eBGP summary into OSPF

[richard.priest]

Hi,

I'm after some advise. Currently I have x3 external routers providing WAN connectivity. All public prefixes are advertised to x3 ISP's via BGP, the providers are importing routes into.

Internally everything is static routed. I've a preference to migrate to OSPF, but what I can't figure out is how to get the external BGP routes from the ISP's (show ip bgp summary shows 864,673 network entries and 2,159,028 path entries), into OSPF without redistributing the lot into the OSPF process.

There is no default route set on the external routers, so I can't redistribute that.

Currently I'm using a static route from the internal LAN to a VIP on the routers (HSRP), but I get no load balancing with this method, in addition, any issue with the VIP subnet means absolutely everything stops working, which I want to avoid!

I've done quite a bit of research, but only seem to get lab examples where you can easily import the 5-10 eBGP routes into OSPF, or where there's a default route  advertised by the external provider which can then be imported into OSPF

Any advise would be most welcome

Many thanks

Rich

[Muhammad Awais Khan]

Hi,

If you want to run OSPF between your Router and LAN and insert default Route also then you can add "default-information orignate always" under Router ospf x

If you want to redistribute specific route into ospf from EBGP then you can take advantage of Route-Map where you will match only interested routes using ACL and call this Route-Map with redistribute command under OSPF process.

[richard.priest]

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@Muhammad Awais Khan wrote:

Hi,

If you want to run OSPF between your Router and LAN and insert default Route also then you can add "default-information orignate always" under Router ospf x

If you want to redistribute specific route into ospf from EBGP then you can take advantage of Route-Map where you will match only interested routes using ACL and call this Route-Map with redistribute command under OSPF process.

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Thanks Muhammad,

There OSPF is the LAN IGP, so there is no default route to originate from it.

I don't want to redistribute specific BGP routes from eBGP into OSPF, I want effectively a default route or a summarised route f all the BGP routes to be redistributed into the OSPF process. I'm aware I can't redistribute all the eBGP routes into OSPF as there are far far too many.

[Vishnu Vardhan S]

I think the "default-information originate always" should help out. 

always command will help you to get a default route even though you dont actually have on in R3

R1------R2(OSPF)-------OSPF(R3)BGP-------BGPR4

Try using the command in R3 which will spread the default route to entire network in OSPF. 

https://www.cisco.com/c/en/us/support/docs/ip/open-shortest-path-first-ospf/47868-ospfdb9.html

[Georg Pauwen]

Hello,

I don't think OSPF is the right choice with what you want to do. Even if you would get the default route from your ISP, you would not be able to redistribute it from BGP into OSPF. Your best option is to redistribute BGP into OSPF and then use (the most restrictive) summary addresses under the OSPF process, but that still would not get you the default route, and you have indicated that this is not what you want to do. So, why not run iBGP in your internal network ? That would make default routing a lot easier, and also give you a lot of options for load balancing...

[richard.priest]

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@Georg Pauwen wrote:

Hello,

I don't think OSPF is the right choice with what you want to do. Even if you would get the default route from your ISP, you would not be able to redistribute it from BGP into OSPF. Your best option is to redistribute BGP into OSPF and then use (the most restrictive) summary addresses under the OSPF process, but that still would not get you the default route, and you have indicated that this is not what you want to do. So, why not run iBGP in your internal network ? That would make default routing a lot easier, and also give you a lot of options for load balancing...

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I've just tested default-information originate always and that seems to allow a default route to be propagated.

However I am curious regarding the iBGP side of it if that's a better setup. I'm still not keen on the core switches having millions of routes in their tables, I'd rather the routers have that job. Is there a standardised method of importing a summary of those routes into the iBGP process?

many thanks

Richard

[Georg Pauwen]

Hello,

the 'default-information originate always' gets the default route to your OSPF neighbors, but not into the routing table of the router on which you issue the command, in your case the router that runs eBGP with your ISP. In order to get any BGP routes into OSPF, you still need to do some sort of redistribution, and then configure summary addresses.

Which routes are you currently getting from your ISP, can you post the output of 'show ip route' from one of the ISP facing routers ? Actually, the easiest would be to ask your ISPs to just send you the default route, is that an option ? Then you would not need to redistribute anything, and you could just originate the default to your OSPF neighbors...