No problem with the splitting of hairs. :-) I have been wondering why Cisco lets you do what the original poster is doing, which most of us misunderstood. He is using the same IP address on 2 serial interfaces on the SAME router.
If you try to use the same IP address on two Ethernet interfaces, you just get an error when you try to configure the second Ethernet interface. With two serial interfaces, you don't get an error. Is this just an oversight? There are many such oversights in Cisco IOS. :-) Or maybe there is a real reason to do it. I said in my original message that there's no ARP on serial interfaces so the router can't easily figure out if anyone else is using its address like it does on Ethernet. On Ethernet the router can send an ARP to see if someone else replies. But that's someone else on the LAN connected to the interface, not another interface on the same router. So, if it gives you an error on Ethernet when you use an address you have already used on another Ethernet interface, why doesn't it give you an error for serial interfaces? Maybe there's an actual technical reason, although probably it's just an oversight. By the way, it lets you configure an Ethernet interface to use an address already in use on a serial interface, but if you try to do it in the other order then you get an error. That's probably just another oversight. Cisco has always given you enough rope to hang yourself. Decent error messages have never been any more important than ease-of-use. :-) Priscilla Peter van Oene wrote: > > At 06:18 PM 2/10/2003 +0000, Priscilla Oppenheimer wrote: > >You can't have duplicate IP addresses anywhere. They have to > be unique. The > >only exceptions would be if you were doing some sort of NAT or > tunneling or > >something and the duplicates were hidden from each other. > > > >You don't get an error when you try to configure it because > it's a lot > >harder for IOS to detect this on a serial interface than on an > Ethernet > >interface. On Ethernet, a Cisco router ARPs for the address > you give it. If > >it receives a reply, then it gives you an error and won't let > you use the > >address. There's no ARP in serial land. > > > >You think you're pinging successfully, but how do you know who > is really > >replying? > > > >Even if you could assign duplicate IP addresses, you > shouldn't. You would > >wreak havoc with all sorts of things. There's no reason to do > it either. If > >you're concerned with running out of addresses, just use > private address. > >The 10.0.0.0 network has 16 million possibilities. > > For what it's worth, duplicating the same IP across a set of > DNS servers in > the same AS can provide an interesting spin on resiliency. So > long as you > configure unique IP's for normal communication. This sort of > thing works > good for protocols that are stateless (UDP DNS) > > Anycast-RP in PIM networks also uses the same IP on multiple > boxes :-) > > >Someone had to get blunt here! :-) > > Someone had to split some hair ! > > > > >_______________________________ > > > >Priscilla Oppenheimer > >www.troubleshootingnetworks.com > >www.priscilla.com > > > > > > > > > >Ladrach, Daniel E. wrote: > > > > > > If you ping you are probably pinging the Local IP.Try debug > ip > > > icmp to > > > verify what you are pinging. > > > > > > Daniel Ladrach > > > CCNP, CCNA > > > WorldCom > > > > > > > > > > > > -----Original Message----- > > > From: Monu Sekhon [mailto:[EMAIL PROTECTED]] > > > Sent: Monday, February 10, 2003 12:03 PM > > > To: [EMAIL PROTECTED] > > > Subject: RE: Simple Ip issue (need help) [7:62728] > > > > > > > > > Hi All, > > > Thanx again for all for contribution > > > confusion still there , > > > I am pinging remote side and I am able too. > > > any comments from all(still confused with answers) > > > > > > Walker, James - Is wrote: > > > > > > > > Only problem is which side are you pinging???????? > > > > > > > > > > > > > > > > > > > > -----Original Message----- > > > > From: John Murphy [mailto:[EMAIL PROTECTED]] > > > > Sent: Monday, February 10, 2003 11:15 AM > > > > To: [EMAIL PROTECTED] > > > > Subject: Re: Simple Ip issue (need help) [7:62728] > > > > > > > > > > > > If you're asking what I think you're asking, then I think > your > > > > answer is > > > > yes, but you won't be able to pass any traffic across the > > > > circuit. Unless > > > > you've confused me (it doesn't seem I would be the only > one), > > > > then the > > > > answer might not be the same. > > > > > > > > > > > > ----- Original Message ----- > > > > From: "Monu Sekhon" > > > > To: > > > > Sent: Monday, February 10, 2003 12:13 AM > > > > Subject: Simple Ip issue (need help) [7:62728] > > > > > > > > > > > > > Hi All, > > > > > I have very simple question, Can we use duplicate ips on > > > > serial interfaces > > > > > among them seleves although we cannot use duplicate ip > on > > > > serial with > > > > > Ethernet(lan interface) or loopback interface. > > > > > > > > > > > > > > > My topology is like this > > > > > > > > > > Client router server router(connected back to back) > > > > > 2 interfaces 2 inetrfaces > > > > > > > > > > > > > > > these routers connected back to back > > > > > > > > > > > > > > > configuration > > > > > int serial 0/0 > > > > > encap hdlc > > > > > ip address 1.1.1.1 255.255.255.0 > > > > > > > > > > int serial 0/1 > > > > > ip address 1.1.1.1 255.255.255.0 > > > > > encap hdlc > > > > > > > > > > > > > > > > > > > > now if all the two interfaces of serial even if given > > > > duplicate ip among > > > > > themselves works fine. no error from cli .interfaces > are up > > > > > and i am able to ping remote side. > > > > > > > > > > > > > > > > > > > > The ques is that > > > > > > > > > > 1) Lan interface also was in different subnet but serial > > > > interface > > > > > doesnot accept that ips as duplicate or of loopback > > > > > > > > > > 2)What Implication such have on my design ,any > limitation it > > > > has > > > > > > > > > > Does this type of design can be used, > > > > > > > > > > This is small thing is confusing me about ip. > > > > > > > > > > Thanx in advance > > Message Posted at: http://www.groupstudy.com/form/read.php?f=7&i=62787&t=62728 -------------------------------------------------- FAQ, list archives, and subscription info: http://www.groupstudy.com/list/cisco.html Report misconduct and Nondisclosure violations to [EMAIL PROTECTED]
