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

Adam Greene maillist at webjogger.net
Tue Sep 27 17:02:28 EDT 2005


I'm an ISP and am currently running BGP to (2) upstream ISP's. The upstream
ISP's send me default routes, which I then inject into OSPF. I take ISP A's
default route and give it a metric of 100. I assign ISP B's default route a
metric of 110. In this way, all traffic outbound from my AS prefers ISP A.
By the same token, I heavily prepend route announcements to ISP B so inbound
traffic also prefers ISP A. If the connection to ISP A goes down, we have
full backup connectivity through ISP B.

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.

Does anyone see any big errors in this reasoning, or have a better way of
accomplishing my objectives?

Thanks y'all,

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