[c-nsp] Re: ISP redundancy - internal routing question

[Mark Kent]

>> The question is. what is the best way for RouterB to announce what
>> routes it knows about to RouterA.  I am going to assume a iBGP
>> session between each other, and then give a weighting to what I
>> want to go to RouterB?

First, if you pass all routes over the ibgp session and let the
default bgp path selection work then that may be adequate for your
needs, and at least it's a good starting point.  Packets that hit
routerA destined for an IP that has a shorter ASpath through isp-D
will hop over to routerB and then out isp-D.

If that doesn't meet your needs (not balanced as you wish for example)
then you put an inbound route-map on one or more bgp sessions
to set local-pref (which is shared in the ibgp world, weight is not)
to encourage stuff to go where you want.

So, suppose none of the ISP are Sprint, and you want Sprint to
go via isp-D then you do this:

ip as-path access-list 9 permit _1239_

route-map ispD-in permit 10
  match as-path 9
  set local-pref 200

router bgp FOO
  nei x.y.z.w soft-reconfiguration inbound
  nei x.y.z.w route-map ispD-in in

For simplicity, I'ld let the default bgp algorithm run for 
a day or two to see where stuff goes, then on the "lesser" 
neighbors bump up local-pref on some as-paths, or on the
dominant neighbors set some as-path down to local-pref 90.

And don't forget to check if the ISP supply bgp communities
that may help you decide which traffic may be best sent out
to which ISP.

-mark