Re: OSPF Hub and Spoke [7:9268]
Hi Chuck, I feel like I have grasped some of OSPF, and understood without any doubt in my mind all but two of your comments.(It's possibly down to all but one after writing this post) The first one was: Do OSPF area numbers have to be unique. I've re-written this about 4 times while I tried to think it out and read through my books. What sort of flow of LSA's are there between the two discontiguous area's with identical area numbers. LSA type 1& 2 flooded between the two? or are they just treated as separate areas. I'm thinking the latter but changing my mind every second. The second was: Do virtual links have to be point to point? What I probably want here is the argument against my logic, or confirmation of it. Using: Area_3-area_2area_1--area_0 Area 2 has a virtual link to area 0. When I did this I noticed that the ABR between area two and one had all of it's interfaces in area 0, as well as the 'actual' area of each interface. (So a router with 3 interfaces had for instance, 3 interfaces in area 0, 2 interfaces in area 2 and 1 interface in area 1) Area 3 has a virtual link into the router above. As the router above *is now* within area 0, the virtual link ends there, and the router in area 3 now has a virtual link to Area 0 (And therefore now becomes part of the backbone area 0) So my answer would be, yes, virtual links have to be point to point. The first virtual link extends area 0. The second virtual link merely connects to the extended area 0. I need to press send quickly before I change it all again. Thanks for your time, Gaz ""Chuck Larrieu"" wrote in message [EMAIL PROTECTED]">news:[EMAIL PROTECTED]... > Agreed that there are few if any written materials that do a decent job of > explaining how OSPF works and why. Disclaimer - I have not read Tom Thomas' > book. But I have read a number of others, including the RFC. Plus my Lab > prep work. > > One problem is that most of us don't understand the difference between > routing and routing protocols. Routing protocols are one means, but only > means, of getting routes into the routing table. Routing forwards or drops > packets based on the contents of the routing table. > > Lack of understanding of how and why OSPF operates can lead to question like > "when there is a virtual link connecting a discontiguous area to the > backbone, does traffic travel across the transit area to the backbone, and > then back into the transit area?" > > Area_2area_1--area_0 > > If there is a virtual link from area 2 into the backbone, does traffic bound > for area 1 from area two have to go to area 0 first? The answer is two fold. > The area 2/area abr is now part of area 0. This allows area 2 to learn about > area 1 routes. And secondly, when a router receives a packet, it is the > routing process ( not the routing protocols ) that determine the forwarding. > > Another question - do OSPF area numbers have to be unique? Answer is no they > don't. in a multiple area network, as long as there is an area 0, all other > areas can have the same number. there is nothing in an LSA that identifies a > route with an area. A corollary of this is that in a single area network, > there does not have to be an area 0. Why? > > Got a good one. Do virtual links have to be point to point? > > Area_3-area_2area_1--area_0 > > You can create a virtual link from area 3 to area 2 and a second link from > area 2 to area 0 - it works. Good example of this in Slattery's book. > > Over time I have come to appreciate the Cert zone / Berkowitz approach, > which is to attain understanding of how things work. one might be surprised > at the number of people with very high level certification who can make, but > who don't know why it works. > > Chuck > > > -Original Message- > From: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED]] On Behalf Of John > Neiberger > Sent: Friday, June 22, 2001 7:32 AM > To: [EMAIL PROTECTED] > Subject: RE: OSPF Hub and Spoke [7:9268] > > Yep, you are correct. I see now that as long as non-zero areas receive > LSAs from a router configured as area zero, then the routing tables will > be built and all should be well. This makes more sense to me, anyway, > but some of what I've read made the waters murkier rather than clearer. > > How many times have you read "All interarea traffic must go through > area zero" ? I've read that in several places. It's almost true, but > not if you want to split hairs, especially since OSPF defines areas as > links, not routers. It should read that for loop-free routing to take > place, all non-zero areas must connect to area zero only. This is more > correct and doesn&#
RE: OSPF Hub and Spoke [7:9268]
>I don't know about the genius part. >:-) But, you've described the >initial scenario I was asking about. In my original post I wasn't >suggesting that this would be a good design by any means. I simply was >wondering if it was possible and how you'd configure it. > >In this particular case, the hub router is participating in 20 areas >which would cause it to be very busy, especially if the network wasn't >very stable. It would be better to extend area zero across the WAN >links to insulate the hub router from any instability in the outlying >areas. > >John You definitely are describing an "it depends" situation. Without considering other factors, the number of areas per physical router isn't a major performance consideration. The reason for restricting it is that in general, with increasing numbers of areas, you are more likely to need to do a SPF calculation simultaneously in more than one area. If the areas are highly stable, or have very few links in them, the effect of the SPF may be minimal. Another consideration is the degree to which inter-area routes propagate to other non-backbone areas. Interarea routes do not necessarily trigger the most intensive part of SPF, the Dijkstra. By "not necessarily," I don't know how Cisco's implementation actually reacts to receiving an inter-area LSA without any accompanying intra-area changes. You also need to consider the CPU power of the routers involved. The most areas I've configured were on 7000's, not all that fast a CPU box, typically with seven nonzero areas each. The nonzero areas were very stable optically wired campuses. Message Posted at: http://www.groupstudy.com/form/read.php?f=7&i=9541&t=9268 -- FAQ, list archives, and subscription info: http://www.groupstudy.com/list/cisco.html Report misconduct and Nondisclosure violations to [EMAIL PROTECTED]
RE: OSPF Hub and Spoke [7:9268]
thank you ,my learn-ed friends. it seems asif i am finally getting it >From: "John Neiberger" >Reply-To: "John Neiberger" >To: [EMAIL PROTECTED] >Subject: RE: OSPF Hub and Spoke [7:9268] >Date: Fri, 22 Jun 2001 10:46:15 -0400 > >I don't know about the genius part. >:-) But, you've described the >initial scenario I was asking about. In my original post I wasn't >suggesting that this would be a good design by any means. I simply was >wondering if it was possible and how you'd configure it. > >In this particular case, the hub router is participating in 20 areas >which would cause it to be very busy, especially if the network wasn't >very stable. It would be better to extend area zero across the WAN >links to insulate the hub router from any instability in the outlying >areas. > >John > > >>> "Stephen Skinner" 6/22/01 4:50:10 AM >>> >Guys, > >lets see how my ospf is going .. > >in this design goal i would have thought you would have done this > > >hub and spokeat the hub you have say 1 router (3620) with one >interface >and 20 sub-interfaces... > >you also have 20 totally stubby area`s which connect into the hub... > >config each stub as area 1 through 20 > >setup each sub int as per area`s 1-20 > >then setup the lan int as area 0 > >this way you have one router (hub) which is in area 0 and all the >other >area`s aswell > >problem solved .. > > >something tells me i`ve just either > >A got it right and am a genius >B completely missed the point and broken every rule of ospf > > >YOU DECIDE > >steve > > > >From: "Chuck Larrieu" > >Reply-To: "Chuck Larrieu" > >To: [EMAIL PROTECTED] > >Subject: RE: OSPF Hub and Spoke [7:9268] > >Date: Fri, 22 Jun 2001 01:15:31 -0400 > > > >John, this one's got me to thinking a little bit. Your kinda right but > > >kinda > >wrong. > > > >The areas are an OSPF structure, used for the building of the SPF >tables. > >It's not that inter area traffic has to go through a discreet area 0, >but > >that in OSPF in order for an area to learn about routes to another >area > >there has to be an area 0 router in between them. It does not matter >if > >there are a number of interfaces that are ABR's, or if there is a >discrete > >and pure area 0. > > > >With OSPF, all that matters is that the appropriate adjacencies are >formed, > >and that the LSA's are processed and that the OSPF database is >created. If > >all that occurs, OSPF routes will be placed into the routing tables. >As far > >as the router itself is concerned, routing is independent of the >routing > >protocols involved. > > > >I've fooled with this in the past. I'll have to do another Q&D lab to > > >gather > >some evidence, and post it here over the weekend. > > > >In the meantime, for those interested in some in-depth discussion of > >routing, Howard's white paper on Certification Zone is definitely >worth > >reading. I have not seen the likes of it in any other source, >including > >Doyle ( although it has been too long since I've read Doyle ) > > > >Chuck > > > > > >-Original Message- > >From:[EMAIL PROTECTED] [mailto:[EMAIL PROTECTED]] On Behalf >Of > >John > >Neiberger > >Sent:Thursday, June 21, 2001 6:55 AM > >To: [EMAIL PROTECTED] > >Subject: Re: OSPF Hub and Spoke [7:9268] > > > >Yes, I'm replying to myself. > > > >While doing some reading it occurred to me why *not* extending area 0 > > >across > >the WAN links should not work. In OSPF, unlike IS-IS, an area is >defined > >by > >links, not routers. The rule states that interarea traffic must go >through > >area 0. Well, if areas are defined by links, then this means that > >interarea > >traffic must at least go across one link that is defined as an area 0 >link. > > > >In a hub-and-spoke environment with a single hub router, it seems to >me > >that > >there just is no good way to use multiarea OSPF if you don't extend >area 0 > >across the WAN links. > > > >At least, that's the way it appears at the moment. > > > >John > > > >| I'm having trouble wrapping my brain around a specific scenario and >I > >| wanted to get your thoughts. Let's say we have a hub and spoke >network > >| with a single router as
RE: OSPF Hub and Spoke [7:9268]
Agreed that there are few if any written materials that do a decent job of explaining how OSPF works and why. Disclaimer - I have not read Tom Thomas' book. But I have read a number of others, including the RFC. Plus my Lab prep work. One problem is that most of us don't understand the difference between routing and routing protocols. Routing protocols are one means, but only means, of getting routes into the routing table. Routing forwards or drops packets based on the contents of the routing table. Lack of understanding of how and why OSPF operates can lead to question like "when there is a virtual link connecting a discontiguous area to the backbone, does traffic travel across the transit area to the backbone, and then back into the transit area?" Area_2area_1--area_0 If there is a virtual link from area 2 into the backbone, does traffic bound for area 1 from area two have to go to area 0 first? The answer is two fold. The area 2/area abr is now part of area 0. This allows area 2 to learn about area 1 routes. And secondly, when a router receives a packet, it is the routing process ( not the routing protocols ) that determine the forwarding. Another question - do OSPF area numbers have to be unique? Answer is no they don't. in a multiple area network, as long as there is an area 0, all other areas can have the same number. there is nothing in an LSA that identifies a route with an area. A corollary of this is that in a single area network, there does not have to be an area 0. Why? Got a good one. Do virtual links have to be point to point? Area_3-area_2area_1--area_0 You can create a virtual link from area 3 to area 2 and a second link from area 2 to area 0 - it works. Good example of this in Slattery's book. Over time I have come to appreciate the Cert zone / Berkowitz approach, which is to attain understanding of how things work. one might be surprised at the number of people with very high level certification who can make, but who don't know why it works. Chuck -Original Message- From: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED]] On Behalf Of John Neiberger Sent: Friday, June 22, 2001 7:32 AM To: [EMAIL PROTECTED] Subject: RE: OSPF Hub and Spoke [7:9268] Yep, you are correct. I see now that as long as non-zero areas receive LSAs from a router configured as area zero, then the routing tables will be built and all should be well. This makes more sense to me, anyway, but some of what I've read made the waters murkier rather than clearer. How many times have you read "All interarea traffic must go through area zero" ? I've read that in several places. It's almost true, but not if you want to split hairs, especially since OSPF defines areas as links, not routers. It should read that for loop-free routing to take place, all non-zero areas must connect to area zero only. This is more correct and doesn't imply that interarea traffic must cross an area zero link. Does that sounds about right? :-) >>> "Chuck Larrieu" 6/21/01 11:15:31 PM >>> John, this one's got me to thinking a little bit. Your kinda right but kinda wrong. The areas are an OSPF structure, used for the building of the SPF tables. It's not that inter area traffic has to go through a discreet area 0, but that in OSPF in order for an area to learn about routes to another area there has to be an area 0 router in between them. It does not matter if there are a number of interfaces that are ABR's, or if there is a discrete and pure area 0. With OSPF, all that matters is that the appropriate adjacencies are formed, and that the LSA's are processed and that the OSPF database is created. If all that occurs, OSPF routes will be placed into the routing tables. As far as the router itself is concerned, routing is independent of the routing protocols involved. I've fooled with this in the past. I'll have to do another Q&D lab to gather some evidence, and post it here over the weekend. In the meantime, for those interested in some in-depth discussion of routing, Howard's white paper on Certification Zone is definitely worth reading. I have not seen the likes of it in any other source, including Doyle ( although it has been too long since I've read Doyle ) Chuck -Original Message- From: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED]] On Behalf Of John Neiberger Sent: Thursday, June 21, 2001 6:55 AM To: [EMAIL PROTECTED] Subject:Re: OSPF Hub and Spoke [7:9268] Yes, I'm replying to myself. While doing some reading it occurred to me why *not* extending area 0 across the WAN links should not work. In OSPF, unlike IS-IS, an area is defined by links, not routers. The rule states that interarea traffic must go through area 0. Well, if areas are defined by links, then this means that interarea traffic must at least go across o
RE: OSPF Hub and Spoke [7:9268]
I don't know about the genius part. >:-) But, you've described the initial scenario I was asking about. In my original post I wasn't suggesting that this would be a good design by any means. I simply was wondering if it was possible and how you'd configure it. In this particular case, the hub router is participating in 20 areas which would cause it to be very busy, especially if the network wasn't very stable. It would be better to extend area zero across the WAN links to insulate the hub router from any instability in the outlying areas. John >>> "Stephen Skinner" 6/22/01 4:50:10 AM >>> Guys, lets see how my ospf is going .. in this design goal i would have thought you would have done this hub and spokeat the hub you have say 1 router (3620) with one interface and 20 sub-interfaces... you also have 20 totally stubby area`s which connect into the hub... config each stub as area 1 through 20 setup each sub int as per area`s 1-20 then setup the lan int as area 0 this way you have one router (hub) which is in area 0 and all the other area`s aswell problem solved .. something tells me i`ve just either A got it right and am a genius B completely missed the point and broken every rule of ospf YOU DECIDE steve >From: "Chuck Larrieu" >Reply-To: "Chuck Larrieu" >To: [EMAIL PROTECTED] >Subject: RE: OSPF Hub and Spoke [7:9268] >Date: Fri, 22 Jun 2001 01:15:31 -0400 > >John, this one's got me to thinking a little bit. Your kinda right but >kinda >wrong. > >The areas are an OSPF structure, used for the building of the SPF tables. >It's not that inter area traffic has to go through a discreet area 0, but >that in OSPF in order for an area to learn about routes to another area >there has to be an area 0 router in between them. It does not matter if >there are a number of interfaces that are ABR's, or if there is a discrete >and pure area 0. > >With OSPF, all that matters is that the appropriate adjacencies are formed, >and that the LSA's are processed and that the OSPF database is created. If >all that occurs, OSPF routes will be placed into the routing tables. As far >as the router itself is concerned, routing is independent of the routing >protocols involved. > >I've fooled with this in the past. I'll have to do another Q&D lab to >gather >some evidence, and post it here over the weekend. > >In the meantime, for those interested in some in-depth discussion of >routing, Howard's white paper on Certification Zone is definitely worth >reading. I have not seen the likes of it in any other source, including >Doyle ( although it has been too long since I've read Doyle ) > >Chuck > > >-Original Message- >From: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED]] On Behalf Of >John >Neiberger >Sent: Thursday, June 21, 2001 6:55 AM >To:[EMAIL PROTECTED] >Subject: Re: OSPF Hub and Spoke [7:9268] > >Yes, I'm replying to myself. > >While doing some reading it occurred to me why *not* extending area 0 >across >the WAN links should not work. In OSPF, unlike IS-IS, an area is defined >by >links, not routers. The rule states that interarea traffic must go through >area 0. Well, if areas are defined by links, then this means that >interarea >traffic must at least go across one link that is defined as an area 0 link. > >In a hub-and-spoke environment with a single hub router, it seems to me >that >there just is no good way to use multiarea OSPF if you don't extend area 0 >across the WAN links. > >At least, that's the way it appears at the moment. > >John > >| I'm having trouble wrapping my brain around a specific scenario and I >| wanted to get your thoughts. Let's say we have a hub and spoke network >| with a single router as the hub. There are five areas attached to the >| backbone. It seems that we would have to extend area 0 across the WAN >| links, but I'm wondering what would happen if we didn't. >| >| If we didn't, the backbone router would have no interfaces in area 0. >| I'm wondering if this would cause some major problems. I bet that it >| would but I'm having a hard time thinking through what actual problems >| might arise. Would this backbone router just "know" that it was area 0 >| because it has interfaces in multiple non-zero areas and hence behave >| correctly? >| >| One obvious problem is that the backbone router would be a member of >| every area and would thus be pretty busy if the network got to be very >| big. If we extended area 0 across the WAN link the backbone r
RE: OSPF Hub and Spoke [7:9268]
Yep, you are correct. I see now that as long as non-zero areas receive LSAs from a router configured as area zero, then the routing tables will be built and all should be well. This makes more sense to me, anyway, but some of what I've read made the waters murkier rather than clearer. How many times have you read "All interarea traffic must go through area zero" ? I've read that in several places. It's almost true, but not if you want to split hairs, especially since OSPF defines areas as links, not routers. It should read that for loop-free routing to take place, all non-zero areas must connect to area zero only. This is more correct and doesn't imply that interarea traffic must cross an area zero link. Does that sounds about right? :-) >>> "Chuck Larrieu" 6/21/01 11:15:31 PM >>> John, this one's got me to thinking a little bit. Your kinda right but kinda wrong. The areas are an OSPF structure, used for the building of the SPF tables. It's not that inter area traffic has to go through a discreet area 0, but that in OSPF in order for an area to learn about routes to another area there has to be an area 0 router in between them. It does not matter if there are a number of interfaces that are ABR's, or if there is a discrete and pure area 0. With OSPF, all that matters is that the appropriate adjacencies are formed, and that the LSA's are processed and that the OSPF database is created. If all that occurs, OSPF routes will be placed into the routing tables. As far as the router itself is concerned, routing is independent of the routing protocols involved. I've fooled with this in the past. I'll have to do another Q&D lab to gather some evidence, and post it here over the weekend. In the meantime, for those interested in some in-depth discussion of routing, Howard's white paper on Certification Zone is definitely worth reading. I have not seen the likes of it in any other source, including Doyle ( although it has been too long since I've read Doyle ) Chuck -Original Message- From: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED]] On Behalf Of John Neiberger Sent: Thursday, June 21, 2001 6:55 AM To: [EMAIL PROTECTED] Subject:Re: OSPF Hub and Spoke [7:9268] Yes, I'm replying to myself. While doing some reading it occurred to me why *not* extending area 0 across the WAN links should not work. In OSPF, unlike IS-IS, an area is defined by links, not routers. The rule states that interarea traffic must go through area 0. Well, if areas are defined by links, then this means that interarea traffic must at least go across one link that is defined as an area 0 link. In a hub-and-spoke environment with a single hub router, it seems to me that there just is no good way to use multiarea OSPF if you don't extend area 0 across the WAN links. At least, that's the way it appears at the moment. John | I'm having trouble wrapping my brain around a specific scenario and I | wanted to get your thoughts. Let's say we have a hub and spoke network | with a single router as the hub. There are five areas attached to the | backbone. It seems that we would have to extend area 0 across the WAN | links, but I'm wondering what would happen if we didn't. | | If we didn't, the backbone router would have no interfaces in area 0. | I'm wondering if this would cause some major problems. I bet that it | would but I'm having a hard time thinking through what actual problems | might arise. Would this backbone router just "know" that it was area 0 | because it has interfaces in multiple non-zero areas and hence behave | correctly? | | One obvious problem is that the backbone router would be a member of | every area and would thus be pretty busy if the network got to be very | big. If we extended area 0 across the WAN link the backbone router | would be protected from running SPF calculations everytime a remote area | had a link change. | | What other problems would arise? Would this even work at all? I don't | really have the tools to try it or I'd just attempt this chaos myself. | As you can guess, we run eigrp everywhere so I'm still clueless to some | of the workings of OSPF in a production environment. | | Regards, | John | | | | ___ Send a cool gift with your E-Card http://www.bluemountain.com/giftcenter/ Message Posted at: http://www.groupstudy.com/form/read.php?f=7&i=9516&t=9268 -- FAQ, list archives, and subscription info: http://www.groupstudy.com/list/cisco.html Report misconduct and Nondisclosure violations to [EMAIL PROTECTED]
RE: OSPF Hub and Spoke [7:9268]
Guys, lets see how my ospf is going .. in this design goal i would have thought you would have done this hub and spokeat the hub you have say 1 router (3620) with one interface and 20 sub-interfaces... you also have 20 totally stubby area`s which connect into the hub... config each stub as area 1 through 20 setup each sub int as per area`s 1-20 then setup the lan int as area 0 this way you have one router (hub) which is in area 0 and all the other area`s aswell problem solved .. something tells me i`ve just either A got it right and am a genius B completely missed the point and broken every rule of ospf YOU DECIDE steve >From: "Chuck Larrieu" >Reply-To: "Chuck Larrieu" >To: [EMAIL PROTECTED] >Subject: RE: OSPF Hub and Spoke [7:9268] >Date: Fri, 22 Jun 2001 01:15:31 -0400 > >John, this one's got me to thinking a little bit. Your kinda right but >kinda >wrong. > >The areas are an OSPF structure, used for the building of the SPF tables. >It's not that inter area traffic has to go through a discreet area 0, but >that in OSPF in order for an area to learn about routes to another area >there has to be an area 0 router in between them. It does not matter if >there are a number of interfaces that are ABR's, or if there is a discrete >and pure area 0. > >With OSPF, all that matters is that the appropriate adjacencies are formed, >and that the LSA's are processed and that the OSPF database is created. If >all that occurs, OSPF routes will be placed into the routing tables. As far >as the router itself is concerned, routing is independent of the routing >protocols involved. > >I've fooled with this in the past. I'll have to do another Q&D lab to >gather >some evidence, and post it here over the weekend. > >In the meantime, for those interested in some in-depth discussion of >routing, Howard's white paper on Certification Zone is definitely worth >reading. I have not seen the likes of it in any other source, including >Doyle ( although it has been too long since I've read Doyle ) > >Chuck > > >-Original Message- >From: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED]] On Behalf Of >John >Neiberger >Sent: Thursday, June 21, 2001 6:55 AM >To:[EMAIL PROTECTED] >Subject: Re: OSPF Hub and Spoke [7:9268] > >Yes, I'm replying to myself. > >While doing some reading it occurred to me why *not* extending area 0 >across >the WAN links should not work. In OSPF, unlike IS-IS, an area is defined >by >links, not routers. The rule states that interarea traffic must go through >area 0. Well, if areas are defined by links, then this means that >interarea >traffic must at least go across one link that is defined as an area 0 link. > >In a hub-and-spoke environment with a single hub router, it seems to me >that >there just is no good way to use multiarea OSPF if you don't extend area 0 >across the WAN links. > >At least, that's the way it appears at the moment. > >John > >| I'm having trouble wrapping my brain around a specific scenario and I >| wanted to get your thoughts. Let's say we have a hub and spoke network >| with a single router as the hub. There are five areas attached to the >| backbone. It seems that we would have to extend area 0 across the WAN >| links, but I'm wondering what would happen if we didn't. >| >| If we didn't, the backbone router would have no interfaces in area 0. >| I'm wondering if this would cause some major problems. I bet that it >| would but I'm having a hard time thinking through what actual problems >| might arise. Would this backbone router just "know" that it was area 0 >| because it has interfaces in multiple non-zero areas and hence behave >| correctly? >| >| One obvious problem is that the backbone router would be a member of >| every area and would thus be pretty busy if the network got to be very >| big. If we extended area 0 across the WAN link the backbone router >| would be protected from running SPF calculations everytime a remote area >| had a link change. >| >| What other problems would arise? Would this even work at all? I don't >| really have the tools to try it or I'd just attempt this chaos myself. >| As you can guess, we run eigrp everywhere so I'm still clueless to some >| of the workings of OSPF in a production environment. >| >| Regards, >| John >| >| >| >| >___ >Send a cool gift with your E-Card >http://www.bluemountain.com/giftcenter/ _ Get Your Private, Free E-mail from MSN Hotmail at http://www.hotmail.com. Message Posted at: http://www.groupstudy.com/form/read.php?f=7&i=9495&t=9268 -- FAQ, list archives, and subscription info: http://www.groupstudy.com/list/cisco.html Report misconduct and Nondisclosure violations to [EMAIL PROTECTED]
RE: OSPF Hub and Spoke [7:9268]
John, this one's got me to thinking a little bit. Your kinda right but kinda wrong. The areas are an OSPF structure, used for the building of the SPF tables. It's not that inter area traffic has to go through a discreet area 0, but that in OSPF in order for an area to learn about routes to another area there has to be an area 0 router in between them. It does not matter if there are a number of interfaces that are ABR's, or if there is a discrete and pure area 0. With OSPF, all that matters is that the appropriate adjacencies are formed, and that the LSA's are processed and that the OSPF database is created. If all that occurs, OSPF routes will be placed into the routing tables. As far as the router itself is concerned, routing is independent of the routing protocols involved. I've fooled with this in the past. I'll have to do another Q&D lab to gather some evidence, and post it here over the weekend. In the meantime, for those interested in some in-depth discussion of routing, Howard's white paper on Certification Zone is definitely worth reading. I have not seen the likes of it in any other source, including Doyle ( although it has been too long since I've read Doyle ) Chuck -Original Message- From: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED]] On Behalf Of John Neiberger Sent: Thursday, June 21, 2001 6:55 AM To: [EMAIL PROTECTED] Subject:Re: OSPF Hub and Spoke [7:9268] Yes, I'm replying to myself. While doing some reading it occurred to me why *not* extending area 0 across the WAN links should not work. In OSPF, unlike IS-IS, an area is defined by links, not routers. The rule states that interarea traffic must go through area 0. Well, if areas are defined by links, then this means that interarea traffic must at least go across one link that is defined as an area 0 link. In a hub-and-spoke environment with a single hub router, it seems to me that there just is no good way to use multiarea OSPF if you don't extend area 0 across the WAN links. At least, that's the way it appears at the moment. John | I'm having trouble wrapping my brain around a specific scenario and I | wanted to get your thoughts. Let's say we have a hub and spoke network | with a single router as the hub. There are five areas attached to the | backbone. It seems that we would have to extend area 0 across the WAN | links, but I'm wondering what would happen if we didn't. | | If we didn't, the backbone router would have no interfaces in area 0. | I'm wondering if this would cause some major problems. I bet that it | would but I'm having a hard time thinking through what actual problems | might arise. Would this backbone router just "know" that it was area 0 | because it has interfaces in multiple non-zero areas and hence behave | correctly? | | One obvious problem is that the backbone router would be a member of | every area and would thus be pretty busy if the network got to be very | big. If we extended area 0 across the WAN link the backbone router | would be protected from running SPF calculations everytime a remote area | had a link change. | | What other problems would arise? Would this even work at all? I don't | really have the tools to try it or I'd just attempt this chaos myself. | As you can guess, we run eigrp everywhere so I'm still clueless to some | of the workings of OSPF in a production environment. | | Regards, | John | | | | ___ Send a cool gift with your E-Card http://www.bluemountain.com/giftcenter/ Message Posted at: http://www.groupstudy.com/form/read.php?f=7&i=9474&t=9268 -- FAQ, list archives, and subscription info: http://www.groupstudy.com/list/cisco.html Report misconduct and Nondisclosure violations to [EMAIL PROTECTED]
Re: OSPF Hub and Spoke [7:9268]
A technique I like to use in hub-and-spoke networks is to declare a LAN interface as area 0.0.0.0. If I subsequently add a backup router, I'll also connect that to the same LAN. It's reasonable to connect TFTP, DHCP, etc., there as well, but not any application servers. If I do have application servers, I'll hang another LAN interface off the core router and declare them in their own area. Just considering backup, there's probably going to be server-to-server communications that doesn't belong in the backbone. But to answer the general question, yes, dynamically routed inter-area routing in OSPF must go through area 0.0.0.0. There are some proposals for alternatives in the IETF, but I'd say they are mostly at the talking stage. When I need to break OSPF hierarchy, it's most often for traffic engineering, and I'll set it up with a static route explicitly NOT redistributed into OSPF, and with an administrative distance lower than OSPF. I can always have OSPF through-the-backbone routing as a backup. Message Posted at: http://www.groupstudy.com/form/read.php?f=7&i=9445&t=9268 -- FAQ, list archives, and subscription info: http://www.groupstudy.com/list/cisco.html Report misconduct and Nondisclosure violations to [EMAIL PROTECTED]
Re: OSPF Hub and Spoke [7:9268]
Gareth, Go directly to the source! RFC 2328, sections 3.1 and 3.2 define area 0 as the backbone and how traffic flows in a multi-area network, respectively. Pamela At 04:53 PM 6/21/01 -0400, you wrote: >Im fairly sure I understand most of the rules regarding area zero, but have >heard a few people referring to "the area zero rule". Is there a fixed >definition of the area zero rule somewhere? > >Thanks, > >Gaz > >""John Neiberger"" wrote in message >[EMAIL PROTECTED]">news:[EMAIL PROTECTED]... > > In this scenario, with a small number of routers an area zero isn't > > really necessary. However, expand this to 100 routers in 20 or so > > groups and an area zero starts to make sense. However, in my original > > post I wasn't concerned with best practices. I was only curious as to > > how OSPF would behave if I were to try to configure it that way. > > > > I was trying to reconcile a couple of things. I've always read that in > > multiarea OSPF, all interarea traffic must go through area zero. If > > areas are defined by links and not routers, then does that mean all > > traffic must flow over a link defined as area zero? That was really the > > issue and Pamela answered that for me. > > > > This scenario occurred to me while reading the area zero rule and then > > thinking about a hub-and-spoke configuration with a single hub router. > > I would never suggest that someone actually configure the network that > > way, I simply was wondering how OSPF would behave in a hub and spoke > > network with no area zero configured. As it turns out, the loopback > > interface can be placed in area zero, thus fulfilling that requirement. > > > > John > > > > >>> "Chuck Larrieu" 6/21/01 10:28:12 AM >>> > > A couple of questions / thoughts > > > > In the scenario mentioned, is an area zero really necessary? I.e. why > > not > > throw all routers into a singe area, whatever it's name? In hub and > > spoke, > > all inter-spoke traffic will have to go through the hub anyway, no > > matter > > what the protocol. > > > > Another thing to keep in mind, is that OSPF does not in and of itself > > change > > the way routing works. When a router receives a packet, it checks the > > destination address, compares this to routes in the routing table, and > > if > > there is a match, forwards the packet out the appropriate interface. It > > does > > not say "hmmm, I have a directly connected interface that matches, but > > this > > is an OSPF router, therefore I will forward the packet to the backbone > > router first" :-> > > > > How's stuff, Pamela? > > > > Chuck > > > > > > -Original Message- > > From: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED]] On Behalf > > Of > > Pamela Forsyth > > Sent: Thursday, June 21, 2001 9:09 AM > > To: [EMAIL PROTECTED] > > Subject: Re: OSPF Hub and Spoke [7:9268] > > > > John, > > > > I just tried this out, and the newer IOS versions (after 11.2) *will* > > let > > you use a loopback interface as area 0 with different non-zero areas > > defined on the spokes. > > > > There is no reason for the traffic actually to travel over the area 0 > > link, > > but area 0 must be in the hub router for the inter-area LSAs to be > > advertised to the spoke routers. OSPF is just populating the IP > > routing > > table; it is not making forwarding decisions. The router in this > > instance > > will not try to send traffic over an extra link just because of an > > OSPF > > rule about backbones. ;-) > > > > Again, your mileage may vary, depending on IOS version. > > > > Pamela > > > > At 09:54 AM 6/21/01 -0400, you wrote: > > >Yes, I'm replying to myself. > > > > > >While doing some reading it occurred to me why *not* extending area 0 > > across > > >the WAN links should not work. In OSPF, unlike IS-IS, an area is > > defined > > by > > >links, not routers. The rule states that interarea traffic must go > > through > > >area 0. Well, if areas are defined by links, then this means that > > interarea > > >traffic must at least go across one link that is defined as an area 0 > > link. > > > > > >In a hub-and-spoke environment with a single hub router, it seems to > > me > > that > > >there just is no good way to use multiarea OSPF if you don't extend > > area 0 > > >across the WAN links. > > > > > >At least, that's the way it appears at the moment. > > > > > >John Message Posted at: http://www.groupstudy.com/form/read.php?f=7&i=9431&t=9268 -- FAQ, list archives, and subscription info: http://www.groupstudy.com/list/cisco.html Report misconduct and Nondisclosure violations to [EMAIL PROTECTED]
Re: OSPF Hub and Spoke [7:9268]
I used that phrase to refer to the fact that non-backbone OSPF areas must use the backbone to communicate; they cannot communicate directly to each other. To save myself some typing--after saying the same thing several times--I used shorthand and referred to this as the area zero rule. There's probably another term for this aspect of OSPF routing but if there is, it's slipping my mind at the moment. >>> "Gareth Hinton" 6/21/01 2:53:35 PM >>> Im fairly sure I understand most of the rules regarding area zero, but have heard a few people referring to "the area zero rule". Is there a fixed definition of the area zero rule somewhere? Thanks, Gaz ""John Neiberger"" wrote in message [EMAIL PROTECTED]">news:[EMAIL PROTECTED]... > In this scenario, with a small number of routers an area zero isn't > really necessary. However, expand this to 100 routers in 20 or so > groups and an area zero starts to make sense. However, in my original > post I wasn't concerned with best practices. I was only curious as to > how OSPF would behave if I were to try to configure it that way. > > I was trying to reconcile a couple of things. I've always read that in > multiarea OSPF, all interarea traffic must go through area zero. If > areas are defined by links and not routers, then does that mean all > traffic must flow over a link defined as area zero? That was really the > issue and Pamela answered that for me. > > This scenario occurred to me while reading the area zero rule and then > thinking about a hub-and-spoke configuration with a single hub router. > I would never suggest that someone actually configure the network that > way, I simply was wondering how OSPF would behave in a hub and spoke > network with no area zero configured. As it turns out, the loopback > interface can be placed in area zero, thus fulfilling that requirement. > > John > > >>> "Chuck Larrieu" 6/21/01 10:28:12 AM >>> > A couple of questions / thoughts > > In the scenario mentioned, is an area zero really necessary? I.e. why > not > throw all routers into a singe area, whatever it's name? In hub and > spoke, > all inter-spoke traffic will have to go through the hub anyway, no > matter > what the protocol. > > Another thing to keep in mind, is that OSPF does not in and of itself > change > the way routing works. When a router receives a packet, it checks the > destination address, compares this to routes in the routing table, and > if > there is a match, forwards the packet out the appropriate interface. It > does > not say "hmmm, I have a directly connected interface that matches, but > this > is an OSPF router, therefore I will forward the packet to the backbone > router first" :-> > > How's stuff, Pamela? > > Chuck > > > -Original Message- > From: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED]] On Behalf > Of > Pamela Forsyth > Sent: Thursday, June 21, 2001 9:09 AM > To: [EMAIL PROTECTED] > Subject: Re: OSPF Hub and Spoke [7:9268] > > John, > > I just tried this out, and the newer IOS versions (after 11.2) *will* > let > you use a loopback interface as area 0 with different non-zero areas > defined on the spokes. > > There is no reason for the traffic actually to travel over the area 0 > link, > but area 0 must be in the hub router for the inter-area LSAs to be > advertised to the spoke routers. OSPF is just populating the IP > routing > table; it is not making forwarding decisions. The router in this > instance > will not try to send traffic over an extra link just because of an > OSPF > rule about backbones. ;-) > > Again, your mileage may vary, depending on IOS version. > > Pamela > > At 09:54 AM 6/21/01 -0400, you wrote: > >Yes, I'm replying to myself. > > > >While doing some reading it occurred to me why *not* extending area 0 > across > >the WAN links should not work. In OSPF, unlike IS-IS, an area is > defined > by > >links, not routers. The rule states that interarea traffic must go > through > >area 0. Well, if areas are defined by links, then this means that > interarea > >traffic must at least go across one link that is defined as an area 0 > link. > > > >In a hub-and-spoke environment with a single hub router, it seems to > me > that > >there just is no good way to use multiarea OSPF if you don't extend > area 0 > >across the WAN links. > > > >At least, that's the way it appears at the moment. > > > >John > > > >| I'm having trouble wrapping my brain around a specific scenario and &g
Re: OSPF Hub and Spoke [7:9268]
Im fairly sure I understand most of the rules regarding area zero, but have heard a few people referring to "the area zero rule". Is there a fixed definition of the area zero rule somewhere? Thanks, Gaz ""John Neiberger"" wrote in message [EMAIL PROTECTED]">news:[EMAIL PROTECTED]... > In this scenario, with a small number of routers an area zero isn't > really necessary. However, expand this to 100 routers in 20 or so > groups and an area zero starts to make sense. However, in my original > post I wasn't concerned with best practices. I was only curious as to > how OSPF would behave if I were to try to configure it that way. > > I was trying to reconcile a couple of things. I've always read that in > multiarea OSPF, all interarea traffic must go through area zero. If > areas are defined by links and not routers, then does that mean all > traffic must flow over a link defined as area zero? That was really the > issue and Pamela answered that for me. > > This scenario occurred to me while reading the area zero rule and then > thinking about a hub-and-spoke configuration with a single hub router. > I would never suggest that someone actually configure the network that > way, I simply was wondering how OSPF would behave in a hub and spoke > network with no area zero configured. As it turns out, the loopback > interface can be placed in area zero, thus fulfilling that requirement. > > John > > >>> "Chuck Larrieu" 6/21/01 10:28:12 AM >>> > A couple of questions / thoughts > > In the scenario mentioned, is an area zero really necessary? I.e. why > not > throw all routers into a singe area, whatever it's name? In hub and > spoke, > all inter-spoke traffic will have to go through the hub anyway, no > matter > what the protocol. > > Another thing to keep in mind, is that OSPF does not in and of itself > change > the way routing works. When a router receives a packet, it checks the > destination address, compares this to routes in the routing table, and > if > there is a match, forwards the packet out the appropriate interface. It > does > not say "hmmm, I have a directly connected interface that matches, but > this > is an OSPF router, therefore I will forward the packet to the backbone > router first" :-> > > How's stuff, Pamela? > > Chuck > > > -Original Message- > From: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED]] On Behalf > Of > Pamela Forsyth > Sent: Thursday, June 21, 2001 9:09 AM > To: [EMAIL PROTECTED] > Subject: Re: OSPF Hub and Spoke [7:9268] > > John, > > I just tried this out, and the newer IOS versions (after 11.2) *will* > let > you use a loopback interface as area 0 with different non-zero areas > defined on the spokes. > > There is no reason for the traffic actually to travel over the area 0 > link, > but area 0 must be in the hub router for the inter-area LSAs to be > advertised to the spoke routers. OSPF is just populating the IP > routing > table; it is not making forwarding decisions. The router in this > instance > will not try to send traffic over an extra link just because of an > OSPF > rule about backbones. ;-) > > Again, your mileage may vary, depending on IOS version. > > Pamela > > At 09:54 AM 6/21/01 -0400, you wrote: > >Yes, I'm replying to myself. > > > >While doing some reading it occurred to me why *not* extending area 0 > across > >the WAN links should not work. In OSPF, unlike IS-IS, an area is > defined > by > >links, not routers. The rule states that interarea traffic must go > through > >area 0. Well, if areas are defined by links, then this means that > interarea > >traffic must at least go across one link that is defined as an area 0 > link. > > > >In a hub-and-spoke environment with a single hub router, it seems to > me > that > >there just is no good way to use multiarea OSPF if you don't extend > area 0 > >across the WAN links. > > > >At least, that's the way it appears at the moment. > > > >John > > > >| I'm having trouble wrapping my brain around a specific scenario and > I > >| wanted to get your thoughts. Let's say we have a hub and spoke > network > >| with a single router as the hub. There are five areas attached to > the > >| backbone. It seems that we would have to extend area 0 across the > WAN > >| links, but I'm wondering what would happen if we didn't. > >| > >| If we didn't, the backbone router would have no interfaces in area > 0. > >| I'm wondering if
RE: OSPF Hub and Spoke [7:9268]
A lot of books recommend a small but high-speed LAN at the hub site as the area 0. This may not apply in your case, but might help you think of other options. Caveat: This may be a recommendation that comes from Cisco who wants to sell more routers!? ;-) Priscilla At 12:52 PM 6/21/01, John Neiberger wrote: >In this scenario, with a small number of routers an area zero isn't >really necessary. However, expand this to 100 routers in 20 or so >groups and an area zero starts to make sense. However, in my original >post I wasn't concerned with best practices. I was only curious as to >how OSPF would behave if I were to try to configure it that way. > >I was trying to reconcile a couple of things. I've always read that in >multiarea OSPF, all interarea traffic must go through area zero. If >areas are defined by links and not routers, then does that mean all >traffic must flow over a link defined as area zero? That was really the >issue and Pamela answered that for me. > >This scenario occurred to me while reading the area zero rule and then >thinking about a hub-and-spoke configuration with a single hub router. >I would never suggest that someone actually configure the network that >way, I simply was wondering how OSPF would behave in a hub and spoke >network with no area zero configured. As it turns out, the loopback >interface can be placed in area zero, thus fulfilling that requirement. > >John > > >>> "Chuck Larrieu" 6/21/01 10:28:12 AM >>> >A couple of questions / thoughts > >In the scenario mentioned, is an area zero really necessary? I.e. why >not >throw all routers into a singe area, whatever it's name? In hub and >spoke, >all inter-spoke traffic will have to go through the hub anyway, no >matter >what the protocol. > >Another thing to keep in mind, is that OSPF does not in and of itself >change >the way routing works. When a router receives a packet, it checks the >destination address, compares this to routes in the routing table, and >if >there is a match, forwards the packet out the appropriate interface. It >does >not say "hmmm, I have a directly connected interface that matches, but >this >is an OSPF router, therefore I will forward the packet to the backbone >router first" :-> > >How's stuff, Pamela? > >Chuck > > >-----Original Message- >From: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED]] On Behalf >Of >Pamela Forsyth >Sent: Thursday, June 21, 2001 9:09 AM >To: [EMAIL PROTECTED] >Subject:Re: OSPF Hub and Spoke [7:9268] > >John, > >I just tried this out, and the newer IOS versions (after 11.2) *will* >let >you use a loopback interface as area 0 with different non-zero areas >defined on the spokes. > >There is no reason for the traffic actually to travel over the area 0 >link, >but area 0 must be in the hub router for the inter-area LSAs to be >advertised to the spoke routers. OSPF is just populating the IP >routing >table; it is not making forwarding decisions. The router in this >instance >will not try to send traffic over an extra link just because of an >OSPF >rule about backbones. ;-) > >Again, your mileage may vary, depending on IOS version. > >Pamela > >At 09:54 AM 6/21/01 -0400, you wrote: > >Yes, I'm replying to myself. > > > >While doing some reading it occurred to me why *not* extending area 0 >across > >the WAN links should not work. In OSPF, unlike IS-IS, an area is >defined >by > >links, not routers. The rule states that interarea traffic must go >through > >area 0. Well, if areas are defined by links, then this means that >interarea > >traffic must at least go across one link that is defined as an area 0 >link. > > > >In a hub-and-spoke environment with a single hub router, it seems to >me >that > >there just is no good way to use multiarea OSPF if you don't extend >area 0 > >across the WAN links. > > > >At least, that's the way it appears at the moment. > > > >John > > > >| I'm having trouble wrapping my brain around a specific scenario and >I > >| wanted to get your thoughts. Let's say we have a hub and spoke >network > >| with a single router as the hub. There are five areas attached to >the > >| backbone. It seems that we would have to extend area 0 across the >WAN > >| links, but I'm wondering what would happen if we didn't. > >| > >| If we didn't, the backbone router would have no interfaces in area >0. > >| I'm wondering if this would cause some major problems. I bet that >it > >| would but I
RE: OSPF Hub and Spoke [7:9268]
In this scenario, with a small number of routers an area zero isn't really necessary. However, expand this to 100 routers in 20 or so groups and an area zero starts to make sense. However, in my original post I wasn't concerned with best practices. I was only curious as to how OSPF would behave if I were to try to configure it that way. I was trying to reconcile a couple of things. I've always read that in multiarea OSPF, all interarea traffic must go through area zero. If areas are defined by links and not routers, then does that mean all traffic must flow over a link defined as area zero? That was really the issue and Pamela answered that for me. This scenario occurred to me while reading the area zero rule and then thinking about a hub-and-spoke configuration with a single hub router. I would never suggest that someone actually configure the network that way, I simply was wondering how OSPF would behave in a hub and spoke network with no area zero configured. As it turns out, the loopback interface can be placed in area zero, thus fulfilling that requirement. John >>> "Chuck Larrieu" 6/21/01 10:28:12 AM >>> A couple of questions / thoughts In the scenario mentioned, is an area zero really necessary? I.e. why not throw all routers into a singe area, whatever it's name? In hub and spoke, all inter-spoke traffic will have to go through the hub anyway, no matter what the protocol. Another thing to keep in mind, is that OSPF does not in and of itself change the way routing works. When a router receives a packet, it checks the destination address, compares this to routes in the routing table, and if there is a match, forwards the packet out the appropriate interface. It does not say "hmmm, I have a directly connected interface that matches, but this is an OSPF router, therefore I will forward the packet to the backbone router first" :-> How's stuff, Pamela? Chuck -Original Message- From: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED]] On Behalf Of Pamela Forsyth Sent: Thursday, June 21, 2001 9:09 AM To: [EMAIL PROTECTED] Subject:Re: OSPF Hub and Spoke [7:9268] John, I just tried this out, and the newer IOS versions (after 11.2) *will* let you use a loopback interface as area 0 with different non-zero areas defined on the spokes. There is no reason for the traffic actually to travel over the area 0 link, but area 0 must be in the hub router for the inter-area LSAs to be advertised to the spoke routers. OSPF is just populating the IP routing table; it is not making forwarding decisions. The router in this instance will not try to send traffic over an extra link just because of an OSPF rule about backbones. ;-) Again, your mileage may vary, depending on IOS version. Pamela At 09:54 AM 6/21/01 -0400, you wrote: >Yes, I'm replying to myself. > >While doing some reading it occurred to me why *not* extending area 0 across >the WAN links should not work. In OSPF, unlike IS-IS, an area is defined by >links, not routers. The rule states that interarea traffic must go through >area 0. Well, if areas are defined by links, then this means that interarea >traffic must at least go across one link that is defined as an area 0 link. > >In a hub-and-spoke environment with a single hub router, it seems to me that >there just is no good way to use multiarea OSPF if you don't extend area 0 >across the WAN links. > >At least, that's the way it appears at the moment. > >John > >| I'm having trouble wrapping my brain around a specific scenario and I >| wanted to get your thoughts. Let's say we have a hub and spoke network >| with a single router as the hub. There are five areas attached to the >| backbone. It seems that we would have to extend area 0 across the WAN >| links, but I'm wondering what would happen if we didn't. >| >| If we didn't, the backbone router would have no interfaces in area 0. >| I'm wondering if this would cause some major problems. I bet that it >| would but I'm having a hard time thinking through what actual problems >| might arise. Would this backbone router just "know" that it was area 0 >| because it has interfaces in multiple non-zero areas and hence behave >| correctly? >| >| One obvious problem is that the backbone router would be a member of >| every area and would thus be pretty busy if the network got to be very >| big. If we extended area 0 across the WAN link the backbone router >| would be protected from running SPF calculations everytime a remote area >| had a link change. >| >| What other problems would arise? Would this even work at all? I don't >| really have the tools to try it or I'd just attempt this chaos myself. >| As you can guess, we run eigrp everywhere so I&
RE: OSPF Hub and Spoke [7:9268]
A couple of questions / thoughts In the scenario mentioned, is an area zero really necessary? I.e. why not throw all routers into a singe area, whatever it's name? In hub and spoke, all inter-spoke traffic will have to go through the hub anyway, no matter what the protocol. Another thing to keep in mind, is that OSPF does not in and of itself change the way routing works. When a router receives a packet, it checks the destination address, compares this to routes in the routing table, and if there is a match, forwards the packet out the appropriate interface. It does not say "hmmm, I have a directly connected interface that matches, but this is an OSPF router, therefore I will forward the packet to the backbone router first" :-> How's stuff, Pamela? Chuck -Original Message- From: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED]] On Behalf Of Pamela Forsyth Sent: Thursday, June 21, 2001 9:09 AM To: [EMAIL PROTECTED] Subject: Re: OSPF Hub and Spoke [7:9268] John, I just tried this out, and the newer IOS versions (after 11.2) *will* let you use a loopback interface as area 0 with different non-zero areas defined on the spokes. There is no reason for the traffic actually to travel over the area 0 link, but area 0 must be in the hub router for the inter-area LSAs to be advertised to the spoke routers. OSPF is just populating the IP routing table; it is not making forwarding decisions. The router in this instance will not try to send traffic over an extra link just because of an OSPF rule about backbones. ;-) Again, your mileage may vary, depending on IOS version. Pamela At 09:54 AM 6/21/01 -0400, you wrote: >Yes, I'm replying to myself. > >While doing some reading it occurred to me why *not* extending area 0 across >the WAN links should not work. In OSPF, unlike IS-IS, an area is defined by >links, not routers. The rule states that interarea traffic must go through >area 0. Well, if areas are defined by links, then this means that interarea >traffic must at least go across one link that is defined as an area 0 link. > >In a hub-and-spoke environment with a single hub router, it seems to me that >there just is no good way to use multiarea OSPF if you don't extend area 0 >across the WAN links. > >At least, that's the way it appears at the moment. > >John > >| I'm having trouble wrapping my brain around a specific scenario and I >| wanted to get your thoughts. Let's say we have a hub and spoke network >| with a single router as the hub. There are five areas attached to the >| backbone. It seems that we would have to extend area 0 across the WAN >| links, but I'm wondering what would happen if we didn't. >| >| If we didn't, the backbone router would have no interfaces in area 0. >| I'm wondering if this would cause some major problems. I bet that it >| would but I'm having a hard time thinking through what actual problems >| might arise. Would this backbone router just "know" that it was area 0 >| because it has interfaces in multiple non-zero areas and hence behave >| correctly? >| >| One obvious problem is that the backbone router would be a member of >| every area and would thus be pretty busy if the network got to be very >| big. If we extended area 0 across the WAN link the backbone router >| would be protected from running SPF calculations everytime a remote area >| had a link change. >| >| What other problems would arise? Would this even work at all? I don't >| really have the tools to try it or I'd just attempt this chaos myself. >| As you can guess, we run eigrp everywhere so I'm still clueless to some >| of the workings of OSPF in a production environment. Message Posted at: http://www.groupstudy.com/form/read.php?f=7&i=9374&t=9268 -- FAQ, list archives, and subscription info: http://www.groupstudy.com/list/cisco.html Report misconduct and Nondisclosure violations to [EMAIL PROTECTED]
Re: OSPF Hub and Spoke [7:9268]
Thank you for clarifying that. I was having some difficulty reconciling this and I was curious about how it might behave in a production environment. I doubt I'd ever configure a network like this, I just wondered if it was even possible to configure. The way our network is designed, if we were running OSPF and I chose not to extend area 0 over the WAN links, the hub router would be participating in 24 areas. I'm guessing that's not a good idea. :-) I'm going to have to unpack another router so I can play with this at home. I was hoping not to have to bring out the 4000 series router, it's so loud! Thanks, John >>> "Pamela Forsyth" 6/21/01 10:08:39 AM >>> John, I just tried this out, and the newer IOS versions (after 11.2) *will* let you use a loopback interface as area 0 with different non-zero areas defined on the spokes. There is no reason for the traffic actually to travel over the area 0 link, but area 0 must be in the hub router for the inter-area LSAs to be advertised to the spoke routers. OSPF is just populating the IP routing table; it is not making forwarding decisions. The router in this instance will not try to send traffic over an extra link just because of an OSPF rule about backbones. ;-) Again, your mileage may vary, depending on IOS version. Pamela At 09:54 AM 6/21/01 -0400, you wrote: >Yes, I'm replying to myself. > >While doing some reading it occurred to me why *not* extending area 0 across >the WAN links should not work. In OSPF, unlike IS-IS, an area is defined by >links, not routers. The rule states that interarea traffic must go through >area 0. Well, if areas are defined by links, then this means that interarea >traffic must at least go across one link that is defined as an area 0 link. > >In a hub-and-spoke environment with a single hub router, it seems to me that >there just is no good way to use multiarea OSPF if you don't extend area 0 >across the WAN links. > >At least, that's the way it appears at the moment. > >John > >| I'm having trouble wrapping my brain around a specific scenario and I >| wanted to get your thoughts. Let's say we have a hub and spoke network >| with a single router as the hub. There are five areas attached to the >| backbone. It seems that we would have to extend area 0 across the WAN >| links, but I'm wondering what would happen if we didn't. >| >| If we didn't, the backbone router would have no interfaces in area 0. >| I'm wondering if this would cause some major problems. I bet that it >| would but I'm having a hard time thinking through what actual problems >| might arise. Would this backbone router just "know" that it was area 0 >| because it has interfaces in multiple non-zero areas and hence behave >| correctly? >| >| One obvious problem is that the backbone router would be a member of >| every area and would thus be pretty busy if the network got to be very >| big. If we extended area 0 across the WAN link the backbone router >| would be protected from running SPF calculations everytime a remote area >| had a link change. >| >| What other problems would arise? Would this even work at all? I don't >| really have the tools to try it or I'd just attempt this chaos myself. >| As you can guess, we run eigrp everywhere so I'm still clueless to some >| of the workings of OSPF in a production environment. Message Posted at: http://www.groupstudy.com/form/read.php?f=7&i=9375&t=9268 -- FAQ, list archives, and subscription info: http://www.groupstudy.com/list/cisco.html Report misconduct and Nondisclosure violations to [EMAIL PROTECTED]
Re: OSPF Hub and Spoke [7:9268]
As far as I can see, both scenarios would work, as long as you have an area 0 defined somewhere. If you put the links into individual area, then you would at least have to have area 0 on the ethernet or a loopback on the hub router. A router does not implicitly assume the role of backbone router through having multiple areas connected unless one of them is area zero. If for example you had one router connected to area 1 and area 2, the router would maintain a database for both areas, but there would be no type 3 LSA's or inter area routes in either database. So other area 1 routers , for instance, would have no knowledge of area 2 routes and vice-versa. By definition, all inter-area routes must go via the backbone. No backbone, no inter area routes. I have been tripped up by this in the lab. While the above would work, it would probably not be advisable as you have one router maintaining 5 link state databases. I agree with the others that the better solution (unless your hub router is madly powerfull in comparison to the spokes) is to extend area 0 across the wan links. As for extending area 0 out onto the spoke ethernets, you can extend area 0 as far as you like, only being limited by the number of participating routers in an area, I forget the numbers. Message Posted at: http://www.groupstudy.com/form/read.php?f=7&i=9367&t=9268 -- FAQ, list archives, and subscription info: http://www.groupstudy.com/list/cisco.html Report misconduct and Nondisclosure violations to [EMAIL PROTECTED]
Re: OSPF Hub and Spoke [7:9268]
John, I just tried this out, and the newer IOS versions (after 11.2) *will* let you use a loopback interface as area 0 with different non-zero areas defined on the spokes. There is no reason for the traffic actually to travel over the area 0 link, but area 0 must be in the hub router for the inter-area LSAs to be advertised to the spoke routers. OSPF is just populating the IP routing table; it is not making forwarding decisions. The router in this instance will not try to send traffic over an extra link just because of an OSPF rule about backbones. ;-) Again, your mileage may vary, depending on IOS version. Pamela At 09:54 AM 6/21/01 -0400, you wrote: >Yes, I'm replying to myself. > >While doing some reading it occurred to me why *not* extending area 0 across >the WAN links should not work. In OSPF, unlike IS-IS, an area is defined by >links, not routers. The rule states that interarea traffic must go through >area 0. Well, if areas are defined by links, then this means that interarea >traffic must at least go across one link that is defined as an area 0 link. > >In a hub-and-spoke environment with a single hub router, it seems to me that >there just is no good way to use multiarea OSPF if you don't extend area 0 >across the WAN links. > >At least, that's the way it appears at the moment. > >John > >| I'm having trouble wrapping my brain around a specific scenario and I >| wanted to get your thoughts. Let's say we have a hub and spoke network >| with a single router as the hub. There are five areas attached to the >| backbone. It seems that we would have to extend area 0 across the WAN >| links, but I'm wondering what would happen if we didn't. >| >| If we didn't, the backbone router would have no interfaces in area 0. >| I'm wondering if this would cause some major problems. I bet that it >| would but I'm having a hard time thinking through what actual problems >| might arise. Would this backbone router just "know" that it was area 0 >| because it has interfaces in multiple non-zero areas and hence behave >| correctly? >| >| One obvious problem is that the backbone router would be a member of >| every area and would thus be pretty busy if the network got to be very >| big. If we extended area 0 across the WAN link the backbone router >| would be protected from running SPF calculations everytime a remote area >| had a link change. >| >| What other problems would arise? Would this even work at all? I don't >| really have the tools to try it or I'd just attempt this chaos myself. >| As you can guess, we run eigrp everywhere so I'm still clueless to some >| of the workings of OSPF in a production environment. Message Posted at: http://www.groupstudy.com/form/read.php?f=7&i=9365&t=9268 -- FAQ, list archives, and subscription info: http://www.groupstudy.com/list/cisco.html Report misconduct and Nondisclosure violations to [EMAIL PROTECTED]
Re: OSPF Hub and Spoke [7:9268]
Yes, I'm replying to myself. While doing some reading it occurred to me why *not* extending area 0 across the WAN links should not work. In OSPF, unlike IS-IS, an area is defined by links, not routers. The rule states that interarea traffic must go through area 0. Well, if areas are defined by links, then this means that interarea traffic must at least go across one link that is defined as an area 0 link. In a hub-and-spoke environment with a single hub router, it seems to me that there just is no good way to use multiarea OSPF if you don't extend area 0 across the WAN links. At least, that's the way it appears at the moment. John | I'm having trouble wrapping my brain around a specific scenario and I | wanted to get your thoughts. Let's say we have a hub and spoke network | with a single router as the hub. There are five areas attached to the | backbone. It seems that we would have to extend area 0 across the WAN | links, but I'm wondering what would happen if we didn't. | | If we didn't, the backbone router would have no interfaces in area 0. | I'm wondering if this would cause some major problems. I bet that it | would but I'm having a hard time thinking through what actual problems | might arise. Would this backbone router just "know" that it was area 0 | because it has interfaces in multiple non-zero areas and hence behave | correctly? | | One obvious problem is that the backbone router would be a member of | every area and would thus be pretty busy if the network got to be very | big. If we extended area 0 across the WAN link the backbone router | would be protected from running SPF calculations everytime a remote area | had a link change. | | What other problems would arise? Would this even work at all? I don't | really have the tools to try it or I'd just attempt this chaos myself. | As you can guess, we run eigrp everywhere so I'm still clueless to some | of the workings of OSPF in a production environment. | | Regards, | John | | | | ___ Send a cool gift with your E-Card http://www.bluemountain.com/giftcenter/ Message Posted at: http://www.groupstudy.com/form/read.php?f=7&i=9298&t=9268 -- FAQ, list archives, and subscription info: http://www.groupstudy.com/list/cisco.html Report misconduct and Nondisclosure violations to [EMAIL PROTECTED]
Re: OSPF Hub and Spoke [7:9268]
think about conguring virtual link(s) on your backbone router with an area border router that has an interface in area zero. Message Posted at: http://www.groupstudy.com/form/read.php?f=7&i=9272&t=9268 -- FAQ, list archives, and subscription info: http://www.groupstudy.com/list/cisco.html Report misconduct and Nondisclosure violations to [EMAIL PROTECTED]
Re: OSPF Hub and Spoke [7:9268]
If you have a number of routers on the end of each WAN link then you might want to extend area 0 to include those links and terminate on the remote routers. You might also be able to get away with not really having an area 0 on an actual network by creating a loopback interface and placing it by itself in area 0. Im just shooting that last one off the top of my head and have never tried it but it might work. Also dont forget that static routing works very well for a hub and spoke topology; this is if you don't have other redundant connections and you can redistribute between static and eigrp. To me it sounds like your hub and spoke wan links are your backbone. I would probably try and extend area 0 over those wan links. It also depends on your remote routers. I've got a 1600 on the end of a 56k line running tcp header compression that stays around 8%-10% processor usage, if youv'e got smaller routers like that and are pushing a lot of traffic through them and doing things like compression or encryption you might want to look at the processor utilization pretty closely. I've found OSPF to be a pretty low bandwidth user so I wouldn't worry about WAN link congestion if only the hub and spoke are in area 0 (of course if you extend the other areas over those WAN links and there are numerous recalculations in that area your network is in effect sending useless data over the WAN link and also possibly affecting all the other areas by slowing the hub router down). In closing, after babbling for quite a while, my gut would say to go with area 0 over the WAN links. blah blah, blah blah blah blah blah. Then again I could be wrong :) Ben, CCNP --- John Neiberger wrote: > I'm having trouble wrapping my brain around a > specific scenario and I > wanted to get your thoughts. Let's say we have a > hub and spoke network > with a single router as the hub. There are five > areas attached to the > backbone. It seems that we would have to extend > area 0 across the WAN > links, but I'm wondering what would happen if we > didn't. > > If we didn't, the backbone router would have no > interfaces in area 0. > I'm wondering if this would cause some major > problems. I bet that it > would but I'm having a hard time thinking through > what actual problems > might arise. Would this backbone router just "know" > that it was area 0 > because it has interfaces in multiple non-zero areas > and hence behave > correctly? > > One obvious problem is that the backbone router > would be a member of > every area and would thus be pretty busy if the > network got to be very > big. If we extended area 0 across the WAN link the > backbone router > would be protected from running SPF calculations > everytime a remote area > had a link change. > > What other problems would arise? Would this even > work at all? I don't > really have the tools to try it or I'd just attempt > this chaos myself. > As you can guess, we run eigrp everywhere so I'm > still clueless to some > of the workings of OSPF in a production environment. > > Regards, > John [EMAIL PROTECTED] __ Do You Yahoo!? Get personalized email addresses from Yahoo! Mail http://personal.mail.yahoo.com/ Message Posted at: http://www.groupstudy.com/form/read.php?f=7&i=9271&t=9268 -- FAQ, list archives, and subscription info: http://www.groupstudy.com/list/cisco.html Report misconduct and Nondisclosure violations to [EMAIL PROTECTED]
OSPF Hub and Spoke [7:9268]
I'm having trouble wrapping my brain around a specific scenario and I wanted to get your thoughts. Let's say we have a hub and spoke network with a single router as the hub. There are five areas attached to the backbone. It seems that we would have to extend area 0 across the WAN links, but I'm wondering what would happen if we didn't. If we didn't, the backbone router would have no interfaces in area 0. I'm wondering if this would cause some major problems. I bet that it would but I'm having a hard time thinking through what actual problems might arise. Would this backbone router just "know" that it was area 0 because it has interfaces in multiple non-zero areas and hence behave correctly? One obvious problem is that the backbone router would be a member of every area and would thus be pretty busy if the network got to be very big. If we extended area 0 across the WAN link the backbone router would be protected from running SPF calculations everytime a remote area had a link change. What other problems would arise? Would this even work at all? I don't really have the tools to try it or I'd just attempt this chaos myself. As you can guess, we run eigrp everywhere so I'm still clueless to some of the workings of OSPF in a production environment. Regards, John Message Posted at: http://www.groupstudy.com/form/read.php?f=7&i=9268&t=9268 -- FAQ, list archives, and subscription info: http://www.groupstudy.com/list/cisco.html Report misconduct and Nondisclosure violations to [EMAIL PROTECTED]
