Broadcasting and the all ones subnet [7:48996]
Hello Group, Three things to confirm about broadcasts. a) the all ones broadcast i.e 255.255.255.255 by default will only be propagated to the local network and is not forwarded by routers b) network and subnet directed broadcasts. If I were to broadcast to 192.168.1.255, and I have subnets 192.168.1.32/27, 192.168.1.64/27 and 192.168.1.96/27, would all the subnets receive it as well? c) referring to scenario b), I believe that broadcasts with destination 192.168.1.255 is forwarded. Is this true? I was going thru this article about the effect of using the all ones subnet. There are somethings that I'm still confused about. The link is http://www.cisco.com/warp/public/105/40.html 1. In the first example, when host 195.1.1.24 sends a local broadcast to 195.1.1.255, will hosts attached to router 2's async lines receive the broadcast? 2. OK, its a directed broadcast and router 2 looks up its routing table and forwards it out using the default route. Router 1 receives the packet. I believe the packet is forwarded out to all 192.1.1.x/26 subnets, right? Will Router 1 forward the packet back to Router 2? I hope not 2a. Another way of looking at it is router 1 thinks that it is a broadcast only for subnet 195.1.1.192 and forwards it out only to router 5. Hmmm I'm definitely confused 3. Router 5 receives the packet from router 1. How will it interpret the packet? I'm guessing that the router sees it as a directed broadcast and send it out via the default route. Is it normal that routers forward a packet out from an interface that it received on? As in its received on e0 and forwarded out e0 as well 4. Once router 1 receives the packet from router 5, will it forward the packet out to all 192.1.1.x/26 subnets again or just to router 5. The article did not detail this part and just specified that it will bounce between routers 1 and 5. It also says that routers 2 thru 4 see the 'broadcast' only once. The way I see it , if all subnets receive the broadcast then routers 2 thru 4 should receive the packets as many times as router 5. I would appreciate all the help I can get. I know you gurus can help me out. Thanks!! Wes Message Posted at: http://www.groupstudy.com/form/read.php?f=7&i=48996&t=48996 -- FAQ, list archives, and subscription info: http://www.groupstudy.com/list/cisco.html Report misconduct and Nondisclosure violations to [EMAIL PROTECTED]
RE: Broadcasting and the all ones subnet [7:48996]
Hi Wesley, a) correct b) no, as 192.168.1.32/27, 192.168.1.64/27 and 192.168.1.96/27 are on a different subnet to the broadcast 192.168.1.255 (this is for the 192.168.1.224/27 subnet). c) from the answer to b), no. Only hosts on the 192.168.1.224/27 subnet will see the broadcast packet of 192.168.1.225. HTH, Mark. -Original Message- From: Wesley [mailto:[EMAIL PROTECTED]] Sent: Wednesday, 17 July 2002 16:49 To: [EMAIL PROTECTED] Subject: Broadcasting and the all ones subnet [7:48996] Hello Group, Three things to confirm about broadcasts. a) the all ones broadcast i.e 255.255.255.255 by default will only be propagated to the local network and is not forwarded by routers b) network and subnet directed broadcasts. If I were to broadcast to 192.168.1.255, and I have subnets 192.168.1.32/27, 192.168.1.64/27 and 192.168.1.96/27, would all the subnets receive it as well? c) referring to scenario b), I believe that broadcasts with destination 192.168.1.255 is forwarded. Is this true? I was going thru this article about the effect of using the all ones subnet. There are somethings that I'm still confused about. The link is http://www.cisco.com/warp/public/105/40.html 1. In the first example, when host 195.1.1.24 sends a local broadcast to 195.1.1.255, will hosts attached to router 2's async lines receive the broadcast? 2. OK, its a directed broadcast and router 2 looks up its routing table and forwards it out using the default route. Router 1 receives the packet. I believe the packet is forwarded out to all 192.1.1.x/26 subnets, right? Will Router 1 forward the packet back to Router 2? I hope not 2a. Another way of looking at it is router 1 thinks that it is a broadcast only for subnet 195.1.1.192 and forwards it out only to router 5. Hmmm I'm definitely confused 3. Router 5 receives the packet from router 1. How will it interpret the packet? I'm guessing that the router sees it as a directed broadcast and send it out via the default route. Is it normal that routers forward a packet out from an interface that it received on? As in its received on e0 and forwarded out e0 as well 4. Once router 1 receives the packet from router 5, will it forward the packet out to all 192.1.1.x/26 subnets again or just to router 5. The article did not detail this part and just specified that it will bounce between routers 1 and 5. It also says that routers 2 thru 4 see the 'broadcast' only once. The way I see it , if all subnets receive the broadcast then routers 2 thru 4 should receive the packets as many times as router 5. I would appreciate all the help I can get. I know you gurus can help me out. Thanks!! Wes Message Posted at: http://www.groupstudy.com/form/read.php?f=7&i=49000&t=48996 -- FAQ, list archives, and subscription info: http://www.groupstudy.com/list/cisco.html Report misconduct and Nondisclosure violations to [EMAIL PROTECTED]
Re: Broadcasting and the all ones subnet [7:48996]
Then how would you define an all /27 subnets broadcast i.e. not just 192.168.1.224 subnet getting the broadcast but all subnets? Thank you for the reply BTW. ""Vicuna, Mark"" wrote in message [EMAIL PROTECTED]">news:[EMAIL PROTECTED]... > Hi Wesley, > > a) correct > > b) no, as 192.168.1.32/27, 192.168.1.64/27 and > 192.168.1.96/27 are on a different subnet to the broadcast 192.168.1.255 > (this is for the 192.168.1.224/27 subnet). > > c) from the answer to b), no. Only hosts on the 192.168.1.224/27 subnet > will see the broadcast packet of 192.168.1.225. > > > HTH, > Mark. > -Original Message- > From: Wesley [mailto:[EMAIL PROTECTED]] > Sent: Wednesday, 17 July 2002 16:49 > To: [EMAIL PROTECTED] > Subject: Broadcasting and the all ones subnet [7:48996] > > > Hello Group, > > Three things to confirm about broadcasts. > > a) the all ones broadcast i.e 255.255.255.255 by default will only be > propagated to the local network and is not forwarded by routers > > b) network and subnet directed broadcasts. If I were to broadcast to > 192.168.1.255, and I have subnets 192.168.1.32/27, 192.168.1.64/27 and > 192.168.1.96/27, would all the subnets receive it as well? > > c) referring to scenario b), I believe that broadcasts with destination > 192.168.1.255 is forwarded. Is this true? > > I was going thru this article about the effect of using the all ones > subnet. > There are somethings that I'm still confused about. The link is > > http://www.cisco.com/warp/public/105/40.html > > 1. In the first example, when host 195.1.1.24 sends a local broadcast to > 195.1.1.255, will hosts attached to router 2's async lines receive the > broadcast? > > 2. OK, its a directed broadcast and router 2 looks up its routing table > and > forwards it out using the default route. Router 1 receives the packet. I > believe the packet is forwarded out to all 192.1.1.x/26 subnets, right? > Will > Router 1 forward the packet back to Router 2? I hope not > > 2a. Another way of looking at it is router 1 thinks that it is a > broadcast > only for subnet 195.1.1.192 and forwards it out only to router 5. Hmmm > > I'm definitely confused > > 3. Router 5 receives the packet from router 1. How will it interpret the > packet? I'm guessing that the router sees it as a directed broadcast and > send it out via the default route. Is it normal that routers forward a > packet out from an interface that it received on? As in its received on > e0 > and forwarded out e0 as well > > 4. Once router 1 receives the packet from router 5, will it forward the > packet out to all 192.1.1.x/26 subnets again or just to router 5. The > article did not detail this part and just specified that it will bounce > between routers 1 and 5. It also says that routers 2 thru 4 see the > 'broadcast' only once. The way I see it , if all subnets receive the > broadcast then routers 2 thru 4 should receive the packets as many times > as > router 5. > > I would appreciate all the help I can get. I know you gurus can help me > out. > Thanks!! > > Wes Message Posted at: http://www.groupstudy.com/form/read.php?f=7&i=49001&t=48996 -- FAQ, list archives, and subscription info: http://www.groupstudy.com/list/cisco.html Report misconduct and Nondisclosure violations to [EMAIL PROTECTED]
RE: Broadcasting and the all ones subnet [7:48996]
It will be the all 1's bit for that subnet eg. for 192.168.1.224/27 it would be 192.168.1.255 and for 192.168.1.32/27 it would be 192.168.1.63. HTH, Mark. -Original Message- From: Wesley [mailto:[EMAIL PROTECTED]] Sent: Wednesday, 17 July 2002 6:21 PM To: [EMAIL PROTECTED] Subject: Re: Broadcasting and the all ones subnet [7:48996] Then how would you define an all /27 subnets broadcast i.e. not just 192.168.1.224 subnet getting the broadcast but all subnets? Thank you for the reply BTW. ""Vicuna, Mark"" wrote in message [EMAIL PROTECTED]">news:[EMAIL PROTECTED]... > Hi Wesley, > > a) correct > > b) no, as 192.168.1.32/27, 192.168.1.64/27 and > 192.168.1.96/27 are on a different subnet to the broadcast 192.168.1.255 > (this is for the 192.168.1.224/27 subnet). > > c) from the answer to b), no. Only hosts on the 192.168.1.224/27 subnet > will see the broadcast packet of 192.168.1.225. > > > HTH, > Mark. > -Original Message- > From: Wesley [mailto:[EMAIL PROTECTED]] > Sent: Wednesday, 17 July 2002 16:49 > To: [EMAIL PROTECTED] > Subject: Broadcasting and the all ones subnet [7:48996] > > > Hello Group, > > Three things to confirm about broadcasts. > > a) the all ones broadcast i.e 255.255.255.255 by default will only be > propagated to the local network and is not forwarded by routers > > b) network and subnet directed broadcasts. If I were to broadcast to > 192.168.1.255, and I have subnets 192.168.1.32/27, 192.168.1.64/27 and > 192.168.1.96/27, would all the subnets receive it as well? > > c) referring to scenario b), I believe that broadcasts with destination > 192.168.1.255 is forwarded. Is this true? > > I was going thru this article about the effect of using the all ones > subnet. > There are somethings that I'm still confused about. The link is > > http://www.cisco.com/warp/public/105/40.html > > 1. In the first example, when host 195.1.1.24 sends a local broadcast to > 195.1.1.255, will hosts attached to router 2's async lines receive the > broadcast? > > 2. OK, its a directed broadcast and router 2 looks up its routing table > and > forwards it out using the default route. Router 1 receives the packet. I > believe the packet is forwarded out to all 192.1.1.x/26 subnets, right? > Will > Router 1 forward the packet back to Router 2? I hope not > > 2a. Another way of looking at it is router 1 thinks that it is a > broadcast > only for subnet 195.1.1.192 and forwards it out only to router 5. Hmmm > > I'm definitely confused > > 3. Router 5 receives the packet from router 1. How will it interpret the > packet? I'm guessing that the router sees it as a directed broadcast and > send it out via the default route. Is it normal that routers forward a > packet out from an interface that it received on? As in its received on > e0 > and forwarded out e0 as well > > 4. Once router 1 receives the packet from router 5, will it forward the > packet out to all 192.1.1.x/26 subnets again or just to router 5. The > article did not detail this part and just specified that it will bounce > between routers 1 and 5. It also says that routers 2 thru 4 see the > 'broadcast' only once. The way I see it , if all subnets receive the > broadcast then routers 2 thru 4 should receive the packets as many times > as > router 5. > > I would appreciate all the help I can get. I know you gurus can help me > out. > Thanks!! > > Wes Message Posted at: http://www.groupstudy.com/form/read.php?f=7&i=49006&t=48996 -- FAQ, list archives, and subscription info: http://www.groupstudy.com/list/cisco.html Report misconduct and Nondisclosure violations to [EMAIL PROTECTED]
Re: Broadcasting and the all ones subnet [7:48996]
Wesley wrote: > > So there isn't a broadcast address for all /27 subnets? I don't think sending to all subnets of a network is something that IP ever defined. > I > basically > understand that the last address of each subnet is reserved for > subnet > broadcast. I was just wondering if the broadcasting > architecture allowed for > all subnets to be broadcasted at once. And Mark, since you are > the only one > replying mind if you check out the CCO link in the original I hope Mark will answer too, but since we're the only ones talking now, I'll jump in. ;-) > post and tell me > your views on the issues that I have highlighted. I'll provide > the link > again > > http://www.cisco.com/warp/public/105/40.html I think the main thing to realize about the article is that it's a very strange case. Notice that the Asynch routers have a bunch of host-specific routes (/32). And then their E0's are configured with a /24 subnet mask, even though they probably should really be /26 to fit the network design. And then to make the problem happen they had to have a host misconfigured for /24 also and have it send a NetBIOS (or other) broadcast to x.x.x.255. I suggest that you set up a more normal situation in your lab and see if you can get the problem to happen. Perhaps TAC ran into a problem matching the scenario they describe. But is the problem reproducible under more normal condistions? (Perhaps TAC just made up the scenario too!? There are parts of it that aren't too believable. ;-) Please see a few more comments below. snip > > > > > > I was going thru this article about the effect of using the > all ones > > > subnet. > > > There are somethings that I'm still confused about. The > link is > > > > > > http://www.cisco.com/warp/public/105/40.html > > > > > > 1. In the first example, when host 195.1.1.24 sends a local > broadcast > > to > > > 195.1.1.255, will hosts attached to router 2's async lines > receive the > > > broadcast? No, the asynch lines are using /32. > > > > > > 2. OK, its a directed broadcast and router 2 looks up its I don't think Router 2 thinks it's a directed broadcast. The destination address doesn't match any of the /32 host routes, so Router 2 sends the packet out the default route. > routing > > table > > > and > > > forwards it out using the default route. Router 1 receives > the packet. > > I > > > believe the packet is forwarded out to all 192.1.1.x/26 > subnets, > > right? No, not all subnets. Router 1 has a specific route for subnet 192.1.1.192. (1100 in the last octet). If a packet comes into that subnet, it's supposed to go to Router 5. See the static route that points to Router 5 (195.1.2.5). Now, Router 1 should recognize that the incoming packet is a directed broadcast for subnet 192 and not forward it if "no ip directed-broadcast" is configured, which is the default these days. > > > Will > > > Router 1 forward the packet back to Router 2? I hope not > > > No. > > > 2a. Another way of looking at it is router 1 thinks that it > is a > > > broadcast > > > only for subnet 195.1.1.192 and forwards it out only to > router 5. That's my interpretation too. > > Hmmm > > > > > > I'm definitely confused > > > > > > 3. Router 5 receives the packet from router 1. How will it > interpret > > the > > > packet? I'm guessing that the router sees it as a directed > broadcast Router 5, like Router 2, has a bunch of /32 host routes. The incoming packet doesn't match any of those, so Router 5 sends it out the default route. > > and > > > send it out via the default route. Is it normal that > routers forward a > > > packet out from an interface that it received on? Well, not too common, but it does happen sometimes. > As in its > received > > on > > > e0 > > > and forwarded out e0 as well > > > > > > 4. Once router 1 receives the packet from router 5, will it > forward > > the > > > packet out to all 192.1.1.x/26 subnets again or just to > router 5. Just to Router 5 > The > > > article did not detail this part and just specified that it > will > > bounce > > > between routers 1 and 5. It also says that routers 2 thru 4 > see the > > > 'broadcast' only once. The way I see it , if all subnets > receive the > > > broadcast then routers 2 thru 4 should receive the packets > as many > > times > > > as > > > router 5. > > > > > > I would appreciate all the help I can get. I know you gurus > can help > > me > > > out. > > > Thanks!! > > > > > > Wes > > Message Posted at: http://www.groupstudy.com/form/read.php?f=7&i=49050&t=48996 -- FAQ, list archives, and subscription info: http://www.groupstudy.com/list/cisco.html Report misconduct and Nondisclosure violations to [EMAIL PROTECTED]
RE: Broadcasting and the all ones subnet [7:48996]
Vicuna, Mark wrote: > > It will be the all 1's bit for that subnet eg. for > 192.168.1.224/27 it > would be 192.168.1.255 and for 192.168.1.32/27 it would be > 192.168.1.63. I think he was asking how would you send to all the subnets. And the answer is, you wouldn't. Why would you want to do such a thing? I can't think of any legitimate application that needs to do that. It's alwasy been a fuzzy area. I know there have been rumors for years that if you use the all ones subnet it will confuse routers into thinking that they should send to all the subnets, but I've never seen that actually happen. Priscilla > > > HTH, > Mark. > > -Original Message- > From: Wesley [mailto:[EMAIL PROTECTED]] > Sent: Wednesday, 17 July 2002 6:21 PM > To: [EMAIL PROTECTED] > Subject: Re: Broadcasting and the all ones subnet [7:48996] > > > Then how would you define an all /27 subnets broadcast i.e. not > just > 192.168.1.224 subnet getting the broadcast but all subnets? > Thank you > for > the reply BTW. > > ""Vicuna, Mark"" wrote in message > [EMAIL PROTECTED]">news:[EMAIL PROTECTED]... > > Hi Wesley, > > > > a) correct > > > > b) no, as 192.168.1.32/27, 192.168.1.64/27 and > > 192.168.1.96/27 are on a different subnet to the broadcast > 192.168.1.255 > > (this is for the 192.168.1.224/27 subnet). > > > > c) from the answer to b), no. Only hosts on the > 192.168.1.224/27 > subnet > > will see the broadcast packet of 192.168.1.225. > > > > > > HTH, > > Mark. > > -Original Message- > > From: Wesley [mailto:[EMAIL PROTECTED]] > > Sent: Wednesday, 17 July 2002 16:49 > > To: [EMAIL PROTECTED] > > Subject: Broadcasting and the all ones subnet [7:48996] > > > > > > Hello Group, > > > > Three things to confirm about broadcasts. > > > > a) the all ones broadcast i.e 255.255.255.255 by default will > only be > > propagated to the local network and is not forwarded by > routers > > > > b) network and subnet directed broadcasts. If I were to > broadcast to > > 192.168.1.255, and I have subnets 192.168.1.32/27, > 192.168.1.64/27 and > > 192.168.1.96/27, would all the subnets receive it as well? > > > > c) referring to scenario b), I believe that broadcasts with > destination > > 192.168.1.255 is forwarded. Is this true? > > > > I was going thru this article about the effect of using the > all ones > > subnet. > > There are somethings that I'm still confused about. The link > is > > > > http://www.cisco.com/warp/public/105/40.html > > > > 1. In the first example, when host 195.1.1.24 sends a local > broadcast > to > > 195.1.1.255, will hosts attached to router 2's async lines > receive the > > broadcast? > > > > 2. OK, its a directed broadcast and router 2 looks up its > routing > table > > and > > forwards it out using the default route. Router 1 receives > the packet. > I > > believe the packet is forwarded out to all 192.1.1.x/26 > subnets, > right? > > Will > > Router 1 forward the packet back to Router 2? I hope not > > > > 2a. Another way of looking at it is router 1 thinks that it > is a > > broadcast > > only for subnet 195.1.1.192 and forwards it out only to > router 5. > Hmmm > > > > I'm definitely confused > > > > 3. Router 5 receives the packet from router 1. How will it > interpret > the > > packet? I'm guessing that the router sees it as a directed > broadcast > and > > send it out via the default route. Is it normal that routers > forward a > > packet out from an interface that it received on? As in its > received > on > > e0 > > and forwarded out e0 as well > > > > 4. Once router 1 receives the packet from router 5, will it > forward > the > > packet out to all 192.1.1.x/26 subnets again or just to > router 5. The > > article did not detail this part and just specified that it > will > bounce > > between routers 1 and 5. It also says that routers 2 thru 4 > see the > > 'broadcast' only once. The way I see it , if all subnets > receive the > > broadcast then routers 2 thru 4 should receive the packets as > many > times > > as > > router 5. > > > > I would appreciate all the help I can get. I know you gurus > can help > me > > out. > > Thanks!! > > > > Wes > > Message Posted at: http://www.groupstudy.com/form/read.php?f=7&i=49045&t=48996 -- FAQ, list archives, and subscription info: http://www.groupstudy.com/list/cisco.html Report misconduct and Nondisclosure violations to [EMAIL PROTECTED]
Re: Broadcasting and the all ones subnet [7:48996]
So there isn't a broadcast address for all /27 subnets? I basically understand that the last address of each subnet is reserved for subnet broadcast. I was just wondering if the broadcasting architecture allowed for all subnets to be broadcasted at once. And Mark, since you are the only one replying mind if you check out the CCO link in the original post and tell me your views on the issues that I have highlighted. I'll provide the link again http://www.cisco.com/warp/public/105/40.html In that example, I have a feeling that routers do forward subnet directed broadcasts. Can anyone else explain the behaviour of the routers in the example i.e. broadcast packets bouncing between Routers 1 and 5. Any comments is greatly appreciated. Thank you. ""Vicuna, Mark"" wrote in message [EMAIL PROTECTED]">news:[EMAIL PROTECTED]... > It will be the all 1's bit for that subnet eg. for 192.168.1.224/27 it > would be 192.168.1.255 and for 192.168.1.32/27 it would be 192.168.1.63. > > > HTH, > Mark. > > -Original Message- > From: Wesley [mailto:[EMAIL PROTECTED]] > Sent: Wednesday, 17 July 2002 6:21 PM > To: [EMAIL PROTECTED] > Subject: Re: Broadcasting and the all ones subnet [7:48996] > > > Then how would you define an all /27 subnets broadcast i.e. not just > 192.168.1.224 subnet getting the broadcast but all subnets? Thank you > for > the reply BTW. > > ""Vicuna, Mark"" wrote in message > [EMAIL PROTECTED]">news:[EMAIL PROTECTED]... > > Hi Wesley, > > > > a) correct > > > > b) no, as 192.168.1.32/27, 192.168.1.64/27 and > > 192.168.1.96/27 are on a different subnet to the broadcast > 192.168.1.255 > > (this is for the 192.168.1.224/27 subnet). > > > > c) from the answer to b), no. Only hosts on the 192.168.1.224/27 > subnet > > will see the broadcast packet of 192.168.1.225. > > > > > > HTH, > > Mark. > > -Original Message- > > From: Wesley [mailto:[EMAIL PROTECTED]] > > Sent: Wednesday, 17 July 2002 16:49 > > To: [EMAIL PROTECTED] > > Subject: Broadcasting and the all ones subnet [7:48996] > > > > > > Hello Group, > > > > Three things to confirm about broadcasts. > > > > a) the all ones broadcast i.e 255.255.255.255 by default will only be > > propagated to the local network and is not forwarded by routers > > > > b) network and subnet directed broadcasts. If I were to broadcast to > > 192.168.1.255, and I have subnets 192.168.1.32/27, 192.168.1.64/27 and > > 192.168.1.96/27, would all the subnets receive it as well? > > > > c) referring to scenario b), I believe that broadcasts with > destination > > 192.168.1.255 is forwarded. Is this true? > > > > I was going thru this article about the effect of using the all ones > > subnet. > > There are somethings that I'm still confused about. The link is > > > > http://www.cisco.com/warp/public/105/40.html > > > > 1. In the first example, when host 195.1.1.24 sends a local broadcast > to > > 195.1.1.255, will hosts attached to router 2's async lines receive the > > broadcast? > > > > 2. OK, its a directed broadcast and router 2 looks up its routing > table > > and > > forwards it out using the default route. Router 1 receives the packet. > I > > believe the packet is forwarded out to all 192.1.1.x/26 subnets, > right? > > Will > > Router 1 forward the packet back to Router 2? I hope not > > > > 2a. Another way of looking at it is router 1 thinks that it is a > > broadcast > > only for subnet 195.1.1.192 and forwards it out only to router 5. > Hmmm > > > > I'm definitely confused > > > > 3. Router 5 receives the packet from router 1. How will it interpret > the > > packet? I'm guessing that the router sees it as a directed broadcast > and > > send it out via the default route. Is it normal that routers forward a > > packet out from an interface that it received on? As in its received > on > > e0 > > and forwarded out e0 as well > > > > 4. Once router 1 receives the packet from router 5, will it forward > the > > packet out to all 192.1.1.x/26 subnets again or just to router 5. The > > article did not detail this part and just specified that it will > bounce > > between routers 1 and 5. It also says that routers 2 thru 4 see the > > 'broadcast' only once. The way I see it , if all subnets receive the > > broadcast then routers 2 thru 4 should receive the packets as many > times > > as > > router 5. > > > > I would appreciate all the help I can get. I know you gurus can help > me > > out. > > Thanks!! > > > > Wes Message Posted at: http://www.groupstudy.com/form/read.php?f=7&i=49043&t=48996 -- FAQ, list archives, and subscription info: http://www.groupstudy.com/list/cisco.html Report misconduct and Nondisclosure violations to [EMAIL PROTECTED]
Re: Broadcasting and the all ones subnet [7:48996]
Heya Priscilla, Thanks for the explaination. Is it safe to assume that Cisco routers do not perform an all subnets broadcast? I found something off Google that I would like to share with you guys. This is an excerpt from "TCP/IP Tutorial" by IBM. All-Subnets-Directed Broadcast Address If the network number is a valid network number, the network is subnetted and the local part is all ones (for example, 128.2.255.255), then the address refers to all hosts on all subnets in the specified network. In principle routers may propagate broadcasts for all subnets but are not required to do so. In practice, they do not; there are few circumstances where such a broadcast would be desirable, and it can lead to problems, particularly if a host has been incorrectly configured with no subnet mask. Consider the wasted resource involved if a host 9.180.214.114 in the subnetted Class A network 9 thought that it was not subnetted and used 9.255.255.255 as a local broadcast address instead of 9.180.214.255 and all of the routers in the network respected the request to forward the request to all clients. If routers do respect all-subnets-directed broadcast address, they use an algorithm called reverse path forwarding to prevent the broadcast messages from multiplying out of control. See RFC 922 for more details on this algorithm. I guess an important point here is whether routers respect the all-subnets directed broadcast. A question on the /32 entries in the routing table. I can understand that each async line can only have one host at the remote end and therefore would have a host route in the routing table. So how do the hosts on async lines receive broadcasts? What would be their broadcast address? It would only make sense that the broadcast packet was not delivered to the hosts connected to Router 5. That is why the packets kept bouncing back and forth. Otherwise it would have been delivered to the hosts and this problem would not have surfaced. Why is it that the all ones subnet was initially excluded as a valid subnet? It would seem perfectly OK to me to use the all zeros and all ones subnet numbers. I mean with prefix routing, these things can be distinguished. Like you said, the problem discussed in the article was pretty 'artsy'. As always, thank you so very much. Wesley ""Priscilla Oppenheimer"" wrote in message [EMAIL PROTECTED]">news:[EMAIL PROTECTED]... > Wesley wrote: > > > > So there isn't a broadcast address for all /27 subnets? > > I don't think sending to all subnets of a network is something that IP ever > defined. > > > I > > basically > > understand that the last address of each subnet is reserved for > > subnet > > broadcast. I was just wondering if the broadcasting > > architecture allowed for > > all subnets to be broadcasted at once. And Mark, since you are > > the only one > > replying mind if you check out the CCO link in the original > > I hope Mark will answer too, but since we're the only ones talking now, I'll > jump in. ;-) > > > post and tell me > > your views on the issues that I have highlighted. I'll provide > > the link > > again > > > > http://www.cisco.com/warp/public/105/40.html > > I think the main thing to realize about the article is that it's a very > strange case. Notice that the Asynch routers have a bunch of host-specific > routes (/32). And then their E0's are configured with a /24 subnet mask, > even though they probably should really be /26 to fit the network design. > > And then to make the problem happen they had to have a host misconfigured > for /24 also and have it send a NetBIOS (or other) broadcast to x.x.x.255. > > I suggest that you set up a more normal situation in your lab and see if you > can get the problem to happen. Perhaps TAC ran into a problem matching the > scenario they describe. But is the problem reproducible under more normal > condistions? (Perhaps TAC just made up the scenario too!? There are parts of > it that aren't too believable. ;-) > > Please see a few more comments below. > > snip > > > > > > > > > I was going thru this article about the effect of using the > > all ones > > > > subnet. > > > > There are somethings that I'm still confused about. The > > link is > > > > > > > > http://www.cisco.com/warp/public/105/40.html > > > > > > > > 1. In the first example, when host 195.1.1.24 sends a local > > broadcast > > > to > > > > 195.1.1.255, will hosts attached to router 2's async lines > > receive the > > > > broadcast? > > No, the asynch lines are using /32. > > > > > > > > > 2. OK, its a directed broadcast and router 2 looks up its > > I don't think Router 2 thinks it's a directed broadcast. The destination > address doesn't match any of the /32 host routes, so Router 2 sends the > packet out the default route. > > > routing > > > table > > > > and > > > > forwards it out using the default route. Router 1 receives > > the packet. > > > I > > > > believe the packet is forwarded out to all 192.1.1.x/26 > > subnets, > > > right? > > No,
Re: Broadcasting and the all ones subnet [7:48996]
Wesley wrote: > > Heya Priscilla, > > Thanks for the explaination. Is it safe to assume that Cisco > routers do not > perform an all subnets broadcast? I've never seen them do it, but I can't say for sure that they never do. > I found something off Google > that I would > like to share with you guys. This is an excerpt from "TCP/IP > Tutorial" by > IBM. Good stuff. Thanks. > > All-Subnets-Directed Broadcast Address > > If the network number is a valid network number, the network is > subnetted > and the local part is all ones (for example, 128.2.255.255), > then the > address refers to all hosts on all subnets in the specified > network. In > principle routers may propagate broadcasts for all subnets but > are not > required to do so. In practice, they do not; there are few > circumstances > where such a broadcast would be desirable, and it can lead to > problems, > particularly if a host has been incorrectly configured with no > subnet mask. > Consider the wasted resource involved if a host 9.180.214.114 > in the > subnetted Class A network 9 thought that it was not subnetted > and used > 9.255.255.255 as a local broadcast address instead of > 9.180.214.255 and all > of the routers in the network respected the request to forward > the request > to all clients. If routers do respect all-subnets-directed > broadcast > address, they use an algorithm called reverse path forwarding > to prevent > the broadcast messages from multiplying out of control. See RFC > 922 for more > details on this algorithm. > > I guess an important point here is whether routers respect the > all-subnets > directed broadcast. > > A question on the /32 entries in the routing table. I can > understand that > each async line can only have one host at the remote end and > therefore would > have a host route in the routing table. So how do the hosts on > async lines > receive broadcasts? Why would the hosts need to receive broadcasts? A broadcast means go to everyone. Well, "everyone" is just one device in this case! ;-) There's no need for anyone other than possibly the router to find the MAC address of these devices, so there's no need for ARP broadcasts (which may not have any meaning for async anyway). You wouldn't want a host on an async link to be announcing things like the existance of its local printer, so there's little need for service announcement-type broadcasts. If you did need service announcements to get forwarded, you could use a helper address. DHCP broadcasts probably aren't necessary. (A lot of async lines use PPP. The IP PPP NCP does IP address assignment). If you did need DHCP, then you could use a helper address. > What would be their broadcast address? I guess it would be the same as the actual address if there were a need for it, but there may not be any need for it. > It > would only > make sense that the broadcast packet was not delivered to the > hosts > connected to Router 5. That is why the packets kept bouncing > back and forth. > Otherwise it would have been delivered to the hosts and this > problem would > not have surfaced. > > Why is it that the all ones subnet was initially excluded as a > valid subnet? Paranoia?? ;-) And that's all I have to say on the subject. Priscilla > It would seem perfectly OK to me to use the all zeros and all > ones subnet > numbers. I mean with prefix routing, these things can be > distinguished. Like > you said, the problem discussed in the article was pretty > 'artsy'. > > As always, thank you so very much. > > Wesley > > ""Priscilla Oppenheimer"" wrote in > message > [EMAIL PROTECTED]">news:[EMAIL PROTECTED]... > > Wesley wrote: > > > > > > So there isn't a broadcast address for all /27 subnets? > > > > I don't think sending to all subnets of a network is > something that IP > ever > > defined. > > > > > I > > > basically > > > understand that the last address of each subnet is reserved > for > > > subnet > > > broadcast. I was just wondering if the broadcasting > > > architecture allowed for > > > all subnets to be broadcasted at once. And Mark, since you > are > > > the only one > > > replying mind if you check out the CCO link in the original > > > > I hope Mark will answer too, but since we're the only ones > talking now, > I'll > > jump in. ;-) > > > > > post and tell me > > > your views on the issues that I have highlighted. I'll > provide > > > the link > > > again > > > > > > http://www.cisco.com/warp/public/105/40.html > > > > I think the main thing to realize about the article is that > it's a very > > strange case. Notice that the Asynch routers have a bunch of > host-specific > > routes (/32). And then their E0's are configured with a /24 > subnet mask, > > even though they probably should really be /26 to fit the > network design. > > > > And then to make the problem happen they had to have a host > misconfigured > > for /24 also and have it send a NetBIOS (or other) broadcast > to x.x.x.255. > > > > I suggest that you set up a m
Re: Broadcasting and the all ones subnet [7:48996]
as always, the answers may be found in the RFC's - specifically in RFC 1812. See in particular pages 93 and 94. http://www.ietf.org/rfc/rfc1812.txt?number=1812 comments below: ""Priscilla Oppenheimer"" wrote in message [EMAIL PROTECTED]">news:[EMAIL PROTECTED]... > Wesley wrote: > > > > Heya Priscilla, > > > > Thanks for the explaination. Is it safe to assume that Cisco > > routers do not > > perform an all subnets broadcast? to quote the RFC: "In a CIDR routing domain, wherein classical IP network numbers are meaningless, the concept of an all-subnets-directed-broadcast is also meaningless. To the knowledge of the working group, the facility was never implemented or deployed, and is now relegated to the dustbin of history." in other words - some pretty smart folks have already been there, done that, and concluded it weren't worth the effort > > I've never seen them do it, but I can't say for sure that they never do. to quote the RFC: "The first version of this memo described an algorithm for distributing a directed broadcast to all the subnets of a classical network number. This algorithm was stated to be "broken," and certain failure cases were specified." the reference seems to be to RFC 1009, which does indeed discuss directed broadcasts and certain exceptions. > > > I found something off Google > > that I would > > like to share with you guys. This is an excerpt from "TCP/IP > > Tutorial" by > > IBM. no doubt written by folks who studied the RFC's > > Good stuff. Thanks. > > > > > All-Subnets-Directed Broadcast Address > > > > If the network number is a valid network number, the network is > > subnetted > > and the local part is all ones (for example, 128.2.255.255), > > then the > > address refers to all hosts on all subnets in the specified > > network. In > > principle routers may propagate broadcasts for all subnets but > > are not > > required to do so. In practice, they do not; there are few > > circumstances > > where such a broadcast would be desirable, and it can lead to > > problems, > > particularly if a host has been incorrectly configured with no > > subnet mask. > > Consider the wasted resource involved if a host 9.180.214.114 > > in the > > subnetted Class A network 9 thought that it was not subnetted > > and used > > 9.255.255.255 as a local broadcast address instead of > > 9.180.214.255 and all > > of the routers in the network respected the request to forward > > the request > > to all clients. If routers do respect all-subnets-directed > > broadcast > > address, they use an algorithm called reverse path forwarding > > to prevent > > the broadcast messages from multiplying out of control. See RFC > > 922 for more > > details on this algorithm. again, if I read my RFC history correctly, this avenue proved more difficult than at first believed, and was eventually abandoned. > > > > I guess an important point here is whether routers respect the > > all-subnets > > directed broadcast. no longer important. it isn't done. > > > > A question on the /32 entries in the routing table. I can > > understand that > > each async line can only have one host at the remote end and > > therefore would > > have a host route in the routing table. So how do the hosts on > > async lines > > receive broadcasts? > > Why would the hosts need to receive broadcasts? A broadcast means go to > everyone. Well, "everyone" is just one device in this case! ;-) There's no > need for anyone other than possibly the router to find the MAC address of > these devices, so there's no need for ARP broadcasts (which may not have any > meaning for async anyway). I suspect that the point here is that in an asynch world there might still be applications that do broadcast, and that all hosts in the same "subnet" e.g. 192.168.1.x receive that broadcast, even if every host is asynch, and every host was in theory a /32 let's say, for example, I'm a brokerage firm, and a lot of my brokers work from home. they dial in, and they run their quote application. the quote server sends nothing but broadcasts. > > You wouldn't want a host on an async link to be announcing things like the > existance of its local printer, so there's little need for service > announcement-type broadcasts. If you did need service announcements to get > forwarded, you could use a helper address. > > DHCP broadcasts probably aren't necessary. (A lot of async lines use PPP. > The IP PPP NCP does IP address assignment). If you did need DHCP, then you > could use a helper address. > > > What would be their broadcast address? > > I guess it would be the same as the actual address if there were a need for > it, but there may not be any need for it. > > > It > > would only > > make sense that the broadcast packet was not delivered to the > > hosts > > connected to Router 5. That is why the packets kept bouncing > > back and forth. > > Otherwise it would have been delivered to the hosts and this > > problem would > > n
