BGP Multihoming 2 providers full or partial?

Mon Jun 1 21:05:55 UTC 2015

William Herrin wrote on 6/1/2015 3:28 PM:
> On Mon, Jun 1, 2015 at 2:40 PM, Blake Hudson <blake at ispn.net> wrote:
>> A gateway of last resort, also called a backup default route, will take care
>> of partitions
> No, Blake, it won't. A partition means one of your ISPs has no route
> to the destination. Route the packet to that ISP via a default route
> and it gets sent to /dev/null. More, during a partition you don't get
> to pick which of your ISPs lack the route.
>
> Regards,
> Bill Herrin
Thanks. I see what you mean. I was coming from the vantage point of 
taking full routes and assuming that the prefix information existed and 
simply hadn't filtered down to the op's equipment yet. It was there, 
just upstream a hop or two. This could be due to a newly advertised 
route, path changes, or initial BGP convergence. In this case, a backup 
route provides the necessary bridge while BGP converges. I see what you 
mean about one ISP having a route and the other not; Taking full routes 
resolves any question about the best (only) path.

After studying failure modes and attempting to optimize BGP using 
partial routing tables, I am of the opinion that BGP with a full routing 
table to directly connected devices is by far the best way to gain the 
availability benefits of BGP. Many attempts to cost save through 
multi-hop BGP or traffic engineering end up breaking down when a fault 
occurs. Some faults, like link state, are easy to detect and work 
around. Other faults, like where a peer is up, but has no outside 
connectivity, are harder to detect if you're taking anything less than 
full routes.

--Blake