BigISP<-->SmallISP peerings

>Hrm. Does it matter if the small ISP already has transit with someone else?
>In such a case, peering does nothing but shorten the path from SmallISP to
>BigISP in order to no longer make it go through SmallISPs transit provider.

Yes, but then from the point of view of large ISP the peering is of zero
value. You see, it has to deliver packets to IXP anyway. OTOH, the
load on routers, bloated configurations and engineering resources to
support the additional peering are quite real.

Not quite zero - if you have direct Small/Big vs Small/Transit/Big,
then you bypass any potential problems with Transit. (Andrew Partan)