Don't look now, but here comes round two of the Net building boom.Though sales of core Internet routers -- the big traffic-management devices at key junction points of communications networks -- have been slowing, total revenue for the segment next year is expected to hit levels not seen since the peak of Net construction in 2000. In the third quarter, core router sales grew 35% from a year ago to $453 million, according to Dell'Oro Group, an industry research shop. But the pace of core router growth has slowed from the recent 50% clip. After the new century collapse of the telecom industry and the sharp slowdown in network-equipment spending, some niches of the gearmaker market have managed to thrive as the march of Internet expansion continues. Shin Umeda, an analyst with Dell'Oro Group, says the growth rate "will begin to taper off a bit" next year, but that is largely a function of how big the market has become. Core router sales likely will hit the $2 billion mark in 2006, equaling the 2000 peak, says Umeda. Two players, Cisco ( CSCO) and Juniper ( JNPR), have dominated the core router market over the past five years. Cisco has been the top seller throughout, but Juniper has gradually chipped away at the lead. For the quarter ended in September, Cisco's core router market share dropped to 55% from 56% in the prior quarter and 59% a year ago. Juniper, meanwhile, has managed to capture 37% of the market. "Over the long term, there has been a trend shifting away from Cisco to Juniper, though this year that has moderated somewhat," says Umeda. A good portion of Cisco's solid core router performance this year has been the moderate success of its CRS-1 mega router.