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