19.07.2013 10:54, Mark Tinka wrote:
Why not prepend the routes out of ur gigE? That way it
>becomes the backup link and u can turn down bgp anytime
>you want on the gigE without impact
Because many ISP's LOCAL_PREF the hell out of customer
routes, where prepending would be ignored. However, this
isn't always the case, so the OP can always try it.
I'm afraid this explanation needs to be expanded a bit. High LP on the
ISP side for customers' routes is a common practice, but this makes the
perpended AS-PATH (and other BGP attributes) ignored only within the ISP
AS. So traffic from the backup ISP customers, DCs, etc will come
directly through their link despite the prepended path. However, when
they advertise the prefixes to the world, AS-PATH will come back into
play, thus making the prepend work. If the 1G-ISP does not support LP
altering communities, it might be a compromise solution to receive the
traffic generated by the 1G-ISP itself directly though 1G link, but keep
the rest of the Internet on the 10GE using AS-PATH prepend.
As Mark said, community signaling for LP is a more ideal way
technically, but the problem is that not all ISP support them. Often
NOCs don't even know what it is. I've seen an ISP (not that small one),
who eventually stopped supporting LP communities silently. Reply to the
customer's complain was: "It's not needed anymore, as we upgraded our
routers to more advanced gear". They fixed it at the end, but it was
worth a week-long scandal. So, I'd recommend to still use prepends
together with the community signaled LP, just in case.
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