Quick OSPF Challenge
I thought of a task today for my next prep lab. I've been asking people this question, so far two out of three people have gotten it correct. Figured it'd be cool to post this on the Cisco forums I frequent as well.
Assume that all routers can communicate. Configure loopback IPs 192.168.1.1/24, 192.168.2.1/24, 192.168.3.1/24 and 192.168.4.1/24 on R6. Inject a summary for these networks into OSPF, ensure that R1 and all routers behind it see this summary and no specific routes. Do not use any summarization commands under the OSPF process to accomplish this. Do not enable another dynamic routing protocol to accomplish this.
Assume that all routers can communicate. Configure loopback IPs 192.168.1.1/24, 192.168.2.1/24, 192.168.3.1/24 and 192.168.4.1/24 on R6. Inject a summary for these networks into OSPF, ensure that R1 and all routers behind it see this summary and no specific routes. Do not use any summarization commands under the OSPF process to accomplish this. Do not enable another dynamic routing protocol to accomplish this.
Comments
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Forsaken_GA
Member Posts: 4,024
Well, he said assume all routers can communicate, so I'm assuming that's already done. I didn't mock up the full setup, what I posted was the first thing that occurred to me, and I just mocked up two routers to prove my concept. Unless there's something I'm seriously overlooking, there's no reason for me to think that R1 wouldn't pass the summary route downstream, and it's not receiving the more specifics, so it can't propagate them -
ColbyG
Member Posts: 1,264
You got it Forsaken. Wish I'd said to PM me or something so other people could answer too. -
Forsaken_GA
Member Posts: 4,024
meh, easily solved. acid, if you're still around, do me a favor and edit my config solution out of your quote so other folks can have a shot at it
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acidsatyr
Member Posts: 111
Ok guys how about this:
1) You can't use what you just did to solve it!
2) R4 must have a default route to reach these destinations (the summary, that is).
3) You can't manually inject a default route anywhere.
4) R5 must still see that summary.
:] -
tim100
Member Posts: 162
Just off the top of my head without getting too specific, it would involve a virtual link with an area range assuming when you say no summarization commands you specifically mean summary-address.
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APA
Member Posts: 959
APA come on down.....
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APA
Member Posts: 959
Just off the top of my head without getting too specific, it would involve a virtual link with an area range assuming when you say no summarization commands you specifically mean summary-address.
I actually assume Colby means no 'area-range' statement either.... as that is really the ABRs way of sending out a summary route into the different areas.
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acidsatyr
Member Posts: 111
Yea, no summary commands of any kind (summary-address, area range, etc).
Since we'r ahead of these other guys
, let me repost my question, lets see if you got this one, slightly different:
All routers behind R1, including R1 must have a route to network 192.168.0.0/21.
R1 and other routers behind it must not have any longer prefix than that /21 for that network, such as 192.168.1.0 or 192.168.2.0 which you configured on loopbacks.
Now, can you configure the topology so that:
1) You don't use what we did last time to solve it.
2) R4 must have a default route to reach this network on R6.
3) This default route can't be injected manually (ie, static route, or using default keyword in OSPF.
4) R5 must still see that network as 192.168.0.0/21.
5) All routers must be able to ping all 4 loopback addresses.
Others rules you said are still up, ie you cant use other routing protocol, you can't use area-range or summary command in OSPF.
Good luck!!) -
ColbyG
Member Posts: 1,264
Are we assuming the virtual link has been created for area 45? I think I have an idea how to do this, but I'm going to lab it first before saying anything, lol.
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tim100
Member Posts: 162
Yea, no summary commands of any kind (summary-address, area range, etc).
Then in that case, again not being too specific as to totally give it away, somewhere along the lines of static, null0, redistribute, prefix-list, route-map, etc... -
ColbyG
Member Posts: 1,264
You got it for the most part, I think. No real need for prefix-lists or a route-map though.
acid: For yours, the easiest way I can think of is a virtual link for R4 to area 0, then a gre tunnel from R5 to a router other than R4. Somehow I think that might be "cheating" though, lol. I'm too tired to lab this up, so if that answer doesn't work I'll get back to you tomorrow with my other idea that I'm not sure of. -
acidsatyr
Member Posts: 111
You got it for the most part, I think. No real need for prefix-lists or a route-map though.
acid: For yours, the easiest way I can think of is a virtual link for R4 to area 0, then a gre tunnel from R5 to a router other than R4. Somehow I think that might be "cheating" though, lol. I'm too tired to lab this up, so if that answer doesn't work I'll get back to you tomorrow with my other idea that I'm not sure of.
That won't work! And you aren't cheating as long as you don't do what we said in those rules.
And, you can't use redistribution. -
acidsatyr
Member Posts: 111
You can't use ip default-network on R4
So, no default keyword, no ip default, no ip route 0.0.0.0...
You still have to let that 192.168.0.0/21 somehow in ospf -
ColbyG
Member Posts: 1,264
That won't work! And you aren't cheating as long as you don't do what we said in those rules.
And, you can't use redistribution.
Why won't it work? You stub area 45, this gives R4 the default, then you create a GRE tunnel to R3 from R5, run OSPF across it, area 35 (or area 0). Then you get your externals on R5. -
acidsatyr
Member Posts: 111
Now that will work. Let me remind you mentioned virtual-link in area 45, which won't work if the area is stub.
What about other reqs ? -
ColbyG
Member Posts: 1,264
Alright, here's my ghetto solution (didn't know/forgot a virtual link couldn't go through a stub).
Area 234 stub, default to R4. GRE tunnel R4 to R1, the tunnel is in area 45, make it a stub. You get the default and no externals. This gives R4 two default (in theory, haven't labbed it, but it sounds right). Then GRE tunnel from R5 to R1, this one in area 0. You get the external routes and see the summary.
That should cover it, no? -
acidsatyr
Member Posts: 111
Alright, here's my ghetto solution (didn't know/forgot a virtual link couldn't go through a stub).
Area 234 stub, default to R4. GRE tunnel R4 to R1, the tunnel is in area 45, make it a stub. You get the default and no externals. This gives R4 two default (in theory, haven't labbed it, but it sounds right). Then GRE tunnel from R5 to R1, this one in area 0. You get the external routes and see the summary.
That should cover it, no?
You'r complicating it; all you need is area 234 as stub (or maybe totally stub ? Since you still haven't mentioned how are you injecting 192.168.0.0/21 into ospf, which is "harder" part then this ), and a tunnel belonging to area 0 connecting any router from Area 0 and R5. Now R5 is part of Area0 and has all the routes.
And yeah, you would need a filter on R5 to stop those routes getting to R4, but that's given. ;D -
ColbyG
Member Posts: 1,264
Oooh, I thought you were saying the summary is already injected. I'll try to think of that solution in a minute.
I don't think your solution works. Are 45 needs a virtual link or tunnel to area 0 on R4 or OSPF won't work. Again, I haven't labbed this, so I'm just talking book stuff and assumptions. You're saying you will connect R5 to Area 0 through a tunnel, but will it even get that far? -
acidsatyr
Member Posts: 111
Oh it works
So the problem area is 45 which needs to be connected to area 0 somehow. Virtual link won't work as LSAs are not flooded accross stub network, which we need to inject default route to R4. Therefore simple tunnel connecting R5 and Area 0 make R5 an ABR and Area 45 connected to area 0.
Also, if that tunnel is Area 0, you need filtering on R5 so that LSAs are not flooded to R4. If tunnel is any other area number (say, 15), R5 won't be ABR anymore (as it is sitting between Area 45 and 15) and won't flood any routes to Area 45. -
ColbyG
Member Posts: 1,264
Oh it works
So the problem area is 45 which needs to be connected to area 0 somehow. Virtual link won't work as LSAs are not flooded accross stub network, which we need to inject default route to R4. Therefore simple tunnel connecting R5 and Area 0 make R5 an ABR and Area 45 connected to area 0.
I get the theory, but you have a broken OSPF domain, so R5 shouldn't have connectivity to area 0. Do you have this up and going now? If so, post some show commands indicating what you're saying. -
acidsatyr
Member Posts: 111
I get the theory, but you have a broken OSPF domain, so R5 shouldn't have connectivity to area 0. Do you have this up and going now? If so, post some show commands indicating what you're saying.
No i don't but it works. All you need is endpoints of tunnel (loop of R1 and R5) pointing to each other (you can use static route for this).
Is that what you mean?
Once the tunnel is up/up it will encapsulate all LSA from R1 and you are golden. -
ColbyG
Member Posts: 1,264
We can use static routes?! Well ****, this just got a lot easier.;)
I can't think of how to get that summary into OSPF. It's driving me nuts. -
acidsatyr
Member Posts: 111
Yeah i didn't say anything about not using static routes. You just can't use static for default, but we'r over that now anyway.
You are CCIP....anything comes to mind?
