ODPF - How to not propagate default gateway between areas?
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Posted by Sailor on December 1, 2007, 11:31 am
Please log in for more thread options Two OSPF areas (0 and 1) are connected with ABR (Area Border Router). Every area has it's own default gateway (DG), that should be propagate only in a scope of this area. Information about networks in area 0 should be propagate to area 1. Information about networks in area 1 should be propagate to area 0. Information about DG from area 0 should not be propagate to area 1. Information about DG from area 1 should not be propagate to area 0. Example diagram (use monospace font): (Internet) | | | (cloud of routers from AREA0) | | | R_AREA0 <----serial----> ABR <----serial----> R_AREA1
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| | | (cloud of routers from AREA1) | | | | (Internet) I try to use filter-list on ABR, but it do not work as I would like to. Information about particular internal networks is easy to filter, but DG are always distribute between areas and this makes a problem. DG is propagate through command "default-information originate" and it's seen by other routers as "OSPF external type 2". I've read somewhere that external routes can't be filter by filter-list in ABR. That explains why DG routes are propagate between areas. But I would like to know, if there is possibility to resolve the problem. Part of routing table in cloud of routers from AREA1 (notice E2 route) O*E2 0.0.0.0/0 [110/1] via 192.168.167.6, 01:15:55, FastEthernet0/0 Part of configuration from ABR: router ospf 1 area 0 filter-list prefix area1nets in area 1 filter-list prefix area0nets in network 192.168.0.0 0.0.127.255 area 0 network 192.168.128.0 0.0.127.255 area 1 ip prefix-list area0nets seq 5 permit 192.168.12.0/24 ip prefix-list area0nets seq 10 permit 192.168.24.0/24 ip prefix-list area0nets seq 15 permit 192.168.23.0/24 ip prefix-list area0nets seq 20 permit 192.168.34.0/24 ip prefix-list area0nets seq 25 deny 0.0.0.0/0 ! ip prefix-list area1nets seq 5 permit 192.168.146.0/24 ip prefix-list area1nets seq 10 permit 192.168.156.0/24 ip prefix-list area1nets seq 15 permit 192.168.167.0/24 ip prefix-list area1nets seq 20 deny 0.0.0.0/0 -- Best regards, Sailor | |||||||||||||
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Posted by pk on December 1, 2007, 12:30 pm
Please log in for more thread options > DG is propagate through command "default-information originate" and it's
> seen by other routers as "OSPF external type 2". I've read somewhere > that external routes can't be filter by filter-list in ABR. That > explains why DG routes are propagate between areas. But I would like to > know, if there is possibility to resolve the problem. Maybe I'm not understanding correctly, but can't you just avoid using "default-information originate"? | |||||||||||||
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Posted by Sailor on December 1, 2007, 1:06 pm
Please log in for more thread options pk pisze:
> Sailor wrote:
> >> DG is propagate through command "default-information originate" and it's
>> seen by other routers as "OSPF external type 2". I've read somewhere >> that external routes can't be filter by filter-list in ABR. That >> explains why DG routes are propagate between areas. But I would like to >> know, if there is possibility to resolve the problem. >
> Maybe I'm not understanding correctly, but can't you just avoid > using "default-information originate"? Do you mean to configuring default gw on each router as alternative? I would like to avoid this. -- Best regards, Sailor | |||||||||||||
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Posted by Sailor on December 1, 2007, 1:12 pm
Please log in for more thread options pk pisze:
> Sailor wrote:
> >> DG is propagate through command "default-information originate" and it's
>> seen by other routers as "OSPF external type 2". I've read somewhere >> that external routes can't be filter by filter-list in ABR. That >> explains why DG routes are propagate between areas. But I would like to >> know, if there is possibility to resolve the problem. >
> Maybe I'm not understanding correctly, but can't you just avoid > using "default-information originate"? Is there another method to propagate information about default gateway to routers in area? -- Best regards, Sailor | |||||||||||||
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Posted by pk on December 2, 2007, 9:57 am
Please log in for more thread options Sailor wrote:
>> Maybe I'm not understanding correctly, but can't you just avoid
>> using "default-information originate"? >
> Is there another method to propagate information about default gateway > to routers in area? Ok, so IIUC the internet-connected routers generate the default routes, and you'd like to block them at the ABR. Topology: internet----r1a0---(area0 routers)---ABR---(area1 routers)---r1a1---internet IIUC, r1a0 and r1a1 originate a default route, which is meant to serve as the default route for the routers of the area. 1) filter-list only filters type 3 LSAs, so in this case it does nothing to default routes. 2) if you are worried about having multiple default routes, then have the ASBRs originate the default route using metric type 1. This way, routers internal to an area will use the "closest" default route (which will probably be the one originated by the area's ASBR; if this is not the case, you'll probably be able to adjust the metrics to make this happen). 3) even if you use the default metric type 2, routers internal to an area will still prefer the default route advertised by the closest ASBR, so the same concepts expressed in 2) apply, but this time you have to adjust the metric *to the ASBR*, not the metric of the default route. 4) you didn't say anything about what default route should be used by the ABR. If you don't want it to load-balance between the two (and you probably don't), then make sure its metrics to the two ASBR are not equal, or that it receives the default routes with different metrics. 5) external LSAs are flooded throughout the whole OSPF domain by definition (unless stub or nssa areas are used, but in your case one of the areas is area 0 so this is not an option). If you still think that you want to filter external LSAs, you might run two different OSPF processes on the ABR, and do mutual route redistribution between them, filtering out the unwanted default routes using a route map. No need to say that this solution is awkward to say the least, making all the routes coming from the other "area" look external. If you need to do that, it's usually a symptom of bad network design, so you better reorganize your network in a different way. | |||||||||||||
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