forward any packets towards a default route for any subnets of a classfull network that the router thinks are local. For instance. Rtr A local net 10.1.0.0/24. 192.168.1.0 interconnects routers a and b RTR B (sends default network only to RTR A and RTR C) 192.168.2.0 interconnects routers b and C RTR C local net 10.2.0.0/24 Note how 10.0.0.0 is split by the 192.168.1.0 and 192.168.2.0 networks. In the above example if RTR A gets a packet destined for 10.2.0.1, but it does not have the IP CLASSLESS command defined then RTR A will drop the packets. If you include the IP CLASSLESS command then the packets will be forwarded to the default router b. I hope this helps Mike Paulson Network Architect Infrastructure Design Systems LLP Hunt Lee wrote: 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 [GroupStudy.com removed an attachment of type text/x-vcard which had a name of michael.paulson.vcf] Message Posted at: http://www.groupstudy.com/form/read.php?f=7i=30087t=30056 -- FAQ, list archives, and subscription info: http://www.groupstudy.com/list/cisco.html Report misconduct and Nondisclosure violations to [EMAIL PROTECTED]
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=7i=30089t=30056 -- FAQ, list archives, and subscription info: http://www.groupstudy.com/list/cisco.html Report misconduct and Nondisclosure violations to [EMAIL PROTECTED]
RE: IP classless command [7:30056]
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=7i=30090t=30056 -- FAQ, list archives, and subscription info: http://www.groupstudy.com/list/cisco.html Report misconduct and Nondisclosure violations to [EMAIL PROTECTED]
RE: IP classless command [7:30056]
For a simple explanation if IP classless, see: http://www.networkking.net/out/ipclassless.htm Bernard -Original Message- From: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED]] Sent: Tuesday, December 25, 2001 12:41 PM To: [EMAIL PROTECTED] Subject: Re: IP classless command [7:30056] IP CLASSLESS is a little hard to understand. Even Cisco is very vague on this. Once a TAC engineer just told me that the command just makes routing better. Here is what I believe happens. If you don't have the IP CLASSLESS command defined in a Cisco router then the router will not forward any packets towards a default route for any subnets of a classfull network that the router thinks are local. For instance. Rtr A local net 10.1.0.0/24. 192.168.1.0 interconnects routers a and b RTR B (sends default network only to RTR A and RTR C) 192.168.2.0 interconnects routers b and C RTR C local net 10.2.0.0/24 Note how 10.0.0.0 is split by the 192.168.1.0 and 192.168.2.0 networks. In the above example if RTR A gets a packet destined for 10.2.0.1, but it does not have the IP CLASSLESS command defined then RTR A will drop the packets. If you include the IP CLASSLESS command then the packets will be forwarded to the default router b. I hope this helps Mike Paulson Network Architect Infrastructure Design Systems LLP Hunt Lee wrote: 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 [GroupStudy.com removed an attachment of type text/x-vcard which had a name of michael.paulson.vcf] Message Posted at: http://www.groupstudy.com/form/read.php?f=7i=30093t=30056 -- FAQ, list archives, and subscription info: http://www.groupstudy.com/list/cisco.html Report misconduct and Nondisclosure violations to [EMAIL PROTECTED]
RE: IP classless command [7:30056]
Brilliant! Pierre-Alex -Original Message- From: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED]]On Behalf Of Bernard Omrani Sent: Tuesday, December 25, 2001 7:21 PM To: [EMAIL PROTECTED] Subject: RE: IP classless command [7:30056] For a simple explanation if IP classless, see: http://www.networkking.net/out/ipclassless.htm Bernard -Original Message- From: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED]] Sent: Tuesday, December 25, 2001 12:41 PM To: [EMAIL PROTECTED] Subject: Re: IP classless command [7:30056] IP CLASSLESS is a little hard to understand. Even Cisco is very vague on this. Once a TAC engineer just told me that the command just makes routing better. Here is what I believe happens. If you don't have the IP CLASSLESS command defined in a Cisco router then the router will not forward any packets towards a default route for any subnets of a classfull network that the router thinks are local. For instance. Rtr A local net 10.1.0.0/24. 192.168.1.0 interconnects routers a and b RTR B (sends default network only to RTR A and RTR C) 192.168.2.0 interconnects routers b and C RTR C local net 10.2.0.0/24 Note how 10.0.0.0 is split by the 192.168.1.0 and 192.168.2.0 networks. In the above example if RTR A gets a packet destined for 10.2.0.1, but it does not have the IP CLASSLESS command defined then RTR A will drop the packets. If you include the IP CLASSLESS command then the packets will be forwarded to the default router b. I hope this helps Mike Paulson Network Architect Infrastructure Design Systems LLP Hunt Lee wrote: 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 [GroupStudy.com removed an attachment of type text/x-vcard which had a name of michael.paulson.vcf] Message Posted at: http://www.groupstudy.com/form/read.php?f=7i=30097t=30056 -- FAQ, list archives, and subscription info: http://www.groupstudy.com/list/cisco.html Report misconduct and Nondisclosure violations to [EMAIL PROTECTED]
Re: IP classless command [7:30056]
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=7i=30058t=30056 -- FAQ, list archives, and subscription info: http://www.groupstudy.com/list/cisco.html Report misconduct and Nondisclosure violations to [EMAIL PROTECTED]
Re: IP classless command [7:30056]
12/24/2001 5:35pm Monday Well said - Original Message - From: Mike Newsgroups: groupstudy.cisco Sent: Monday, December 24, 2001 5:35 PM Subject: Re: IP classless command [7:30056] 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=7i=30063t=30056 -- FAQ, list archives, and subscription info: http://www.groupstudy.com/list/cisco.html Report misconduct and Nondisclosure violations to [EMAIL PROTECTED]
