[c-nsp] Three ISPs - Three Edge Routers - iBGP Mesh

[Kevin Loch]

Mark Mason wrote:
> Two of our DC's are about to get their 3rd internet drop. Each ISP connection has its own edge router. HSRP is running facing on the LAN side. Please see https://supportforums.cisco.com/message/3496562#3496562 for topology and further discussions. I expect that packets leaving the DC will hit the HSRP active, perform the route lookup and exit via the best path BGP has selected (and/or the best path my PfR setup has installed). Does anyone see any gotcha's with just letting BGP do its thing; no local-pref changing, no path prepending?
> 

Given the flatt-ish topology of the Internet these days you will see
most of your traffic use the local transit on the active hsrp node.
This is because for the same route with equal as-path length and
local-preference the router will prefer the ebgp (local) route
over the ibgp routes.

If you want to roughly balance outbound traffic across all three
transit links, you will need to use local-pref to prefer some 
routes/as-paths over others regardless of whether they are on the
local router or not.  The common way to do this is to make a short list
of large ISP/backbone AS's, prefer some of them on each link and
adjust until you get the preferred traffic distribution.

- Kevin