[c-nsp] primary & backup b/w w/ OSPF & BGP

[Kristofer Sigurdsson]

Hi,

On Tue, 2005-09-27 at 17:02 -0400, Adam Greene wrote:

- snip -

> What I need to do now is move from a purely primary / backup bandwidth
> configuration with my ISP's to one in which part of my network prefers ISP A
> inbound & outbound, while the other part prefers ISP B. However, should one
> ISP fail, the section of my network which prefers it should still fail over
> to the other ISP.
> 
> I am considering dividing my single OSPF area 0 into two areas to accomplish
> this. Area 1 would have an eBGP session with ISP A, and area 0 with ISP B. I
> would assign a metric of 100 to the default routes redistributed from eBGP
> into OSPF, and a metric of 110 to default routes learned via inter-area
> redistribution. I'd prepend area 0 routes when advertising to ISP A, and
> prepend area 1 routes when advertising to ISP B.

Are your two uplinks connected to the same edge router?  If so, the 
simplest way to do this would be to apply route-maps on your outbound
BGP announcements to prepend hops for the correct announcements and use
policy routing to direct traffic to the right next-hop (if it is up).
That way, you would get rid of the extra complications of having two 
OSPF areas.

Also, how big are your announcements?  If you are announcing to two 
different ISPs, say /24 to each, your routes will most likely be 
filtered at some point, which would mean that part of your inbound 
traffic would go the "wrong" way.

-- 
Kristófer Sigurðsson           Tel: +354 414 1600
Netrekstur/Network Operations  IP Fjarskipti ehf.