eBGP with iBGP with the same AS

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Aug 13th, 2008
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Hi All,


I have a network running OSPF I have 2 internet exit points with the same AS in different locations, OSPF is redistributed at this routers into BGP.


So one of the Routers is the exit for the internet.


In in the OSFP cloud we have about dozen Routers.


My question is how do I get both internet exit points to work at the same time?


Do I need to convert the OSPF cloud into iBGP for it to work?


Many thanks for any help

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rais Wed, 08/13/2008 - 12:00
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For outbound traffic you can inject BGP-learned default-route into OSPF so that OSPF cost is reflected internally. Sites closer to exit A would take that exit, vice versa for B.


Thanks.

richard.gosling Wed, 08/13/2008 - 12:11
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Hi


Will that overload the routing tables of the OSPF Routers.


Is there a document with a configuration guide.


Can we Mulithome also?


Many thanks Richard

Giuseppe Larosa Wed, 08/13/2008 - 12:02
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Hello Richard,

for optimal routing adding iBGP is required


but it is wise to do it if you receive full tables.

If you just receive an eBGP default route at each exit point there's no need for eBGP but just to originate an E1 type default-route using a route-map conditioned by the presence of the BGP 0.0.0.0/0

default-information originate route-map checkBGP type E1 metric 50


route-map checkBGP permit 10

match ip address prefix-list only-default

match ip next-hop 12


Hope to help

Giuseppe

Edison Ortiz Wed, 08/13/2008 - 12:34
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My question is how do I get both internet exit points to work at the same time?


You can add the default information-originate under the OSPF process on each ASBR.


The default information-originate will only advertise a default route if the router itself has a 0.0.0.0 route.


However, this design is only taking care of the outbound traffic. If you want the inbound traffic to take the same path (avoid asymmetrical routing) you need to talk to your ISP provider.


HTH,


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Edison.


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richard.gosling Wed, 08/13/2008 - 12:48
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Thanks for the posts so far, sorry if the questions are a bit basic.


So if both our PE routers are advetising a default route into our OSPF cloud, if a BGP neighbour drops on one PE and the other PE has a Route will that propagate onto the OSPF Network?


Is multihoming a option is it better?


Many thanks Richard



Edison Ortiz Wed, 08/13/2008 - 12:52
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So if both our PE routers are advetising a default route into our OSPF cloud, if a BGP neighbour drops on one PE and the other PE has a Route will that propagate onto the OSPF Network?


No.


OSPF only knows about the default route from those devices unless you redistribute BGP into OSPF.


However, you mentioned those BGP routers are receiving internet routes. Are they getting full internet routes, partial internet routes or just a default route?


Depending upon the type of route being received from the ISP, it can affect how the design should be implemented.


HTH,


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Edison.

richard.gosling Wed, 08/13/2008 - 13:00
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Not to sure how would you tell, the routing table is very big on our router and recieve about 275000 prefixes rough guess.

Edison Ortiz Wed, 08/13/2008 - 13:06
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It sounds like a full internet table. I don't recommend redistributing BGP into OSPF.


Go with the default information-originate on both routers and also have an iBGP between the 2 routers.


If one of the routers loses its connection to the internet, it can use the other router as the exit point to the network.


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Edison.

richard.gosling Wed, 08/13/2008 - 13:17
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Thats the other problem, we have about 15 Routers in the OSPF cloud the 2 PE routers are about 4 hops away from each other.


Don't I need a full mesh?

Can I just create iBGP between the 2 PE's?



Thanks Richard

Edison Ortiz Wed, 08/13/2008 - 13:21
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The 15 OSPF routers will find the closest exit point (any of the two BGP routers).


The BGP routers will handle the traffic towards the internet. If one of the routers loses its connection to the internet, it can use the other router as the exit point.


This can be accomplished with iBGP peering between these 2 routers.


If you aren't sure about the whole design, I highly recommend you bring some outside help.


There are a lot of details that can't be covered in a single post in a forum. A total evaluation of your network must be done for the best optimal design. This evaluation can take weeks.


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