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, not all subnets. Router 1 has a specific route for subnet 192.1.1.192. > (11000000 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=49091&t=48996 -------------------------------------------------- FAQ, list archives, and subscription info: http://www.groupstudy.com/list/cisco.html Report misconduct and Nondisclosure violations to [EMAIL PROTECTED]
