Re: About OSPF and Loopback port
Ok, I'll join the party... Of course we'll make some assumptions(without making an .. of myself or anyone else) My first thought here is ask another question...? "They share an Ethernet" you say. From the looks of it this could mean R1 E1 -- R2 E1 is connected or R1 E2 -- R2 E2 is connected which would of course change the outcome of this little "fendish scenario". I choose do the first option simply because you mention a device on R1 E2 being able to reach a host on R2 E2 My fist stab at this.. Assuming that R1 E1 -- R2 E1 is the common shared medium for these two devices with R1's E1 = 10.5.0.2 being the highest address(noting that E0 is in a down state and will not be used) should become the DR, but when R2's E1 interface comes online with an identical address no OSPF relationships should be formed because the RID would be identical on both devices, as well (with no IP connectivity between) these devices would not be able to see each other. Now, once the "heroine" corrects the problem R2's E1 interface would in effect be the DR after the IP connection is established. This being the case, I don't see the need for clearing the ospf process. A thought as to what happens once the neighbor relationships is formed and both routers begin to advertise a route to the 10.2.0.0 network...? Thoughts, well within each router there will be a connected interface for this network (10.2.0.0 and 10.5.0.0 )which would logically replace any learned route for the neighboring router. The other obvious thought is that a host trying to ping/reach another device at R2's E2 interface would try to ping a device on the local segment which would not be there in effect time-out..! So the answer... No.. Now, I'm off to mock this up on the home lab to see what other fenish occurrences I can find So, just how far off am I.and just how much of a fool did I make of myself... Nigel - Original Message - From: Howard C. Berkowitz [EMAIL PROTECTED] To: [EMAIL PROTECTED] Sent: Wednesday, November 29, 2000 8:37 PM Subject: RE: About OSPF and Loopback port At 10:45 AM -0800 11/29/2000, Healis, Jim wrote: Use loopback ports in OSPF so you can set the Router OSPF ID, otherwise it will take the highest IP address. Jim Healis CCNP, CCDP Senior Network Administrator Virata Unless there have been recent IOS changes (I'm really most current in 11-something), it's even more unpredictable than just the highest IP address: At the time of OSPF initialization, the router ID is: if there are multiple loopback interfaces, the highest IP address on any loopback (i.e., not highest loopback interface number) if there is a single loopback interface, use its address if there are no loopback interfaces, use the highest IP address on any active interface (i.e., if all interfaces are in shutdown, OSPF can't initialize. Using loopbacks avoids this because a loopback cannot be down.) A fiendish troubleshooting scenario: R1 comes up first, then R2. They share an Ethernet. Neither has any loopbacks. Scenario 1 (R1 is initially misconfigured) R1 E0: 10.6.0.1 DOWN R2 E0: 10.1.0.1 UP/UP E1: 10.5.0.2 UP/UP E1: 10.5.0.2 UP/UP E2: 10.2.0.1 UP/UP E2: 10.2.0.2 UP/UP Admin discovers that R1 E1 is misconfigured and should have been 10.5.0.1. Our Heroine corrects that interface to 10.5.0.1. Assuming both routers had OSPF configured with network 10.0.0.0 0.255.255.255 area 0.0.0.1 Will a device on R1 E2 be able to ping a host on R2 E2? -Original Message- From: Moerdo [mailto:[EMAIL PROTECTED]] Sent: Wednesday, November 29, 2000 8:26 AM To: [EMAIL PROTECTED] Subject: About OSPF and Loopback port Does anyone here can explain to me, why me must use loopback port for OSPF configuration. Thank you for the answer for this stupid question. Thank you. moerdo. _ FAQ, list archives, and subscription info: http://www.groupstudy.com/list/cisco.html Report misconduct and Nondisclosure violations to [EMAIL PROTECTED] _ FAQ, list archives, and subscription info: http://www.groupstudy.com/list/cisco.html Report misconduct and Nondisclosure violations to [EMAIL PROTECTED]
RE: About OSPF and Loopback port
Hello, I am a little late on this one, but is R1E2 and R2E2 on the same Lan? (10.2.0.0). If so any device on that lan with the correct IP will be able to ping any other device with the correct IP. I think a simple ARP will allow that. No Router needed for the ping to succeed. Even if I have missed the gist of the story, please don't stop. I love this OSPF stuff. And the next time I will read all 400 of my emails before I jump in. Winston. -Original Message- From: Chuck Larrieu [SMTP:[EMAIL PROTECTED]] Sent: Thursday, November 30, 2000 5:47 AM To: Howard C. Berkowitz; [EMAIL PROTECTED] Subject: RE: About OSPF and Loopback port OK Howard. I'm not afraid to look foolish in front of everyone. --- Howard's scenario: Scenario 1 (R1 is initially misconfigured) R1 E0: 10.6.0.1 DOWN R2 E0: 10.1.0.1 UP/UP E1: 10.5.0.2 UP/UP E1: 10.5.0.2 UP/UP E2: 10.2.0.1 UP/UP E2: 10.2.0.2 UP/UP Admin discovers that R1 E1 is misconfigured and should have been 10.5.0.1. Our Heroine corrects that interface to 10.5.0.1. Assuming both routers had OSPF configured with network 10.0.0.0 0.255.255.255 area 0.0.0.1 Will a device on R1 E2 be able to ping a host on R2 E2? -- My feeble attempt at cleverness: Assuming that there is no typo in your addressing scheme, I say you have a real mess here and that the answer is no. 1) you have the same subnet appearing on two different routers 10.0.5.0/whatever on R1E1 and R2E1 While OSPF can handle discontiguous subnets, I would be surprised if it can handle duplicate subnets with any aplomb 2) on R1 the interface with the address of 10.0.5.2 has been placed into area 0.0.0.1. when the ip address for that interface is changed, without either reloading the router or performing a clear ip ospf process the interface with the address 10.5.0.1 will NOT have been placed into the OSPF process. 3) I have not done this on my routers yet. I want to see if I am in the right ballpark so far. ( well, to be honest, as soon as I send this message I am setting up a scenario, upon which I will report subsequently ) Well, gang - am I wise or still a fool? Chuck -Original Message- From: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED]] On Behalf Of Howard C. Berkowitz Sent: Wednesday, November 29, 2000 12:37 PM To: [EMAIL PROTECTED] Subject: RE: About OSPF and Loopback port At 10:45 AM -0800 11/29/2000, Healis, Jim wrote: Use loopback ports in OSPF so you can set the Router OSPF ID, otherwise it will take the highest IP address. Jim Healis CCNP, CCDP Senior Network Administrator Virata Unless there have been recent IOS changes (I'm really most current in 11-something), it's even more unpredictable than just the highest IP address: At the time of OSPF initialization, the router ID is: if there are multiple loopback interfaces, the highest IP address on any loopback (i.e., not highest loopback interface number) if there is a single loopback interface, use its address if there are no loopback interfaces, use the highest IP address on any active interface (i.e., if all interfaces are in shutdown, OSPF can't initialize. Using loopbacks avoids this because a loopback cannot be down.) A fiendish troubleshooting scenario: R1 comes up first, then R2. They share an Ethernet. Neither has any loopbacks. Scenario 1 (R1 is initially misconfigured) R1 E0: 10.6.0.1 DOWN R2 E0: 10.1.0.1 UP/UP E1: 10.5.0.2 UP/UP E1: 10.5.0.2 UP/UP E2: 10.2.0.1 UP/UP E2: 10.2.0.2 UP/UP Admin discovers that R1 E1 is misconfigured and should have been 10.5.0.1. Our Heroine corrects that interface to 10.5.0.1. Assuming both routers had OSPF configured with network 10.0.0.0 0.255.255.255 area 0.0.0.1 Will a device on R1 E2 be able to ping a host on R2 E2? -Original Message- From:Moerdo [mailto:[EMAIL PROTECTED]] Sent:Wednesday, November 29, 2000 8:26 AM To: [EMAIL PROTECTED] Subject: About OSPF and Loopback port Does anyone here can explain to me, why me must use loopback port for OSPF configuration. Thank you for the answer for this stupid question. Thank you. moerdo. _ FAQ, list archives, and subscription info: http://www.groupstudy.com/list/cisco.html Report misconduct and Nondisclosure violations to [EMAIL PROTECTED] _ FAQ, list archives, and subscription info: http://www.groupstudy.com/list/cisco.html Report misconduct and Nondisclosure violations to [EMAIL PROTECTED] _ FAQ, list archives, and subscription info: http://www.groupstudy.com/list/cisco.html Report misconduct and Nondisclosure violations to [EMAIL PROTECTED]
RE: RE: About OSPF and Loopback port
Howard, Chuck et al, Even more strange is if you configure the loopback interface as a point-to-point network - e.g. int lo0 ip address 172.16.1.1 255.255.255.0 ip osdpf network-type point-to-point router ospf 10 network 172.16.1.1 0.0.0.0 area 0 In this configuration, the OSPF process won't use this loopback address as the router id. (See Advanced IP Network Design - ISBN 1-57870-097-3, Retana, SLice and White - Cisco Press p108 for more info) Hope this adds some to the party - I'll have a go at Howard's challenge later when I can get onto my lab, but I suspect the output from "debug ip ospf adjency" might help here. I have included Howard's OSPF router ID selection table as a pointer to why I think this will help. Regards Pete S. SNIPed some stuff, this is HCB's bit Unless there have been recent IOS changes (I'm really most current in 11-something), it's even more unpredictable than just the highest IP address: At the time of OSPF initialization, the router ID is: if there are multiple loopback interfaces, the highest IP address on any loopback (i.e., not highest loopback interface number) if there is a single loopback interface, use its address if there are no loopback interfaces, use the highest IP address on any active interface (i.e., if all interfaces are in shutdown, OSPF can't initialize. Using loopbacks avoids this because a loopback cannot be down.) A fiendish troubleshooting scenario: R1 comes up first, then R2. They share an Ethernet. Neither has any loopbacks. Scenario 1 (R1 is initially misconfigured) R1 E0: 10.6.0.1 DOWN R2 E0: 10.1.0.1 UP/UP E1: 10.5.0.2 UP/UP E1: 10.5.0.2 UP/UP E2: 10.2.0.1 UP/UP E2: 10.2.0.2 UP/UP Admin discovers that R1 E1 is misconfigured and should have been 10.5.0.1. Our Heroine corrects that interface to 10.5.0.1. Assuming both routers had OSPF configured with network 10.0.0.0 0.255.255.255 area 0.0.0.1 Will a device on R1 E2 be able to ping a host on R2 E2? - Sent using MailStart.com ( http://MailStart.Com/welcome.html ) The FREE way to access your mailbox via any web browser, anywhere! _ FAQ, list archives, and subscription info: http://www.groupstudy.com/list/cisco.html Report misconduct and Nondisclosure violations to [EMAIL PROTECTED]
RE: About OSPF and Loopback port
Interesting what draws comment and what doesn't. Howard's puzzle is quite a challenge, when one approaches it with reason rather than with routers. In general I was 2/3's right ( arguably ) but I missed a major point when I stated my predictions. --- Howard's scenario: Scenario 1 (R1 is initially misconfigured) R1 E0: 10.6.0.1 DOWN R2 E0: 10.1.0.1 UP/UP E1: 10.5.0.2 UP/UP E1: 10.5.0.2 UP/UP E2: 10.2.0.1 UP/UP E2: 10.2.0.2 UP/UP Admin discovers that R1 E1 is misconfigured and should have been 10.5.0.1. Our Heroine corrects that interface to 10.5.0.1. Assuming both routers had OSPF configured with network 10.0.0.0 0.255.255.255 area 0.0.0.1 Will a device on R1 E2 be able to ping a host on R2 E2? -- My feeble attempt at cleverness: ( and actual results noted ) [ I had to use loopbacks to simulate the multiple ethernet ports ] Assuming that there is no typo in your addressing scheme, I say you have a real mess here and that the answer is no. CL: this much was true. But this was a case of me being right, but for the wrong reason. See below. 1) you have the same subnet appearing on two different routers 10.0.5.0/whatever on R1E1 and R2E1 While OSPF can handle discontiguous subnets, I would be surprised if it can handle duplicate subnets with any aplomb CL: OSPF definitely did NOT like duplicate networks. Name a routing protocol that would? 2) on R1 the interface with the address of 10.0.5.2 has been placed into area 0.0.0.1. when the ip address for that interface is changed, without either reloading the router or performing a clear ip ospf process the interface with the address 10.5.0.1 will NOT have been placed into the OSPF process. CL: absolutely WRONG. CL: What really happens is that the routers in question never form adjacency because they both have the same router i.d. RID's must be unique. With the duplicate RID's, the routers do not form neighbor relationships. The LSA's aren't processed. A DR and a BDR are not formed. No routing information is propagated. End of story CL: my thanks to Kevin W. for his observation, which caused me to see the error of my initial QD scenario. The key piece I overlooked was the interface with the address of 10.6.0.1, which should have been the RID on router one. However, with the interface down, it was not. The next highest address - 10.5.0.2 became the RID. Trouble begins. CL: BTW, even after correcting the interface IP address, the problem continues. So, then, I have a further question for all of you. How do you correct this problem? Or rather, what are some things one might suppose would correct this problem. Does that work? Is there a "clear" command one can use? Or must one resort to blowing away the OSPF process, and then restoring it? Hhhm does this qualify as a Friday Folly? :- Chuck -Original Message- From: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED]] On Behalf Of Chuck Larrieu Sent: Wednesday, November 29, 2000 9:47 PM To: Chuck Larrieu; Howard C. Berkowitz; [EMAIL PROTECTED] Subject: RE: About OSPF and Loopback port OK, I have run a couple of QD's. I have some observations, but I will save them until a few of you have taken a stab at Howard's puzzle for yourselves. No fair touching routers first. You gotta test your knowledge and your reasoning. I'll share my results Thursday evening. Chuck -Original Message- From: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED]] On Behalf Of Chuck Larrieu Sent: Wednesday, November 29, 2000 8:47 PM To: Howard C. Berkowitz; [EMAIL PROTECTED] Subject: RE: About OSPF and Loopback port OK Howard. I'm not afraid to look foolish in front of everyone. --- Howard's scenario: Scenario 1 (R1 is initially misconfigured) R1 E0: 10.6.0.1 DOWN R2 E0: 10.1.0.1 UP/UP E1: 10.5.0.2 UP/UP E1: 10.5.0.2 UP/UP E2: 10.2.0.1 UP/UP E2: 10.2.0.2 UP/UP Admin discovers that R1 E1 is misconfigured and should have been 10.5.0.1. Our Heroine corrects that interface to 10.5.0.1. Assuming both routers had OSPF configured with network 10.0.0.0 0.255.255.255 area 0.0.0.1 Will a device on R1 E2 be able to ping a host on R2 E2? -- My feeble attempt at cleverness: Assuming that there is no typo in your addressing scheme, I say you have a real mess here and that the answer is no. 1) you have the same subnet appearing on two different routers 10.0.5.0/whatever on R1E1 and R2E1 While OSPF can handle discontiguous subnets, I would be surprised if it can handle duplicate subnets with any aplomb 2) on R1 the interface with the address of 10.0.5.2 has been placed into area 0.0.0.1. when the ip address for that interface is changed, without either reloading the router or performing a clear ip ospf process the interface with the address 10.5.0.1 will NOT have been placed into the OS
About OSPF and Loopback port
Does anyone here can explain to me, why me must use loopback port for OSPF configuration. Thank you for the answer for this stupid question. Thank you. moerdo. _ FAQ, list archives, and subscription info: http://www.groupstudy.com/list/cisco.html Report misconduct and Nondisclosure violations to [EMAIL PROTECTED]
RE: About OSPF and Loopback port
Use loopback ports in OSPF so you can set the Router OSPF ID, otherwise it will take the highest IP address. Jim Healis CCNP, CCDP Senior Network Administrator Virata -Original Message- From: Moerdo [mailto:[EMAIL PROTECTED]] Sent: Wednesday, November 29, 2000 8:26 AM To: [EMAIL PROTECTED] Subject:About OSPF and Loopback port Does anyone here can explain to me, why me must use loopback port for OSPF configuration. Thank you for the answer for this stupid question. Thank you. moerdo. _ FAQ, list archives, and subscription info: http://www.groupstudy.com/list/cisco.html Report misconduct and Nondisclosure violations to [EMAIL PROTECTED] _ FAQ, list archives, and subscription info: http://www.groupstudy.com/list/cisco.html Report misconduct and Nondisclosure violations to [EMAIL PROTECTED]
RE: About OSPF and Loopback port
At 10:45 AM -0800 11/29/2000, Healis, Jim wrote: Use loopback ports in OSPF so you can set the Router OSPF ID, otherwise it will take the highest IP address. Jim Healis CCNP, CCDP Senior Network Administrator Virata Unless there have been recent IOS changes (I'm really most current in 11-something), it's even more unpredictable than just the highest IP address: At the time of OSPF initialization, the router ID is: if there are multiple loopback interfaces, the highest IP address on any loopback (i.e., not highest loopback interface number) if there is a single loopback interface, use its address if there are no loopback interfaces, use the highest IP address on any active interface (i.e., if all interfaces are in shutdown, OSPF can't initialize. Using loopbacks avoids this because a loopback cannot be down.) A fiendish troubleshooting scenario: R1 comes up first, then R2. They share an Ethernet. Neither has any loopbacks. Scenario 1 (R1 is initially misconfigured) R1 E0: 10.6.0.1 DOWN R2 E0: 10.1.0.1 UP/UP E1: 10.5.0.2 UP/UP E1: 10.5.0.2 UP/UP E2: 10.2.0.1 UP/UP E2: 10.2.0.2 UP/UP Admin discovers that R1 E1 is misconfigured and should have been 10.5.0.1. Our Heroine corrects that interface to 10.5.0.1. Assuming both routers had OSPF configured with network 10.0.0.0 0.255.255.255 area 0.0.0.1 Will a device on R1 E2 be able to ping a host on R2 E2? -Original Message- From: Moerdo [mailto:[EMAIL PROTECTED]] Sent: Wednesday, November 29, 2000 8:26 AM To:[EMAIL PROTECTED] Subject: About OSPF and Loopback port Does anyone here can explain to me, why me must use loopback port for OSPF configuration. Thank you for the answer for this stupid question. Thank you. moerdo. _ FAQ, list archives, and subscription info: http://www.groupstudy.com/list/cisco.html Report misconduct and Nondisclosure violations to [EMAIL PROTECTED]
RE: About OSPF and Loopback port
OK Howard. I'm not afraid to look foolish in front of everyone. --- Howard's scenario: Scenario 1 (R1 is initially misconfigured) R1 E0: 10.6.0.1 DOWN R2 E0: 10.1.0.1 UP/UP E1: 10.5.0.2 UP/UP E1: 10.5.0.2 UP/UP E2: 10.2.0.1 UP/UP E2: 10.2.0.2 UP/UP Admin discovers that R1 E1 is misconfigured and should have been 10.5.0.1. Our Heroine corrects that interface to 10.5.0.1. Assuming both routers had OSPF configured with network 10.0.0.0 0.255.255.255 area 0.0.0.1 Will a device on R1 E2 be able to ping a host on R2 E2? -- My feeble attempt at cleverness: Assuming that there is no typo in your addressing scheme, I say you have a real mess here and that the answer is no. 1) you have the same subnet appearing on two different routers 10.0.5.0/whatever on R1E1 and R2E1 While OSPF can handle discontiguous subnets, I would be surprised if it can handle duplicate subnets with any aplomb 2) on R1 the interface with the address of 10.0.5.2 has been placed into area 0.0.0.1. when the ip address for that interface is changed, without either reloading the router or performing a clear ip ospf process the interface with the address 10.5.0.1 will NOT have been placed into the OSPF process. 3) I have not done this on my routers yet. I want to see if I am in the right ballpark so far. ( well, to be honest, as soon as I send this message I am setting up a scenario, upon which I will report subsequently ) Well, gang - am I wise or still a fool? Chuck -Original Message- From: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED]] On Behalf Of Howard C. Berkowitz Sent: Wednesday, November 29, 2000 12:37 PM To: [EMAIL PROTECTED] Subject:RE: About OSPF and Loopback port At 10:45 AM -0800 11/29/2000, Healis, Jim wrote: Use loopback ports in OSPF so you can set the Router OSPF ID, otherwise it will take the highest IP address. Jim Healis CCNP, CCDP Senior Network Administrator Virata Unless there have been recent IOS changes (I'm really most current in 11-something), it's even more unpredictable than just the highest IP address: At the time of OSPF initialization, the router ID is: if there are multiple loopback interfaces, the highest IP address on any loopback (i.e., not highest loopback interface number) if there is a single loopback interface, use its address if there are no loopback interfaces, use the highest IP address on any active interface (i.e., if all interfaces are in shutdown, OSPF can't initialize. Using loopbacks avoids this because a loopback cannot be down.) A fiendish troubleshooting scenario: R1 comes up first, then R2. They share an Ethernet. Neither has any loopbacks. Scenario 1 (R1 is initially misconfigured) R1 E0: 10.6.0.1 DOWN R2 E0: 10.1.0.1 UP/UP E1: 10.5.0.2 UP/UP E1: 10.5.0.2 UP/UP E2: 10.2.0.1 UP/UP E2: 10.2.0.2 UP/UP Admin discovers that R1 E1 is misconfigured and should have been 10.5.0.1. Our Heroine corrects that interface to 10.5.0.1. Assuming both routers had OSPF configured with network 10.0.0.0 0.255.255.255 area 0.0.0.1 Will a device on R1 E2 be able to ping a host on R2 E2? -Original Message- From: Moerdo [mailto:[EMAIL PROTECTED]] Sent: Wednesday, November 29, 2000 8:26 AM To:[EMAIL PROTECTED] Subject: About OSPF and Loopback port Does anyone here can explain to me, why me must use loopback port for OSPF configuration. Thank you for the answer for this stupid question. Thank you. moerdo. _ FAQ, list archives, and subscription info: http://www.groupstudy.com/list/cisco.html Report misconduct and Nondisclosure violations to [EMAIL PROTECTED] _ FAQ, list archives, and subscription info: http://www.groupstudy.com/list/cisco.html Report misconduct and Nondisclosure violations to [EMAIL PROTECTED]
RE: About OSPF and Loopback port
OK, I have run a couple of QD's. I have some observations, but I will save them until a few of you have taken a stab at Howard's puzzle for yourselves. No fair touching routers first. You gotta test your knowledge and your reasoning. I'll share my results Thursday evening. Chuck -Original Message- From: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED]] On Behalf Of Chuck Larrieu Sent: Wednesday, November 29, 2000 8:47 PM To: Howard C. Berkowitz; [EMAIL PROTECTED] Subject:RE: About OSPF and Loopback port OK Howard. I'm not afraid to look foolish in front of everyone. --- Howard's scenario: Scenario 1 (R1 is initially misconfigured) R1 E0: 10.6.0.1 DOWN R2 E0: 10.1.0.1 UP/UP E1: 10.5.0.2 UP/UP E1: 10.5.0.2 UP/UP E2: 10.2.0.1 UP/UP E2: 10.2.0.2 UP/UP Admin discovers that R1 E1 is misconfigured and should have been 10.5.0.1. Our Heroine corrects that interface to 10.5.0.1. Assuming both routers had OSPF configured with network 10.0.0.0 0.255.255.255 area 0.0.0.1 Will a device on R1 E2 be able to ping a host on R2 E2? -- My feeble attempt at cleverness: Assuming that there is no typo in your addressing scheme, I say you have a real mess here and that the answer is no. 1) you have the same subnet appearing on two different routers 10.0.5.0/whatever on R1E1 and R2E1 While OSPF can handle discontiguous subnets, I would be surprised if it can handle duplicate subnets with any aplomb 2) on R1 the interface with the address of 10.0.5.2 has been placed into area 0.0.0.1. when the ip address for that interface is changed, without either reloading the router or performing a clear ip ospf process the interface with the address 10.5.0.1 will NOT have been placed into the OSPF process. 3) I have not done this on my routers yet. I want to see if I am in the right ballpark so far. ( well, to be honest, as soon as I send this message I am setting up a scenario, upon which I will report subsequently ) Well, gang - am I wise or still a fool? Chuck -Original Message- From: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED]] On Behalf Of Howard C. Berkowitz Sent: Wednesday, November 29, 2000 12:37 PM To: [EMAIL PROTECTED] Subject:RE: About OSPF and Loopback port At 10:45 AM -0800 11/29/2000, Healis, Jim wrote: Use loopback ports in OSPF so you can set the Router OSPF ID, otherwise it will take the highest IP address. Jim Healis CCNP, CCDP Senior Network Administrator Virata Unless there have been recent IOS changes (I'm really most current in 11-something), it's even more unpredictable than just the highest IP address: At the time of OSPF initialization, the router ID is: if there are multiple loopback interfaces, the highest IP address on any loopback (i.e., not highest loopback interface number) if there is a single loopback interface, use its address if there are no loopback interfaces, use the highest IP address on any active interface (i.e., if all interfaces are in shutdown, OSPF can't initialize. Using loopbacks avoids this because a loopback cannot be down.) A fiendish troubleshooting scenario: R1 comes up first, then R2. They share an Ethernet. Neither has any loopbacks. Scenario 1 (R1 is initially misconfigured) R1 E0: 10.6.0.1 DOWN R2 E0: 10.1.0.1 UP/UP E1: 10.5.0.2 UP/UP E1: 10.5.0.2 UP/UP E2: 10.2.0.1 UP/UP E2: 10.2.0.2 UP/UP Admin discovers that R1 E1 is misconfigured and should have been 10.5.0.1. Our Heroine corrects that interface to 10.5.0.1. Assuming both routers had OSPF configured with network 10.0.0.0 0.255.255.255 area 0.0.0.1 Will a device on R1 E2 be able to ping a host on R2 E2? -Original Message- From: Moerdo [mailto:[EMAIL PROTECTED]] Sent: Wednesday, November 29, 2000 8:26 AM To:[EMAIL PROTECTED] Subject: About OSPF and Loopback port Does anyone here can explain to me, why me must use loopback port for OSPF configuration. Thank you for the answer for this stupid question. Thank you. moerdo. _ FAQ, list archives, and subscription info: http://www.groupstudy.com/list/cisco.html Report misconduct and Nondisclosure violations to [EMAIL PROTECTED] _ FAQ, list archives, and subscription info: http://www.groupstudy.com/list/cisco.html Report misconduct and Nondisclosure violations to [EMAIL PROTECTED] _ FAQ, list archives, and subscription info: http://www.groupstudy.com/list/cisco.html Report misconduct and Nondisclosure violations to [EMAIL PROTECTED]
