[j-nsp] BGP Multipath

[Pavel Lunin]

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.