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Webcast-Catalyst9k
New Member

eBGP with iBGP with the same AS

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

10 REPLIES
Silver

Re: eBGP with iBGP with the same AS

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.

New Member

Re: eBGP with iBGP with the same AS

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

Hall of Fame Super Silver

Re: eBGP with iBGP with the same AS

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

Hall of Fame Super Bronze

Re: eBGP with iBGP with the same AS

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,

__

Edison.

Please rate helpful posts

New Member

Re: eBGP with iBGP with the same AS

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

Hall of Fame Super Bronze

Re: eBGP with iBGP with the same AS

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,

__

Edison.

New Member

Re: eBGP with iBGP with the same AS

Not to sure how would you tell, the routing table is very big on our router and recieve about 275000 prefixes rough guess.

Hall of Fame Super Bronze

Re: eBGP with iBGP with the same AS

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.

__

Edison.

New Member

Re: eBGP with iBGP with the same AS

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

Hall of Fame Super Bronze

Re: eBGP with iBGP with the same AS

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.

__

Edison.

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