routing loop

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May 10th, 2010
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guys we have two router (redundant WAN connection to same service provider). we have bunch of network in core using ospf (WAN router is config with OSPF LAN side) now there was requirement by customer that bunch the network together and advertise them in BGP however there was a special requirement that specif network shd take specific router for in-coming traffic

the solution was that we have done prepend and aggregate the routes in BGP

so we have done

agregation with /16 under BGP (no network command was used as router was getting all the routes from OSPF)

and then i have done two porefix list

perix list from ospf to bgp

prefix list from BGP (not to ospf but from service provider)

in prefix list in bgp i have allowed default route (on both routers) and some specific routes for network management

in prefix list from ospf to bgp i have put /16 networks for redistribution (so in future if they add oteh routes they shd not get propagated to WAN)

now in as-path prepend they wanted that shd come from WAN router 1 and shd take router 2.and if one router fails the other network shd take the same.

in redistribution from BGP i have done

redistribute bgp subnets and second command was default information originate under ospf.

now the customer is saying that they have routing loop and told us that we shd put filters on redistribution of BGP into ospf

All i am saying that we are only allowing default routerplus specific routes how come they see there own network

which is

he is saying that since we are redistributing BGP into ospf with command redistribute the router is redistribution all subnet in BGP routing table is it possible as i guess we are advertising those networks with aggregation how come bgp is again redistributing them again into ospf. I think that BGP shd be only redistributing routes which is learned by BGP not advertising by BGP...

guys help me required.

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bgandhi Tue, 05/11/2010 - 00:41
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Just confirm that you have used summary-only while aggregating in BGP, else it would also advertise specific routes as well.

If this does not address your issue, please share topology diagram for more clarity with configurations.



The_guroo_2 Tue, 05/11/2010 - 01:26
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Thanks ya it has a summary-only as i was aware of that....the main point is where i am confused....from service provider we are getting default route on both routers plys some specfic routes now we are redistributing them back in ospf so the million dollar question is will the router advertise back the routes to ospf (the routes which are learned from ospf and advertised in bgp) as under ospf we are using

redistribute bgp subnets

default information orginate

the customer enginer is saying that since the routes are in bgp table (as its redisttrbued from ospf to bgp) so when u r redistributing back it will forward the same routes with default route which i am not beleieving and thats why i am here

bgandhi Tue, 05/11/2010 - 01:40
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yes. It is very much possible as I understand you have OSPF neighborship within the customer network as well. best way to prevent it is by specifically tagging the routes being learnt and filter while redistributing.

1.) Just identify the routes learnt from customer and tag them with let's say 110.

2.) while doing redistribution on other router just filter them with route-map identifying tag 110.

Hope this would help.




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