William Herrin wrote on 6/1/2015 3:28 PM:
On Mon, Jun 1, 2015 at 2:40 PM, Blake Hudson <blake@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